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# std.typecons

This module implements a variety of type constructors, i.e., templates that allow construction of new, useful general-purpose types.

Category | Functions
--- | ---
Tuple | isTuple Tuple tuple reverse
Flags | BitFlags isBitFlagEnum Flag No Yes
Memory allocation | RefCounted refCounted RefCountedAutoInitialize scoped Unique
Code generation | AutoImplement BlackHole generateAssertTrap generateEmptyFunction WhiteHole
Nullable | Nullable nullable NullableRef nullableRef
Proxies | Proxy rebindable Rebindable ReplaceType unwrap wrap
Types | alignForSize Ternary Typedef TypedefType UnqualRef

License:
Boost License 1.0.

Source std/typecons.d

Authors:
Andrei Alexandrescu, Bartosz Milewski, Don Clugston, Shin Fujishiro, Kenji Hara
Examples:

    // value tuples
    alias Coord = Tuple!(int, "x", int, "y", int, "z");
    Coord c;
    c[1] = 1;       // access by index
    c.z = 1;        // access by given name
    writeln(c); // Coord(0, 1, 1)

    // names can be omitted
    alias DicEntry = Tuple!(string, string);

    // tuples can also be constructed on instantiation
    writeln(tuple(2, 3, 4)[1]); // 3
    // construction on instantiation works with names too
    writeln(tuple!("x", "y", "z")(2, 3, 4).y); // 3

    // Rebindable references to const and immutable objects
    {
        class Widget { void foo() const @safe {} }
        const w1 = new Widget, w2 = new Widget;
        w1.foo();
        // w1 = w2 would not work; can't rebind const object
        auto r = Rebindable!(const Widget)(w1);
        // invoke method as if r were a Widget object
        r.foo();
        // rebind r to refer to another object
        r = w2;
    }

struct `Unique`(T);
  Encapsulates unique ownership of a resource.
  When a `Unique`!T goes out of scope it will call destroy on the resource T that it manages, unless it is transferred. One important consequence of destroy is that it will call the destructor of the resource T. GC-managed references are not guaranteed to be valid during a destructor call, but other members of T, such as file handles or pointers to malloc memory, will still be valid during the destructor call. This allows the resource T to deallocate or clean up any non-GC resources.
  If it is desirable to persist a `Unique`!T outside of its original scope, then it can be transferred. The transfer can be explicit, by calling release, or implicit, when returning Unique from a function. The resource T can be a polymorphic class object or instance of an interface, in which case Unique behaves polymorphically too.
  If T is a value type, then `Unique`!T will be implemented as a reference to a T.
  Examples:

      static struct S
      {
          int i;
          this(int i){this.i = i;}
      }
      Unique!S produce()
      {
          // Construct a unique instance of S on the heap
          Unique!S ut = new S(5);
          // Implicit transfer of ownership
          return ut;
      }
      // Borrow a unique resource by ref
      void increment(ref Unique!S ur)
      {
          ur.i++;
      }
      void consume(Unique!S u2)
      {
          writeln(u2.i); // 6
          // Resource automatically deleted here
      }
      Unique!S u1;
      assert(u1.isEmpty);
      u1 = produce();
      increment(u1);
      writeln(u1.i); // 6
      //consume(u1); // Error: u1 is not copyable
      // Transfer ownership of the resource
      consume(u1.release);
      assert(u1.isEmpty);

  alias `RefT` = T;
    Represents a reference to T. Resolves to T* if T is a value type.
  Unique!T `create`(A...)(auto ref A `args`)
  if (__traits(compiles, new T(`args`)));
    Allows safe construction of Unique. It creates the resource and guarantees unique ownership of it (unless T publishes aliases of this).

    Note Nested structs/classes cannot be created.

    Parameters:

    A `args` | Arguments to pass to T's constructor.
        static class C {}
        auto u = Unique!(C).create();

  this(RefT `p`);
    Constructor that takes an rvalue. It will ensure uniqueness, as long as the rvalue isn't just a view on an lvalue (e.g., a cast). Typical usage:

        Unique!Foo f = new Foo;

  this(ref RefT `p`);
    Constructor that takes an lvalue. It nulls its source. The nulling will ensure uniqueness as long as there are no previous aliases to the source.
  this(U)(Unique!U `u`)
  if (is(`u`.RefT : RefT));
    Constructor that takes a Unique of a type that is convertible to our type.
    Typically used to transfer a Unique rvalue of derived type to a Unique of base type.

    Example

        class C : Object {}

        Unique!C uc = new C;
        Unique!Object uo = uc.release;

  void `opAssign`(U)(Unique!U `u`)
  if (is(`u`.RefT : RefT));
    Transfer ownership from a Unique of a type that is convertible to our type.
  const @property bool `isEmpty`();
    Returns whether the resource exists.
  Unique `release`();
    Transfer ownership to a Unique rvalue. Nullifies the current contents. Same as calling std.algorithm.move on it.
struct `Tuple`(Specs...) if (distinctFieldNames!Specs);
  Tuple of values, for example Tuple!(int, string) is a record that stores an int and a string. `Tuple` can be used to bundle values together, notably when returning multiple values from a function. If obj is a `Tuple`, the individual members are accessible with the syntax obj[0] for the first field, obj[1] for the second, and so on.
  See Also:
  tuple.
  Parameters:

  Specs | A list of types (and optionally, member names) that the `Tuple` contains.

  Examples:

      Tuple!(int, int) point;
      // assign coordinates
      point[0] = 5;
      point[1] = 6;
      // read coordinates
      auto x = point[0];
      auto y = point[1];

  Examples:
  `Tuple` members can be named. It is legal to mix named and unnamed members. The method above is still applicable to all fields.

      alias Entry = Tuple!(int, "index", string, "value");
      Entry e;
      e.index = 4;
      e.value = "Hello";
      writeln(e[1]); // "Hello"
      writeln(e[0]); // 4

  Examples:
  A `Tuple` with named fields is a distinct type from a `Tuple` with unnamed fields, i.e. each naming imparts a separate type for the `Tuple`. Two `Tuple`s differing in naming only are still distinct, even though they might have the same structure.

      Tuple!(int, "x", int, "y") point1;
      Tuple!(int, int) point2;
      assert(!is(typeof(point1) == typeof(point2)));

  Examples:
  Use tuples as ranges

      import std.algorithm.iteration : sum;
      import std.range : only;
      auto t = tuple(1, 2);
      writeln(t.expand.only.sum); // 3

  Examples:
  Concatenate tuples

      import std.meta : AliasSeq;
      auto t = tuple(1, "2") ~ tuple(ushort(42), true);
      static assert(is(t.Types == AliasSeq!(int, string, ushort, bool)));
      writeln(t[1]); // "2"
      writeln(t[2]); // 42
      writeln(t[3]); // true

  alias `Types` = staticMap!(extractType, fieldSpecs);
    The types of the Tuple's components.
  alias `fieldNames` = staticMap!(extractName, fieldSpecs);
    The names of the Tuple's components. Unnamed fields have empty names.
    Examples:

        import std.meta : AliasSeq;
        alias Fields = Tuple!(int, "id", string, float);
        static assert(Fields.fieldNames == AliasSeq!("id", "", ""));

  Types `expand`;
    Use t.`expand` for a Tuple t to expand it into its components. The result of `expand` acts as if the Tuple's components were listed as a list of values. (Ordinarily, a Tuple acts as a single value.)
    Examples:

        auto t1 = tuple(1, " hello ", 'a');
        writeln(t1.toString()); // `Tuple!(int, string, char)(1, " hello ", 'a')`

        void takeSeveralTypes(int n, string s, bool b)
        {
            assert(n == 4 && s == "test" && b == false);
        }

        auto t2 = tuple(4, "test", false);
        //t.expand acting as a list of values
        takeSeveralTypes(t2.expand);

  this(Types `values`);
    Constructor taking one value for each field.
    Parameters:

    Types `values` | A list of values that are either the same types as those given by the Types field of this Tuple, or can implicitly convert to those types. They must be in the same order as they appear in Types.

    Examples:

        alias ISD = Tuple!(int, string, double);
        auto tup = ISD(1, "test", 3.2);
        writeln(tup.toString()); // `Tuple!(int, string, double)(1, "test", 3.2)`

  this(U, size_t n)(U[n] `values`)
  if (n == Types.length && allSatisfy!(isBuildableFrom!U, Types));
    Constructor taking a compatible array.
    Parameters:

    U[n] `values` | A compatible static array to build the Tuple from. Array slices are not supported.

    Examples:

        int[2] ints;
        Tuple!(int, int) t = ints;

  this(U)(U `another`)
  if (areBuildCompatibleTuples!(typeof(this), U));
    Constructor taking a compatible Tuple. Two Tuples are compatible iff they are both of the same length, and, for each type T on the left-hand side, the corresponding type U on the right-hand side can implicitly convert to T.
    Parameters:

    U `another` | A compatible Tuple to build from. Its type must be compatible with the target Tuple's type.

