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# std.meta

Templates to manipulate template argument lists (also known as type lists).
Some operations on alias sequences are built in to the language, such as TL[n] which gets the nth type from the alias sequence. TL[lwr .. upr] returns a new type list that is a slice of the old one.

Several templates in this module use or operate on eponymous templates that take a single argument and evaluate to a boolean constant. Such templates are referred to as template predicates.

References: Based on ideas in Table 3.1 from Modern C++ Design, Andrei Alexandrescu (Addison-Wesley Professional, 2001)

License:
Boost License 1.0.
Authors:
Walter Bright, David Nadlinger

Source: std/meta.d

template AliasSeq(TList...)
  Creates a sequence of zero or more aliases. This is most commonly used as template parameters or arguments.
  Examples:

      import std.meta;
      alias TL = AliasSeq!(int, double);

      int foo(TL td)  // same as int foo(int, double);
      {
          return td[0] + cast(int)td[1];
      }

  Examples:

      alias TL = AliasSeq!(int, double);

      alias Types = AliasSeq!(TL, char);
      static assert(is(Types == AliasSeq!(int, double, char)));

enum auto staticIndexOf(T, TList...);
enum auto staticIndexOf(alias T, TList...);
  Returns the index of the first occurrence of type T in the sequence of zero or more types TList. If not found, -1 is returned.
  Examples:

      import std.stdio;

      void foo()
      {
          writefln("The index of long is %s",
                   staticIndexOf!(long, AliasSeq!(int, long, double)));
          // prints: The index of long is 1
      }

alias IndexOf = staticIndexOf(T, TList...);
  Kept for backwards compatibility
template Erase(T, TList...)
template Erase(alias T, TList...)
  Returns a typetuple created from TList with the first occurrence, if any, of T removed.
  Examples:

      alias Types = AliasSeq!(int, long, double, char);
      alias TL = Erase!(long, Types);
      static assert(is(TL == AliasSeq!(int, double, char)));

template EraseAll(T, TList...)
template EraseAll(alias T, TList...)
  Returns a typetuple created from TList with the all occurrences, if any, of T removed.
  Examples:

      alias Types = AliasSeq!(int, long, long, int);

      alias TL = EraseAll!(long, Types);
      static assert(is(TL == AliasSeq!(int, int)));

template NoDuplicates(TList...)
  Returns a typetuple created from TList with the all duplicate types removed.
  Examples:

      alias Types = AliasSeq!(int, long, long, int, float);

      alias TL = NoDuplicates!(Types);
      static assert(is(TL == AliasSeq!(int, long, float)));

template Replace(T, U, TList...)
template Replace(alias T, U, TList...)
template Replace(T, alias U, TList...)
template Replace(alias T, alias U, TList...)
  Returns a typetuple created from TList with the first occurrence of type T, if found, replaced with type U.
  Examples:

      alias Types = AliasSeq!(int, long, long, int, float);

      alias TL = Replace!(long, char, Types);
      static assert(is(TL == AliasSeq!(int, char, long, int, float)));

template ReplaceAll(T, U, TList...)
template ReplaceAll(alias T, U, TList...)
template ReplaceAll(T, alias U, TList...)
template ReplaceAll(alias T, alias U, TList...)
  Returns a typetuple created from TList with all occurrences of type T, if found, replaced with type U.
  Examples:

      alias Types = AliasSeq!(int, long, long, int, float);

      alias TL = ReplaceAll!(long, char, Types);
      static assert(is(TL == AliasSeq!(int, char, char, int, float)));

template Reverse(TList...)
  Returns a typetuple created from TList with the order reversed.
  Examples:

      alias Types = AliasSeq!(int, long, long, int, float);

      alias TL = Reverse!(Types);
      static assert(is(TL == AliasSeq!(float, int, long, long, int)));

template MostDerived(T, TList...)
  Returns the type from TList that is the most derived from type T. If none are found, T is returned.
  Examples:

      class A { }
      class B : A { }
      class C : B { }
      alias Types = AliasSeq!(A, C, B);

      MostDerived!(Object, Types) x;  // x is declared as type C
      static assert(is(typeof(x) == C));

template DerivedToFront(TList...)
  Returns the typetuple TList with the types sorted so that the most derived types come first.
  Examples:

      class A { }
      class B : A { }
      class C : B { }
      alias Types = AliasSeq!(A, C, B);

      alias TL = DerivedToFront!(Types);
      static assert(is(TL == AliasSeq!(C, B, A)));

template staticMap(alias F, T...)
  Evaluates to AliasSeq!(F!(T[0]), F!(T[1]), ..., F!(T[$ - 1])).
  Examples:

      import std.traits : Unqual;
      alias TL = staticMap!(Unqual, int, const int, immutable int);
      static assert(is(TL == AliasSeq!(int, int, int)));

template allSatisfy(alias F, T...)
  Tests whether all given items satisfy a template predicate, i.e. evaluates to F!(T[0]) && F!(T[1]) && ... && F!(T[$ - 1]).
  Evaluation is not short-circuited if a false result is encountered; the template predicate must be instantiable with all the given items.
  Examples:

      import std.traits : isIntegral;

      static assert(!allSatisfy!(isIntegral, int, double));
      static assert( allSatisfy!(isIntegral, int, long));

template anySatisfy(alias F, T...)
  Tests whether any given items satisfy a template predicate, i.e. evaluates to F!(T[0]) || F!(T[1]) || ... || F!(T[$ - 1]).
  Evaluation is not short-circuited if a true result is encountered; the template predicate must be instantiable with all the given items.
  Examples:

      import std.traits : isIntegral;

      static assert(!anySatisfy!(isIntegral, string, double));
      static assert( anySatisfy!(isIntegral, int, double));

template Filter(alias pred, TList...)
  Filters an AliasSeq using a template predicate. Returns a AliasSeq of the elements which satisfy the predicate.
  Examples:

      import std.traits : isNarrowString, isUnsigned;

      alias Types1 = AliasSeq!(string, wstring, dchar[], char[], dstring, int);
      