    Examples:

        alias IntVec = Tuple!(int, int, int);
        alias DubVec = Tuple!(double, double, double);

        IntVec iv = tuple(1, 1, 1);

        //Ok, int can implicitly convert to double
        DubVec dv = iv;
        //Error: double cannot implicitly convert to int
        //IntVec iv2 = dv;

  bool `opEquals`(R)(R `rhs`)
  if (areCompatibleTuples!(typeof(this), R, "=="));
  const bool `opEquals`(R)(R `rhs`)
  if (areCompatibleTuples!(typeof(this), R, "=="));
  bool `opEquals`(R...)(auto ref R `rhs`)
  if (R.length > 1 && areCompatibleTuples!(typeof(this), Tuple!R, "=="));
    Comparison for equality. Two Tuples are considered equal iff they fulfill the following criteria:
    - Each Tuple is the same length.
    - For each type T on the left-hand side and each type U on the right-hand side, values of type T can be compared with values of type U.
    - For each value v1 on the left-hand side and each value v2 on the right-hand side, the expression v1 == v2 is true.
    Parameters:

    R `rhs` | The Tuple to compare against. It must meeting the criteria for comparison between Tuples.

    Returns:
    true if both Tuples are equal, otherwise false.
    Examples:

        Tuple!(int, string) t1 = tuple(1, "test");
        Tuple!(double, string) t2 =  tuple(1.0, "test");
        //Ok, int can be compared with double and
        //both have a value of 1
        writeln(t1); // t2

  int `opCmp`(R)(R `rhs`)
  if (areCompatibleTuples!(typeof(this), R, "<"));
  const int `opCmp`(R)(R `rhs`)
  if (areCompatibleTuples!(typeof(this), R, "<"));
    Comparison for ordering.
    Parameters:

    R `rhs` | The Tuple to compare against. It must meet the criteria for comparison between Tuples.

    Returns:
    For any values v1 on the right-hand side and v2 on the left-hand side:
    - A negative integer if the expression v1 < v2 is true.
    - A positive integer if the expression v1 > v2 is true.
    - 0 if the expression v1 == v2 is true.
    Examples:
    The first v1 for which v1 > v2 is true determines the result. This could lead to unexpected behaviour.

        auto tup1 = tuple(1, 1, 1);
        auto tup2 = tuple(1, 100, 100);
        assert(tup1 < tup2);

        //Only the first result matters for comparison
        tup1[0] = 2;
        assert(tup1 > tup2);

  auto `opBinary`(string op, T)(auto ref T `t`)
  if (op == "~" && !(is(T : U[], U) && isTuple!U));
  auto `opBinaryRight`(string op, T)(auto ref T `t`)
  if (op == "~" && !(is(T : U[], U) && isTuple!U));
    Concatenate Tuples. Tuple concatenation is only allowed if all named fields are distinct (no named field of this tuple occurs in `t` and no named field of `t` occurs in this tuple).
    Parameters:

    T `t` | The Tuple to concatenate with

    Returns:
    A concatenation of this tuple and `t`
  ref Tuple `opAssign`(R)(auto ref R `rhs`)
  if (areCompatibleTuples!(typeof(this), R, "="));
    Assignment from another Tuple.
    Parameters:

    R `rhs` | The source Tuple to assign from. Each element of the source Tuple must be implicitly assignable to each respective element of the target Tuple.

  inout ref auto `rename`(names...)() return
  if (names.length == 0 || allSatisfy!(isSomeString, typeof(names)));
    Renames the elements of a Tuple.
    `rename` uses the passed names and returns a new Tuple using these names, with the content unchanged. If fewer names are passed than there are members of the Tuple then those trailing members are unchanged. An empty string will remove the name for that member. It is an compile-time error to pass more names than there are members of the Tuple.
    Examples:

        auto t0 = tuple(4, "hello");

        auto t0Named = t0.rename!("val", "tag");
        writeln(t0Named.val); // 4
        writeln(t0Named.tag); // "hello"

        Tuple!(float, "dat", size_t[2], "pos") t1;
        t1.pos = [2, 1];
        auto t1Named = t1.rename!"height";
        t1Named.height = 3.4f;
        writeln(t1Named.height); // 3.4f
        writeln(t1Named.pos); // [2, 1]
        t1Named.rename!"altitude".altitude = 5;
        writeln(t1Named.height); // 5

        Tuple!(int, "a", int, int, "c") t2;
        t2 = tuple(3,4,5);
        auto t2Named = t2.rename!("", "b");
        // "a" no longer has a name
        static assert(!__traits(hasMember, typeof(t2Named), "a"));
        writeln(t2Named[0]); // 3
        writeln(t2Named.b); // 4
        writeln(t2Named.c); // 5

        // not allowed to specify more names than the tuple has members
        static assert(!__traits(compiles, t2.rename!("a","b","c","d")));

        // use it in a range pipeline
        import std.range : iota, zip;
        import std.algorithm.iteration : map, sum;
        auto res = zip(iota(1, 4), iota(10, 13))
            .map!(t => t.rename!("a", "b"))
            .map!(t => t.a * t.b)
            .sum;
        writeln(res); // 68

        const tup = Tuple!(int, "a", int, "b")(2, 3);
        const renamed = tup.rename!("c", "d");
        writeln(renamed.c + renamed.d); // 5

  inout ref auto `rename`(alias translate)()
  if (is(typeof(translate) : V[K], V, K) && isSomeString!V && (isSomeString!K || is(K : size_t)));
    Overload of rename that takes an associative array translate as a template parameter, where the keys are either the names or indices of the members to be changed and the new names are the corresponding values. Every key in translate must be the name of a member of the tuple. The same rules for empty strings apply as for the variadic template overload of rename.
    Examples:

        //replacing names by their current name

        Tuple!(float, "dat", size_t[2], "pos") t1;
        t1.pos = [2, 1];
        auto t1Named = t1.rename!(["dat": "height"]);
        t1Named.height = 3.4;
        writeln(t1Named.pos); // [2, 1]
        t1Named.rename!(["height": "altitude"]).altitude = 5;
        writeln(t1Named.height); // 5

        Tuple!(int, "a", int, "b") t2;
        t2 = tuple(3, 4);
        auto t2Named = t2.rename!(["a": "b", "b": "c"]);
        writeln(t2Named.b); // 3
        writeln(t2Named.c); // 4

        const t3 = Tuple!(int, "a", int, "b")(3, 4);
        const t3Named = t3.rename!(["a": "b", "b": "c"]);
        writeln(t3Named.b); // 3
        writeln(t3Named.c); // 4

    Examples:

        //replace names by their position

        Tuple!(float, "dat", size_t[2], "pos") t1;
        t1.pos = [2, 1];
        auto t1Named = t1.rename!([0: "height"]);
        t1Named.height = 3.4;
        writeln(t1Named.pos); // [2, 1]
        t1Named.rename!([0: "altitude"]).altitude = 5;
        writeln(t1Named.height); // 5

        Tuple!(int, "a", int, "b", int, "c") t2;
        t2 = tuple(3, 4, 5);
        auto t2Named = t2.rename!([0: "c", 2: "a"]);
        writeln(t2Named.a); // 5
        writeln(t2Named.b); // 4
        writeln(t2Named.c); // 3

  inout @property ref @trusted inout(Tuple!(sliceSpecs!(from, to))) `slice`(size_t from, size_t to)()
  if (from <= to && (to <= Types.length));
    Takes a slice by-reference of this Tuple.
    Parameters:

    from | A size_t designating the starting position of the slice.
    to | A size_t designating the ending position (exclusive) of the slice.

    Returns:
    A new Tuple that is a slice from [from, to) of the original. It has the same types and values as the range [from, to) in the original.
    Examples:

        Tuple!(int, string, float, double) a;
        a[1] = "abc";
        a[2] = 4.5;
        auto s = a.slice!(1, 3);
        static assert(is(typeof(s) == Tuple!(string, float)));
        assert(s[0] == "abc" && s[1] == 4.5);

        // https://issues.dlang.org/show_bug.cgi?id=15645
        Tuple!(int, short, bool, double) b;
        static assert(!__traits(compiles, b.slice!(2, 4)));

  const nothrow @safe size_t `toHash`();
    Creates a hash of this Tuple.
    Returns:
    A size_t representing the hash of this Tuple.
  const string `toString`()();
    Converts to string.
    Returns:
    The string representation of this Tuple.
  const void `toString`(DG)(scope DG `sink`);
  const void `toString`(DG, Char)(scope DG `sink`, ref scope const FormatSpec!Char `fmt`);
    Formats Tuple with either %s, %(inner%) or %(inner%|sep%).

    Formats supported by Tuple
    Format | Description
    --- | ---
    %s | Format like Tuple!(types)(elements formatted with %s each).
    %(inner%) | The format inner is applied the expanded Tuple, so it may contain as many formats as the Tuple has fields.
    %(inner%|sep%) | The format inner is one format, that is applied on all fields of the Tuple. The inner format must be compatible to all of them.

    Parameters:

    DG `sink` | A char accepting delegate
    FormatSpec!Char `fmt` | A std.format.FormatSpec

    Examples:

        import std.format : format;

        Tuple!(int, double)[3] tupList = [ tuple(1, 1.0), tuple(2, 4.0), tuple(3, 9.0) ];

        // Default format
        writeln(format("%s", tuple("a", 1))); // `Tuple!(string, int)("a", 1)`

        // One Format for each individual component
        writeln(format("%(%#x v %.4f w %#x%)", tuple(1, 1.0, 10))); // `0x1 v 1.0000 w 0xa`
        writeln(format("%#x v %.4f w %#x", tuple(1, 1.0, 10).expand)); // `0x1 v 1.0000 w 0xa`

        // One Format for all components
        // `>abc< & >1< & >2.3< & >[4, 5]<`
        writeln(format("%(>%s<%| & %)", tuple("abc", 1, 2.3, [4, 5])));

        // Array of Tuples
        writeln(format("%(%(f(%d) = %.1f%);  %)", tupList)); // `f(1) = 1.0;  f(2) = 4.0;  f(3) = 9.0`

    Examples:

        import std.exception : assertThrown;
        import std.format : format, FormatException;

        // Error: %( %) missing.
        assertThrown!FormatException(
            format("%d, %f", tuple(1, 2.0)) == `1, 2.0`
        );

        // Error: %( %| %) missing.
        assertThrown!FormatException(
            format("%d", tuple(1, 2)) == `1, 2`
        );

        // Error: %d inadequate for double
        assertThrown!FormatException(
            format("%(%d%|, %)", tuple(1, 2.0)) == `1, 2.0`
        );

auto `reverse`(T)(T `t`)
if (isTuple!T);
  Creates a copy of a Tuple with its fields in reverse order.
  Parameters:

  T `t` | The Tuple to copy.

  Returns:
  A new Tuple.
  Examples:

      auto tup = tuple(1, "2");
      writeln(tup.reverse); // tuple("2", 1)

template `tuple`(Names...)
  Constructs a Tuple object instantiated and initialized according to the given arguments.
  Parameters:

  Names | An optional list of strings naming each successive field of the Tuple or a list of types that the elements are being casted to. For a list of names, each name matches up with the corresponding field given by Args. A name does not have to be provided for every field, but as the names must proceed in order, it is not possible to skip one field and name the next after it. For a list of types, there must be exactly as many types as parameters.

  Examples:

      auto value = tuple(5, 6.7, "hello");
      writeln(value[0]); // 5
      writeln(value[1]); // 6.7
      writeln(value[2]); // "hello"

      // Field names can be provided.
      auto entry = tuple!("index", "value")(4, "Hello");
      writeln(entry.index); // 4
      writeln(entry.value); // "Hello"

  auto `tuple`(Args...)(Args `args`);
    Parameters:

    Args `args` | Values to initialize the Tuple with. The Tuple's type will be inferred from the types of the values given.

    Returns:
    A new Tuple with its type inferred from the arguments given.
enum auto `isTuple`(T);
  Returns true if and only if T is an instance of std.typecons.Tuple.
  Parameters:

  T | The type to check.

  Returns:
  true if T is a Tuple type, false otherwise.
  Examples:

      static assert(isTuple!(Tuple!()));
      static assert(isTuple!(Tuple!(int)));
      static assert(isTuple!(Tuple!(int, real, string)));
      static assert(isTuple!(Tuple!(int, "x", real, "y")));
      static assert(isTuple!(Tuple!(int, Tuple!(real), string)));

template `Rebindable`(T) if (is(T == class) || is(T == interface) || isDynamicArray!T || isAssociativeArray!T)
  `Rebindable`!(T) is a simple, efficient wrapper that behaves just like an object of type T, except that you can reassign it to refer to another object. For completeness, `Rebindable`!(T) aliases itself away to T if T is a non-const object type.
  You may want to use `Rebindable` when you want to have mutable storage referring to const objects, for example an array of references that must be sorted in place. `Rebindable` does not break the soundness of D's type system and does not incur any of the risks usually associated with cast.
  Parameters:

  T | An object, interface, array slice type, or associative array type.

  Examples:
  Regular const object references cannot be reassigned.

      class Widget { int x; int y() @safe const { return x; } }
      const a = new Widget;
      // Fine
      a.y();
      // error! can't modify const a
      // a.x = 5;
      // error! can't modify const a
      // a = new Widget;

  Examples:
  However, `Rebindable`!(Widget) does allow reassignment, while otherwise behaving exactly like a const Widget.

      class Widget { int x; int y() const @safe { return x; } }
      auto a = Rebindable!(const Widget)(new Widget);
      // Fine
      a.y();
      // error! can't modify const a
      // a.x = 5;
      // Fine
      a = new Widget;

Rebindable!T `rebindable`(T)(T `obj`)
if (is(T == class) || is(T == interface) || isDynamicArray!T || isAssociativeArray!T);
  Convenience function for creating a Rebindable using automatic type inference.
  Parameters:

  T `obj` | A reference to an object, interface, associative array, or an array slice to initialize the Rebindable with.

  Returns:
  A newly constructed Rebindable initialized with the given reference.
  Examples:

      class C
      {
          int payload;
          this(int p) { payload = p; }
      }
      const c = new C(1);

      auto c2 = c.rebindable;
      writeln(c2.payload); // 1
      // passing Rebindable to rebindable
      c2 = c2.rebindable;

      c2 = new C(2);
      writeln(c2.payload); // 2

      const c3 = c2.get;
      writeln(c3.payload); // 2

Rebindable!T `rebindable`(T)(Rebindable!T `obj`);
  This function simply returns the Rebindable object passed in. It's useful in generic programming cases when a given object may be either a regular class or a Rebindable.
  Parameters:

  Rebindable!T `obj` | An instance of Rebindable!T.

  Returns:
  `obj` without any modification.
  Examples:

      class C
      {
          int payload;
          this(int p) { payload = p; }
      }
      const c = new C(1);

      auto c2 = c.rebindable;
      writeln(c2.payload); // 1
      // passing Rebindable to rebindable
      c2 = c2.rebindable;
      writeln(c2.payload); // 1

template `UnqualRef`(T) if (is(T == class) || is(T == interface))
  Similar to Rebindable!(T) but strips all qualifiers from the reference as opposed to just constness / immutability. Primary intended use case is with shared (having thread-local reference to shared class data)
  Parameters:

  T | A class or interface type.

  Examples:

      class Data {}

      static shared(Data) a;
      static UnqualRef!(shared Data) b;

      import core.thread;

      auto thread = new core.thread.Thread({
          a = new shared Data();
          b = new shared Data();
      });

      thread.start();
      thread.join();

      assert(a !is null);
      assert(b is null);

string `alignForSize`(E...)(const char[][] `names`...);
  Order the provided members to minimize size while preserving alignment. Alignment is not always optimal for 80-bit reals, nor for structs declared as align(1).
  Parameters:

  E | A list of the types to be aligned, representing fields of an aggregate such as a struct or class.
  char[][] `names` | The names of the fields that are to be aligned.

  Returns:
  A string to be mixed in to an aggregate, such as a struct or class.
  Examples:

      struct Banner {
          mixin(alignForSize!(byte[6], double)(["name", "height"]));
      }

struct `Nullable`(T);
auto `nullable`(T)(T `t`);
  Defines a value paired with a distinctive "null" state that denotes the absence of a value. If default constructed, a Nullable!T object starts in the null state. Assigning it renders it non-null. Calling nullify can nullify it again.
  Practically `Nullable`!T stores a T and a bool.
  Examples:

      struct CustomerRecord
      {
          string name;
          string address;
          int customerNum;
      }

      Nullable!CustomerRecord getByName(string name)
      {
          //A bunch of hairy stuff

          return Nullable!CustomerRecord.init;
      }

      auto queryResult = getByName("Doe, John");
      if (!queryResult.isNull)
      {
          //Process Mr. Doe's customer record
          auto address = queryResult.get.address;
          auto customerNum = queryResult.get.customerNum;

          //Do some things with this customer's info
      }
      else
      {
          //Add the customer to the database
      }

  Examples:

      import std.exception : assertThrown;

      auto a = 42.nullable;
      assert(!a.isNull);
      writeln(a.get); // 42

      a.nullify();
      assert(a.isNull);
      assertThrown!Throwable(a.get);

  inout this(inout T `value`);
    Constructor initializing this with `value`.
    Parameters:

    T `value` | The value to initialize this Nullable with.

  const bool `opEquals`()(auto ref const(typeof(this)) `rhs`);
  const bool `opEquals`(U)(auto ref const(U) `rhs`)
  if (!is(U : typeof(this)) && is(typeof(this.get == `rhs`)));
    If they are both null, then they are equal. If one is null and the other is not, then they are not equal. If they are both non-null, then they are equal if their values are equal.
    Examples:

        Nullable!int empty;
        Nullable!int a = 42;
        Nullable!int b = 42;
        Nullable!int c = 27;

        writeln(empty); // empty
        writeln(empty); // Nullable!int.init
        assert(empty != a);
        assert(empty != b);
        assert(empty != c);

        writeln(a); // b
        assert(a != c);

        assert(empty != 42);
        writeln(a); // 42
        assert(c != 42);

  string `toString`();
  const string `toString`();
  void `toString`(W)(ref W `writer`, ref scope const FormatSpec!char `fmt`)
  if (isOutputRange!(W, char));
  const void `toString`(W)(ref W `writer`, ref scope const FormatSpec!char `fmt`)
  if (isOutputRange!(W, char));
    Gives the string "Nullable.null" if isNull is true. Otherwise, the result is equivalent to calling std.format.formattedWrite on the underlying value.
    Parameters:

    W `writer` | A char accepting output range
    FormatSpec!char `fmt` | A std.format.FormatSpec which is used to represent the value if this Nullable is not null

    Returns:
    A string if `writer` and `fmt` are not set; void otherwise.
  const pure nothrow @property @safe bool `isNull`();
    Check if this is in the null state.
    Returns:
    true iff this is in the null state, otherwise false.
    Examples:

        Nullable!int ni;
        assert(ni.isNull);

        ni = 0;
        assert(!ni.isNull);

  void `nullify`()();
    Forces this to the null state.
    Examples:

        Nullable!int ni = 0;
        assert(!ni.isNull);

        ni.nullify();
        assert(ni.isNull);

  void `opAssign`()(T `value`);
    Assigns `value` to the internally-held state. If the assignment succeeds, this becomes non-null.
    Parameters:

    T `value` | A value of type T to assign to this Nullable.

    Examples:
    If this Nullable wraps a type that already has a null value (such as a pointer), then assigning the null value to this Nullable is no different than assigning any other value of type T, and the resulting code will look very strange. It is strongly recommended that this be avoided by instead using the version of Nullable that takes an additional nullValue template argument.

        //Passes
        Nullable!(int*) npi;
        assert(npi.isNull);

        //Passes?!
        npi = null;
        assert(!npi.isNull);

  inout pure nothrow @property ref @safe inout(T) `get`();
  inout pure nothrow @property @safe inout(T) `get`()(inout(T) `fallback`);
  inout pure nothrow @property @safe auto `get`(U)(inout(U) `fallback`);
    Gets the value if not null. If this is in the null state, and the optional parameter `fallback` was provided, it will be returned. Without `fallback`, calling `get` with a null state is invalid.
    When the fallback type is different from the Nullable type, `get`(T) returns the common type.
    Parameters:

    inout(T) `fallback` | the value to return in case the Nullable is null.

    Returns:
    The value held internally by this Nullable.
struct `Nullable`(T, T nullValue);
auto `nullable`(alias nullValue, T)(T `t`)
if (is(typeof(nullValue) == T));
  Just like `Nullable`!T, except that the null state is defined as a particular value. For example, Nullable!(uint, uint.max) is an uint that sets aside the value uint.max to denote a null state. Nullable!(T, nullValue) is more storage-efficient than Nullable!T because it does not need to store an extra bool.
  Parameters:

  T | The wrapped type for which Nullable provides a null value.
  nullValue | The null value which denotes the null state of this `Nullable`. Must be of type T.

  Examples:

      Nullable!(size_t, size_t.max) indexOf(string[] haystack, string needle)
      {
          //Find the needle, returning -1 if not found

          return Nullable!(size_t, size_t.max).init;
      }

      void sendLunchInvite(string name)
      {
      }

      //It's safer than C...
      auto coworkers = ["Jane", "Jim", "Marry", "Fred"];
      auto pos = indexOf(coworkers, "Bob");
      if (!pos.isNull)
      {
          //Send Bob an invitation to lunch
          sendLunchInvite(coworkers[pos]);
      }
      else
      {
          //Bob not found; report the error
      }

      //And there's no overhead
      static assert(Nullable!(size_t, size_t.max).sizeof == size_t.sizeof);

  Examples:

      import std.exception : assertThrown;

      Nullable!(int, int.min) a;
      assert(a.isNull);
      assertThrown!Throwable(a.get);
      a = 5;
      assert(!a.isNull);
      writeln(a); // 5
      static assert(a.sizeof == int.sizeof);

  Examples:

      auto a = nullable!(int.min)(8);
      writeln(a); // 8
      a.nullify();
      assert(a.isNull);

  this(T `value`);
    Constructor initializing this with `value`.
    Parameters:

    T `value` | The value to initialize this Nullable with.

  const @property bool `isNull`();
    Check if this is in the null state.
    Returns:
    true iff this is in the null state, otherwise false.
    Examples:

        Nullable!(int, -1) ni;
        //Initialized to "null" state
        assert(ni.isNull);

        ni = 0;
        assert(!ni.isNull);

  void `nullify`()();
    Forces this to the null state.
    Examples:

        Nullable!(int, -1) ni = 0;
        assert(!ni.isNull);

        ni = -1;
        assert(ni.isNull);

  void `opAssign`()(T `value`);
    Assigns `value` to the internally-held state. If the assignment succeeds, this becomes non-null. No null checks are made. Note that the assignment may leave this in the null state.
    Parameters:

    T `value` | A value of type T to assign to this Nullable. If it is nullvalue, then the internal state of this Nullable will be set to null.

    Examples:
    If this Nullable wraps a type that already has a null value (such as a pointer), and that null value is not given for nullValue, then assigning the null value to this Nullable is no different than assigning any other value of type T, and the resulting code will look very strange. It is strongly recommended that this be avoided by using T's "built in" null value for nullValue.

        //Passes
        enum nullVal = cast(int*) 0xCAFEBABE;
        Nullable!(int*, nullVal) npi;
        assert(npi.isNull);

        //Passes?!
        npi = null;
        assert(!npi.isNull);

  inout @property ref inout(T) `get`();
    Gets the value. this must not be in the null state. This function is also called for the implicit conversion to T.

    Preconditions isNull must be false.

    Returns:
    The value held internally by this Nullable.
    Examples:

        import std.exception : assertThrown, assertNotThrown;

        Nullable!(int, -1) ni;
        //`get` is implicitly called. Will throw
        //an error in non-release mode
        assertThrown!Throwable(ni == 0);

        ni = 0;
        assertNotThrown!Throwable(ni == 0);

template `apply`(alias fun)
  Unpacks the content of a Nullable, performs an operation and packs it again. Does nothing if isNull.
  When called on a Nullable, `apply` will unpack the value contained in the Nullable, pass it to the function you provide and wrap the result in another Nullable (if necessary). If the Nullable is null, `apply` will return null itself.
  Parameters:

  T t | a Nullable
  fun | a function operating on the content of the nullable

  Returns:
  fun(t.get).nullable if !t.isNull, else Nullable.init.
  See also: The Maybe monad
  Examples:

      alias toFloat = i => cast(float) i;

      Nullable!int sample;

      // apply(null) results in a null `Nullable` of the function's return type.
      Nullable!float f = sample.apply!toFloat;
      assert(sample.isNull && f.isNull);

      sample = 3;

      // apply(non-null) calls the function and wraps the result in a `Nullable`.
      f = sample.apply!toFloat;
      assert(!sample.isNull && !f.isNull);
      writeln(f.get); // 3.0f

  Examples:

      alias greaterThree = i => (i > 3) ? i.nullable : Nullable!(typeof(i)).init;

      Nullable!int sample;

      // when the function already returns a `Nullable`, that `Nullable` is not wrapped.
      auto result = sample.apply!greaterThree;
      assert(sample.isNull && result.isNull);

      // The function may decide to return a null `Nullable`.
      sample = 3;
      result = sample.apply!greaterThree;
      assert(!sample.isNull && result.isNull);

      // Or it may return a value already wrapped in a `Nullable`.
      sample = 4;
      result = sample.apply!greaterThree;
      assert(!sample.isNull && !result.isNull);
      writeln(result.get); // 4

struct `NullableRef`(T);
auto `nullableRef`(T)(T* `t`);
  Just like Nullable!T, except that the object refers to a value sitting elsewhere in memory. This makes assignments overwrite the initially assigned value. Internally `NullableRef`!T only stores a pointer to T (i.e., Nullable!T.sizeof == (T*).sizeof).
  Examples:

      import std.exception : assertThrown;

      int x = 5, y = 7;
      auto a = nullableRef(&x);
      assert(!a.isNull);
      writeln(a); // 5
      writeln(x); // 5
      a = 42;
      writeln(x); // 42
      assert(!a.isNull);
      writeln(a); // 42
      a.nullify();
      writeln(x); // 42
      assert(a.isNull);
      assertThrown!Throwable(a.get);
      assertThrown!Throwable(a = 71);
      a.bind(&y);
      writeln(a); // 7
      y = 135;
      writeln(a); // 135

  pure nothrow @safe this(T* `value`);
    Constructor binding this to `value`.
    Parameters:

    T* `value` | The value to bind to.

  pure nothrow @safe void `bind`(T* `value`);
    Binds the internal state to `value`.
    Parameters:

    T* `value` | A pointer to a value of type T to bind this NullableRef to.

    Examples:

        NullableRef!int nr = new int(42);
        writeln(nr); // 42

        int* n = new int(1);
        nr.bind(n);
        writeln(nr); // 1

  const pure nothrow @property @safe bool `isNull`();
    Returns true if and only if this is in the null state.
    Returns:
    true if this is in the null state, otherwise false.
    Examples:

        NullableRef!int nr;
        assert(nr.isNull);

        int* n = new int(42);
        nr.bind(n);
        assert(!nr.isNull && nr == 42);

  pure nothrow @safe void `nullify`();
    Forces this to the null state.
    Examples:

        NullableRef!int nr = new int(42);
        assert(!nr.isNull);

        nr.nullify();
        assert(nr.isNull);

  void `opAssign`()(T `value`)
  if (isAssignable!T);
    Assigns `value` to the internally-held state.
    Parameters:

    T `value` | A value of type T to assign to this NullableRef. If the internal state of this NullableRef has not been initialized, an error will be thrown in non-release mode.

    Examples:

        import std.exception : assertThrown, assertNotThrown;

        NullableRef!int nr;
        assert(nr.isNull);
        assertThrown!Throwable(nr = 42);

        nr.bind(new int(0));
        assert(!nr.isNull);
        assertNotThrown!Throwable(nr = 42);
        writeln(nr); // 42

  inout pure nothrow @property ref @safe inout(T) `get`();
    Gets the value. this must not be in the null state. This function is also called for the implicit conversion to T.
    Examples:

        import std.exception : assertThrown, assertNotThrown;

        NullableRef!int nr;
        //`get` is implicitly called. Will throw
        //an error in non-release mode
        assertThrown!Throwable(nr == 0);

        nr.bind(new int(0));
        assertNotThrown!Throwable(nr == 0);

template `BlackHole`(Base)
  `BlackHole`!Base is a subclass of Base which automatically implements all abstract member functions in Base as do-nothing functions. Each auto-implemented function just returns the default value of the return type without doing anything.
  The name came from Class::BlackHole Perl module by Sean M. Burke.
  Parameters:

  Base | A non-final class for `BlackHole` to inherit from.

  See Also:
  AutoImplement, generateEmptyFunction
  Examples:

      import std.math : isNaN;

      static abstract class C
      {
          int m_value;
          this(int v) { m_value = v; }
          int value() @property { return m_value; }

          abstract real realValue() @property;
          abstract void doSomething();
      }

      auto c = new BlackHole!C(42);
      writeln(c.value); // 42

      // Returns real.init which is NaN
      assert(c.realValue.isNaN);
      // Abstract functions are implemented as do-nothing
      c.doSomething();

template `WhiteHole`(Base)
  `WhiteHole`!Base is a subclass of Base which automatically implements all abstract member functions as functions that always fail. These functions simply throw an Error and never return. Whitehole is useful for trapping the use of class member functions that haven't been implemented.
  The name came from Class::WhiteHole Perl module by Michael G Schwern.
  Parameters:

  Base | A non-final class for `WhiteHole` to inherit from.

  See Also:
  AutoImplement, generateAssertTrap
  Examples:

      import std.exception : assertThrown;

      static class C
      {
          abstract void notYetImplemented();
      }

      auto c = new WhiteHole!C;
      assertThrown!NotImplementedError(c.notYetImplemented()); // throws an Error

class `AutoImplement`(Base, alias how, alias what = isAbstractFunction) if (!is(how == class)): Base;
class `AutoImplement`(Interface, BaseClass, alias how, alias what = isAbstractFunction) if (is(Interface == interface) && is(BaseClass == class)): BaseClass, Interface;
  `AutoImplement` automatically implements (by default) all abstract member functions in the class or interface Base in specified way.
  The second version of `AutoImplement` automatically implements Interface, while deriving from BaseClass.
  Parameters:

  how | template which specifies how functions will be implemented/overridden. Two arguments are passed to how: the type Base and an alias to an implemented function. Then how must return an implemented function body as a string. The generated function body can use these keywords: a0, a1, …: arguments passed to the function; args: a tuple of the arguments; self: an alias to the function itself; parent: an alias to the overridden function (if any). You may want to use templated property functions (instead of Implicit Template Properties) to generate complex functions:
      // Prints log messages for each call to overridden functions.
      string generateLogger(C, alias fun)() @property
      {
          import std.traits;
          enum qname = C.stringof ~ "." ~ __traits(identifier, fun);
          string stmt;

          stmt ~= q{ struct Importer { import std.stdio; } };
          stmt ~= `Importer.writeln("Log: ` ~ qname ~ `(", args, ")");`;
          static if (!__traits(isAbstractFunction, fun))
          {
              static if (is(ReturnType!fun == void))
                  stmt ~= q{ parent(args); };
              else
                  stmt ~= q{
                      auto r = parent(args);
                      Importer.writeln("--> ", r);
                      return r;
                  };
          }
          return stmt;
      }
  what | template which determines what functions should be implemented/overridden. An argument is passed to what: an alias to a non-final member function in Base. Then what must return a boolean value. Return true to indicate that the passed function should be implemented/overridden.
      // Sees if fun returns something.
      enum bool hasValue(alias fun) = !is(ReturnType!(fun) == void);

  Note Generated code is inserted in the scope of std.typecons module. Thus, any useful functions outside std.typecons cannot be used in the generated code. To workaround this problem, you may import necessary things in a local struct, as done in the generateLogger() template in the above example.

  Bugs:
  - Variadic arguments to constructors are not forwarded to super.
  - Deep interface inheritance causes compile error with messages like "Error: function std.typecons.AutoImplement!(Foo).AutoImplement.bar does not override any function". [Bugzilla 2525]
  - The parent keyword is actually a delegate to the super class' corresponding member function. [Bugzilla 2540]
  - Using alias template parameter in how and/or what may cause strange compile error. Use template tuple parameter instead to workaround this problem. [Bugzilla 4217]
  Examples:

      interface PackageSupplier
      {
          int foo();
          int bar();
      }

      static abstract class AbstractFallbackPackageSupplier : PackageSupplier
      {
          protected PackageSupplier default_, fallback;

          this(PackageSupplier default_, PackageSupplier fallback)
          {
              this.default_ = default_;
              this.fallback = fallback;
          }

          abstract int foo();
          abstract int bar();
      }

      template fallback(T, alias func)
      {
          import std.format : format;
          // for all implemented methods:
          // - try default first
          // - only on a failure run & return fallback
          enum fallback = q{
              scope (failure) return fallback.%1$s(args);
              return default_.%1$s(args);
          }.format(__traits(identifier, func));
      }

      // combines two classes and use the second one as fallback
      alias FallbackPackageSupplier = AutoImplement!(AbstractFallbackPackageSupplier, fallback);

      class FailingPackageSupplier : PackageSupplier
      {
          int foo(){ throw new Exception("failure"); }
          int bar(){ return 2;}
      }

      class BackupPackageSupplier : PackageSupplier
      {
          int foo(){ return -1; }
          int bar(){ return -1;}
      }

      auto registry = new FallbackPackageSupplier(new FailingPackageSupplier(), new BackupPackageSupplier());

      writeln(registry.foo()); // -1
      writeln(registry.bar()); // 2

template `generateEmptyFunction`(C, func...)
enum string `generateAssertTrap`(C, func...);
  Predefined how-policies for AutoImplement. These templates are also used by BlackHole and WhiteHole, respectively.
  Examples:

      alias BlackHole(Base) = AutoImplement!(Base, generateEmptyFunction);

      interface I
      {
          int foo();
          string bar();
      }

      auto i = new BlackHole!I();
      // generateEmptyFunction returns the default value of the return type without doing anything
      writeln(i.foo); // 0
      assert(i.bar is null);

  Examples:

      import std.exception : assertThrown;

      alias WhiteHole(Base) = AutoImplement!(Base, generateAssertTrap);

      interface I
      {
          int foo();
          string bar();
      }

      auto i = new WhiteHole!I();
      // generateAssertTrap throws an exception for every unimplemented function of the interface
      assertThrown!NotImplementedError(i.foo);
      assertThrown!NotImplementedError(i.bar);

template `wrap`(Targets...) if (Targets.length >= 1 && allSatisfy!(isMutable, Targets))
template `wrap`(Targets...) if (Targets.length >= 1 && !allSatisfy!(isMutable, Targets))
template `unwrap`(Target) if (isMutable!Target)
template `unwrap`(Target) if (!isMutable!Target)
  Supports structural based typesafe conversion.
  If Source has structural conformance with the interface Targets, wrap creates an internal wrapper class which inherits Targets and wraps the src object, then returns it.
  `unwrap` can be used to extract objects which have been wrapped by `wrap`.
  Examples:

      interface Quack
      {
          int quack();
          @property int height();
      }
      interface Flyer
      {
          @property int height();
      }
      class Duck : Quack
      {
          int quack() { return 1; }
          @property int height() { return 10; }
      }
      class Human
      {
          int quack() { return 2; }
          @property int height() { return 20; }
      }

      Duck d1 = new Duck();
      Human h1 = new Human();

      interface Refleshable
      {
          int reflesh();
      }

      // does not have structural conformance
      static assert(!__traits(compiles, d1.wrap!Refleshable));
      static assert(!__traits(compiles, h1.wrap!Refleshable));

      // strict upcast
      Quack qd = d1.wrap!Quack;
      assert(qd is d1);
      assert(qd.quack() == 1);    // calls Duck.quack
      // strict downcast
      Duck d2 = qd.unwrap!Duck;
      assert(d2 is d1);

      // structural upcast
      Quack qh = h1.wrap!Quack;
      assert(qh.quack() == 2);    // calls Human.quack
      // structural downcast
      Human h2 = qh.unwrap!Human;
      assert(h2 is h1);

      // structural upcast (two steps)
      Quack qx = h1.wrap!Quack;   // Human -> Quack
      Flyer fx = qx.wrap!Flyer;   // Quack -> Flyer
      assert(fx.height == 20);    // calls Human.height
      // structural downcast (two steps)
      Quack qy = fx.unwrap!Quack; // Flyer -> Quack
      Human hy = qy.unwrap!Human; // Quack -> Human
      assert(hy is h1);
      // structural downcast (one step)
      Human hz = fx.unwrap!Human; // Flyer -> Human
      assert(hz is h1);

  Examples:

      import std.traits : FunctionAttribute, functionAttributes;
      interface A { int run(); }
      interface B { int stop(); @property int status(); }
      class X
      {
          int run() { return 1; }
          int stop() { return 2; }
          @property int status() { return 3; }
      }

      auto x = new X();
      auto ab = x.wrap!(A, B);
      A a = ab;
      B b = ab;
      writeln(a.run()); // 1
      writeln(b.stop()); // 2
      writeln(b.status); // 3
      static assert(functionAttributes!(typeof(ab).status) & FunctionAttribute.property);

enum `RefCountedAutoInitialize`: int;
  Options regarding auto-initialization of a RefCounted object (see the definition of RefCounted below).
  Examples:

      import core.exception : AssertError;
      import std.exception : assertThrown;

      struct Foo
      {
          int a = 42;
      }

      RefCounted!(Foo, RefCountedAutoInitialize.yes) rcAuto;
      RefCounted!(Foo, RefCountedAutoInitialize.no) rcNoAuto;

      writeln(rcAuto.refCountedPayload.a); // 42

      assertThrown!AssertError(rcNoAuto.refCountedPayload);
      rcNoAuto.refCountedStore.ensureInitialized;
      writeln(rcNoAuto.refCountedPayload.a); // 42

  `no`
    Do not auto-initialize the object
  `yes`
    Auto-initialize the object
struct `RefCounted`(T, RefCountedAutoInitialize autoInit = RefCountedAutoInitialize.yes) if (!is(T == class) && !is(T == interface));
  Defines a reference-counted object containing a T value as payload.
  An instance of `RefCounted` is a reference to a structure, which is referred to as the store, or storage implementation struct in this documentation. The store contains a reference count and the T payload. `RefCounted` uses malloc to allocate the store. As instances of `RefCounted` are copied or go out of scope, they will automatically increment or decrement the reference count. When the reference count goes down to zero, `RefCounted` will call destroy against the payload and call free to deallocate the store. If the T payload contains any references to GC-allocated memory, then `RefCounted` will add it to the GC memory that is scanned for pointers, and remove it from GC scanning before free is called on the store.
  One important consequence of destroy is that it will call the destructor of the T payload. GC-managed references are not guaranteed to be valid during a destructor call, but other members of T, such as file handles or pointers to malloc memory, will still be valid during the destructor call. This allows the T to deallocate or clean up any non-GC resources immediately after the reference count has reached zero.
  `RefCounted` is unsafe and should be used with care. No references to the payload should be escaped outside the `RefCounted` object.
  The autoInit option makes the object ensure the store is automatically initialized. Leaving autoInit == RefCountedAutoInitialize.yes (the default option) is convenient but has the cost of a test whenever the payload is accessed. If autoInit == RefCountedAutoInitialize.no, user code must call either refCountedStore.isInitialized or refCountedStore.ensureInitialized before attempting to access the payload. Not doing so results in null pointer dereference.
  Examples:

      // A pair of an `int` and a `size_t` - the latter being the
      // reference count - will be dynamically allocated
      auto rc1 = RefCounted!int(5);
      writeln(rc1); // 5
      // No more allocation, add just one extra reference count
      auto rc2 = rc1;
      // Reference semantics
      rc2 = 42;
      writeln(rc1); // 42
      // the pair will be freed when rc1 and rc2 go out of scope

  struct `RefCountedStore`;
    RefCounted storage implementation.
    const pure nothrow @nogc @property @safe bool `isInitialized`();
      Returns true if and only if the underlying store has been allocated and initialized.
    const pure nothrow @nogc @property @safe size_t `refCount`();
      Returns underlying reference count if it is allocated and initialized (a positive integer), and 0 otherwise.
    void `ensureInitialized`();
      Makes sure the payload was properly initialized. Such a call is typically inserted before using the payload.
  inout nothrow @property ref @safe inout(RefCountedStore) `refCountedStore`();
    Returns storage implementation struct.
  this(A...)(auto ref A `args`)
  if (A.length > 0);
  this(T `val`);
    Constructor that initializes the payload.

    Postcondition refCountedStore.isInitialized

  void `opAssign`(typeof(this) `rhs`);
  void `opAssign`(T `rhs`);
    Assignment operators
  @property ref @trusted T `refCountedPayload`() return;
  inout pure nothrow @nogc @property ref @safe inout(T) `refCountedPayload`() return;
    Returns a reference to the payload. If (autoInit == RefCountedAutoInitialize.yes), calls refCountedStore.ensureInitialized. Otherwise, just issues assert(refCountedStore.isInitialized). Used with alias refCountedPayload this;, so callers can just use the RefCounted object as a T.
    The first overload exists only if autoInit == RefCountedAutoInitialize.yes. So if autoInit == RefCountedAutoInitialize.no or called for a constant or immutable object, then `refCountedPayload` will also be qualified as safe and nothrow (but will still assert if not initialized).
RefCounted!(T, RefCountedAutoInitialize.no) `refCounted`(T)(T `val`);
  Initializes a RefCounted with `val`. The template parameter T of RefCounted is inferred from `val`. This function can be used to move non-copyable values to the heap. It also disables the autoInit option of RefCounted.
  Parameters:

  T `val` | The value to be reference counted

  Returns:
  An initialized RefCounted containing `val`.
  See Also:
  C++'s make_shared
  Examples:

      static struct File
      {
          string name;
          @disable this(this); // not copyable
          ~this() { name = null; }
      }

      auto file = File("name");
      writeln(file.name); // "name"
      // file cannot be copied and has unique ownership
      static assert(!__traits(compiles, {auto file2 = file;}));

      // make the file refcounted to share ownership
      import std.algorithm.mutation : move;
      auto rcFile = refCounted(move(file));
      writeln(rcFile.name); // "name"
      writeln(file.name); // null
      auto rcFile2 = rcFile;
      writeln(rcFile.refCountedStore.refCount); // 2
      // file gets properly closed when last reference is dropped

template `Proxy`(alias a)
  Creates a proxy for the value a that will forward all operations while disabling implicit conversions. The aliased item a must be an lvalue. This is useful for creating a new type from the "base" type (though this is not a subtype-supertype relationship; the new type is not related to the old type in any way, by design).
  The new type supports all operations that the underlying type does, including all operators such as +, --, <, [], etc.
  Parameters:

  a | The value to act as a proxy for all operations. It must be an lvalue.

  Examples:

      struct MyInt
      {
          private int value;
          mixin Proxy!value;

          this(int n){ value = n; }
      }

      MyInt n = 10;

      // Enable operations that original type has.
      ++n;
      writeln(n); // 11
      writeln(n * 2); // 22

      void func(int n) { }

      // Disable implicit conversions to original type.
      //int x = n;
      //func(n);

  Examples:
  The proxied value must be an lvalue.

      struct NewIntType
      {
          //Won't work; the literal '1'
          //is an rvalue, not an lvalue
          //mixin Proxy!1;

          //Okay, n is an lvalue
          int n;
          mixin Proxy!n;

          this(int n) { this.n = n; }
      }

      NewIntType nit = 0;
      nit++;
      writeln(nit); // 1

      struct NewObjectType
      {
          Object obj;
          //Ok, obj is an lvalue
          mixin Proxy!obj;

          this (Object o) { obj = o; }
      }

      NewObjectType not = new Object();
      assert(__traits(compiles, not.toHash()));

  Examples:
  There is one exception to the fact that the new type is not related to the old type. Pseudo-member functions are usable with the new type; they will be forwarded on to the proxied value.

      import std.math;

      float f = 1.0;
      assert(!f.isInfinity);

      struct NewFloat
      {
          float _;
          mixin Proxy!_;

          this(float f) { _ = f; }
      }

      NewFloat nf = 1.0f;
      assert(!nf.isInfinity);

struct `Typedef`(T, T init = T.init, string cookie = null);
  Typedef allows the creation of a unique type which is based on an existing type. Unlike the alias feature, Typedef ensures the two types are not considered as equals.
  Parameters:

  init | Optional initial value for the new type.
  cookie | Optional, used to create multiple unique types which are based on the same origin type T

  Note If a library routine cannot handle the Typedef type, you can use the TypedefType template to extract the type which the Typedef wraps.

  Examples:

      alias MyInt = Typedef!int;
      MyInt foo = 10;
      foo++;
      writeln(foo); // 11

  Examples:
  custom initialization values

      alias MyIntInit = Typedef!(int, 42);
      static assert(is(TypedefType!MyIntInit == int));
      static assert(MyIntInit() == 42);

  Examples:
  Typedef creates a new type

      alias MyInt = Typedef!int;
      static void takeInt(int) {}
      static void takeMyInt(MyInt) {}

      int i;
      takeInt(i);    // ok
      static assert(!__traits(compiles, takeMyInt(i)));

      MyInt myInt;
      static assert(!__traits(compiles, takeInt(myInt)));
      takeMyInt(myInt);  // ok

  Examples:
  Use the optional cookie argument to create different types of the same base type

      alias TypeInt1 = Typedef!int;
      alias TypeInt2 = Typedef!int;

      // The two Typedefs are the same type.
      static assert(is(TypeInt1 == TypeInt2));

      alias MoneyEuros = Typedef!(float, float.init, "euros");
      alias MoneyDollars = Typedef!(float, float.init, "dollars");

      // The two Typedefs are _not_ the same type.
      static assert(!is(MoneyEuros == MoneyDollars));

  string `toString`(this T)();
  void `toString`(this T, W)(ref W `writer`, ref scope const FormatSpec!char `fmt`)
  if (isOutputRange!(W, char));
    Convert wrapped value to a human readable string
    Examples:

        import std.conv : to;

        int i = 123;
        auto td = Typedef!int(i);
        writeln(i.to!string); // td.to!string

template `TypedefType`(T)
  Get the underlying type which a Typedef wraps. If T is not a Typedef it will alias itself to T.
  Examples:

      import std.conv : to;

      alias MyInt = Typedef!int;
      static assert(is(TypedefType!MyInt == int));

      /// Instantiating with a non-Typedef will return that type
      static assert(is(TypedefType!int == int));

      string num = "5";

      // extract the needed type
      MyInt myInt = MyInt( num.to!(TypedefType!MyInt) );
      writeln(myInt); // 5

      // cast to the underlying type to get the value that's being wrapped
      int x = cast(TypedefType!MyInt) myInt;

      alias MyIntInit = Typedef!(int, 42);
      static assert(is(TypedefType!MyIntInit == int));
      static assert(MyIntInit() == 42);

template `scoped`(T) if (is(T == class))
  Allocates a class object right inside the current scope, therefore avoiding the overhead of new. This facility is unsafe; it is the responsibility of the user to not escape a reference to the object outside the scope.
  The class destructor will be called when the result of `scoped`() is itself destroyed.
  Scoped class instances can be embedded in a parent class or struct, just like a child struct instance. Scoped member variables must have type typeof(`scoped`!Class(args)), and be initialized with a call to scoped. See below for an example.

  Note It's illegal to move a class instance even if you are sure there are no pointers to it. As such, it is illegal to move a scoped object.

  Examples:

      class A
      {
          int x;
          this()     {x = 0;}
          this(int i){x = i;}
          ~this()    {}
      }

      // Standard usage, constructing A on the stack
      auto a1 = scoped!A();
      a1.x = 42;

      // Result of `scoped` call implicitly converts to a class reference
      A aRef = a1;
      writeln(aRef.x); // 42

      // Scoped destruction
      {
          auto a2 = scoped!A(1);
          writeln(a2.x); // 1
          aRef = a2;
          // a2 is destroyed here, calling A's destructor
      }
      // aRef is now an invalid reference

      // Here the temporary scoped A is immediately destroyed.
      // This means the reference is then invalid.
      version (Bug)
      {
          // Wrong, should use `auto`
          A invalid = scoped!A();
      }

      // Restrictions
      version (Bug)
      {
          import std.algorithm.mutation : move;
          auto invalid = a1.move; // illegal, scoped objects can't be moved
      }
      static assert(!is(typeof({
          auto e1 = a1; // illegal, scoped objects can't be copied
          assert([a1][0].x == 42); // ditto
      })));
      static assert(!is(typeof({
          alias ScopedObject = typeof(a1);
          auto e2 = ScopedObject();  // illegal, must be built via scoped!A
          auto e3 = ScopedObject(1); // ditto
      })));

      // Use with alias
      alias makeScopedA = scoped!A;
      auto a3 = makeScopedA();
      auto a4 = makeScopedA(1);

      // Use as member variable
      struct B
      {
          typeof(scoped!A()) a; // note the trailing parentheses

          this(int i)
          {
              // construct member
              a = scoped!A(i);
          }
      }

      // Stack-allocate
      auto b1 = B(5);
      aRef = b1.a;
      writeln(aRef.x); // 5
      destroy(b1); // calls A's destructor for b1.a
      // aRef is now an invalid reference

      // Heap-allocate
      auto b2 = new B(6);
      writeln(b2.a.x); // 6
      destroy(*b2); // calls A's destructor for b2.a

  @system auto `scoped`(Args...)(auto ref Args `args`);
    Returns the scoped object.
    Parameters:

    Args `args` | Arguments to pass to T's constructor.

template `Flag`(string name)
  Defines a simple, self-documenting yes/no flag. This makes it easy for APIs to define functions accepting flags without resorting to bool, which is opaque in calls, and without needing to define an enumerated type separately. Using `Flag`!"Name" instead of bool makes the flag's meaning visible in calls. Each yes/no flag has its own type, which makes confusions and mix-ups impossible.

  Example Code calling getLine (usually far away from its definition) can't be understood without looking at the documentation, even by users familiar with the API:

      string getLine(bool keepTerminator)
      {
          ...
          if (keepTerminator) ...
          ...
      }
      ...
      auto line = getLine(false);

  Assuming the reverse meaning (i.e. "ignoreTerminator") and inserting the wrong code compiles and runs with erroneous results.
  After replacing the boolean parameter with an instantiation of `Flag`, code calling getLine can be easily read and understood even by people not fluent with the API:

      string getLine(Flag!"keepTerminator" keepTerminator)
      {
          ...
          if (keepTerminator) ...
          ...
      }
      ...
      auto line = getLine(Yes.keepTerminator);

  The structs Yes and No are provided as shorthand for `Flag`!"Name".yes and `Flag`!"Name".no and are preferred for brevity and readability. These convenience structs mean it is usually unnecessary and counterproductive to create an alias of a `Flag` as a way of avoiding typing out the full type while specifying the affirmative or negative options.
  Passing categorical data by means of unstructured bool parameters is classified under "simple-data coupling" by Steve McConnell in the Code Complete book, along with three other kinds of coupling. The author argues citing several studies that coupling has a negative effect on code quality. `Flag` offers a simple structuring method for passing yes/no flags to APIs.
  Examples:

      Flag!"abc" flag;

      writeln(flag); // Flag!"abc".no
      writeln(flag); // No.abc
      assert(!flag);
      if (flag) assert(0);

  Examples:

      auto flag = Yes.abc;

      assert(flag);
      writeln(flag); // Yes.abc
      if (!flag) assert(0);
      if (flag) {} else assert(0);

  enum `Flag`: bool;
    `no`
      When creating a value of type Flag!"Name", use Flag!"Name".no for the negative option. When using a value of type Flag!"Name", compare it against Flag!"Name".no or just false or 0.
    `yes`
      When creating a value of type Flag!"Name", use Flag!"Name".yes for the affirmative option. When using a value of type Flag!"Name", compare it against Flag!"Name".yes.
struct `Yes`;
struct `No`;
  Convenience names that allow using e.g. `Yes`.encryption instead of Flag!"encryption".yes and `No`.encryption instead of Flag!"encryption".no.
  Examples:

      Flag!"abc" flag;

      writeln(flag); // Flag!"abc".no
      writeln(flag); // No.abc
      assert(!flag);
      if (flag) assert(0);

  Examples:

      auto flag = Yes.abc;

      assert(flag);
      writeln(flag); // Yes.abc
      if (!flag) assert(0);
      if (flag) {} else assert(0);

template `isBitFlagEnum`(E)
  Detect whether an enum is of integral type and has only "flag" values (i.e. values with a bit count of exactly 1). Additionally, a zero value is allowed for compatibility with enums including a "None" value.
  Examples:

      enum A
      {
          None,
          A = 1 << 0,
          B = 1 << 1,
          C = 1 << 2,
          D = 1 << 3,
      }

      static assert(isBitFlagEnum!A);

  Examples:
  Test an enum with default (consecutive) values

      enum B
      {
          A,
          B,
          C,
          D // D == 3
      }

      static assert(!isBitFlagEnum!B);

  Examples:
  Test an enum with non-integral values

      enum C: double
      {
          A = 1 << 0,
          B = 1 << 1
      }

      static assert(!isBitFlagEnum!C);

struct `BitFlags`(E, Flag!"unsafe" unsafe = No.unsafe) if (unsafe || isBitFlagEnum!E);
  A typesafe structure for storing combinations of enum values.
  This template defines a simple struct to represent bitwise OR combinations of enum values. It can be used if all the enum values are integral constants with a bit count of at most 1, or if the unsafe parameter is explicitly set to Yes. This is much safer than using the enum itself to store the OR combination, which can produce surprising effects like this:

      enum E
      {
          A = 1 << 0,
          B = 1 << 1
      }
      E e = E.A | E.B;
      // will throw SwitchError
      final switch (e)
      {
          case E.A:
              return;
          case E.B:
              return;
      }

  Examples:
  Set values with the | operator and test with &

      enum Enum
      {
          A = 1 << 0,
      }

      // A default constructed BitFlags has no value set
      immutable BitFlags!Enum flags_empty;
      assert(!flags_empty.A);

      // Value can be set with the | operator
      immutable flags_A = flags_empty | Enum.A;

      // and tested using property access
      assert(flags_A.A);

      // or the & operator
      assert(flags_A & Enum.A);
      // which commutes.
      assert(Enum.A & flags_A);

  Examples:
  A default constructed BitFlags has no value set

      enum Enum
      {
          None,
          A = 1 << 0,
          B = 1 << 1,
          C = 1 << 2
      }

      immutable BitFlags!Enum flags_empty;
      assert(!(flags_empty & (Enum.A | Enum.B | Enum.C)));
      assert(!(flags_empty & Enum.A) && !(flags_empty & Enum.B) && !(flags_empty & Enum.C));

  Examples:
  Binary operations: subtracting and intersecting flags

      enum Enum
      {
          A = 1 << 0,
          B = 1 << 1,
          C = 1 << 2,
      }
      immutable BitFlags!Enum flags_AB = BitFlags!Enum(Enum.A, Enum.B);
      immutable BitFlags!Enum flags_BC = BitFlags!Enum(Enum.B, Enum.C);

      // Use the ~ operator for subtracting flags
      immutable BitFlags!Enum flags_B = flags_AB & ~BitFlags!Enum(Enum.A);
      assert(!flags_B.A && flags_B.B && !flags_B.C);

      // use & between BitFlags for intersection
      writeln(flags_B); // (flags_BC & flags_AB)

  Examples:
  All the binary operators work in their assignment version

      enum Enum
      {
          A = 1 << 0,
          B = 1 << 1,
      }

      BitFlags!Enum flags_empty, temp, flags_AB;
      flags_AB = Enum.A | Enum.B;

      temp |= flags_AB;
      writeln(temp); // (flags_empty | flags_AB)

      temp = flags_empty;
      temp |= Enum.B;
      writeln(temp); // (flags_empty | Enum.B)

      temp = flags_empty;
      temp &= flags_AB;
      writeln(temp); // (flags_empty & flags_AB)

      temp = flags_empty;
      temp &= Enum.A;
      writeln(temp); // (flags_empty & Enum.A)

  Examples:
  Conversion to bool and int

      enum Enum
      {
          A = 1 << 0,
          B = 1 << 1,
      }

      BitFlags!Enum flags;

      // BitFlags with no value set evaluate to false
      assert(!flags);

      // BitFlags with at least one value set evaluate to true
      flags |= Enum.A;
      assert(flags);

      // This can be useful to check intersection between BitFlags
      BitFlags!Enum flags_AB = Enum.A | Enum.B;
      assert(flags & flags_AB);
      assert(flags & Enum.A);

      // You can of course get you raw value out of flags
      auto value = cast(int) flags;
      writeln(value); // Enum.A

  Examples:
  You need to specify the unsafe parameter for enums with custom values

      enum UnsafeEnum
      {
          A = 1,
          B = 2,
          C = 4,
          BC = B|C
      }
      static assert(!__traits(compiles, { BitFlags!UnsafeEnum flags; }));
      BitFlags!(UnsafeEnum, Yes.unsafe) flags;

      // property access tests for exact match of unsafe enums
      flags.B = true;
      assert(!flags.BC); // only B
      flags.C = true;
      assert(flags.BC); // both B and C
      flags.B = false;
      assert(!flags.BC); // only C

      // property access sets all bits of unsafe enum group
      flags = flags.init;
      flags.BC = true;
      assert(!flags.A && flags.B && flags.C);
      flags.A = true;
      flags.BC = false;
      assert(flags.A && !flags.B && !flags.C);

template `ReplaceType`(From, To, T...)
  Replaces all occurrences of From into To, in one or more types T. For example, `ReplaceType`!(int, uint, Tuple!(int, float)[string]) yields Tuple!(uint, float)[string]. The types in which replacement is performed may be arbitrarily complex, including qualifiers, built-in type constructors (pointers, arrays, associative arrays, functions, and delegates), and template instantiations; replacement proceeds transitively through the type definition. However, member types in structs or classes are not replaced because there are no ways to express the types resulting after replacement.
  This is an advanced type manipulation necessary e.g. for replacing the placeholder type This in std.variant.Algebraic.
  Returns:
  `ReplaceType` aliases itself to the type(s) that result after replacement.
  Examples:

      static assert(
          is(ReplaceType!(int, string, int[]) == string[]) &&
          is(ReplaceType!(int, string, int[int]) == string[string]) &&
          is(ReplaceType!(int, string, const(int)[]) == const(string)[]) &&
          is(ReplaceType!(int, string, Tuple!(int[], float))
              == Tuple!(string[], float))
      );

template `ReplaceTypeUnless`(alias pred, From, To, T...)
  Like ReplaceType, but does not perform replacement in types for which pred evaluates to true.
  Examples:

      import std.traits : isArray;

      static assert(
          is(ReplaceTypeUnless!(isArray, int, string, int*) == string*) &&
          is(ReplaceTypeUnless!(isArray, int, string, int[]) == int[]) &&
          is(ReplaceTypeUnless!(isArray, int, string, Tuple!(int, int[]))
              == Tuple!(string, int[]))
      );

struct `Ternary`;
  Ternary type with three truth values:
  - `Ternary`.yes for true
  - `Ternary`.no for false
  - `Ternary`.unknown as an unknown state
  Also known as trinary, trivalent, or trilean.
  See Also:
  Three Valued Logic on Wikipedia
  Examples:

      Ternary a;
      writeln(a); // Ternary.unknown

      writeln(~Ternary.yes); // Ternary.no
      writeln(~Ternary.no); // Ternary.yes
      writeln(~Ternary.unknown); // Ternary.unknown

  enum Ternary `no`;
  enum Ternary `yes`;
  enum Ternary `unknown`;
    The possible states of the Ternary
  pure nothrow @nogc @safe this(bool `b`);
  pure nothrow @nogc @safe void `opAssign`(bool `b`);
    Construct and assign from a bool, receiving no for false and yes for true.
  pure nothrow @nogc @safe this(const Ternary `b`);
    Construct a ternary value from another ternary value
  Ternary `opUnary`(string s)()
  if (s == "~");
  Ternary `opBinary`(string s)(Ternary `rhs`)
  if (s == "|");
  Ternary `opBinary`(string s)(Ternary `rhs`)
  if (s == "&");
  Ternary `opBinary`(string s)(Ternary `rhs`)
  if (s == "^");
  Ternary `opBinary`(string s)(bool `rhs`)
  if (s == "|" || s == "&" || s == "^");

    Truth table for logical operations
    a | b | ˜a | a | b | a & b | a ^ b
    --- | --- | --- | --- | --- | ---
    no | no | yes | no | no | no
    no | yes |  | yes | no | yes
    no | unknown |  | unknown | no | unknown
    yes | no | no | yes | no | yes
    yes | yes |  | yes | yes | no
    yes | unknown |  | yes | unknown | unknown
    unknown | no | unknown | unknown | no | unknown
    unknown | yes |  | yes | unknown | unknown
    unknown | unknown |  | unknown | unknown | unknown

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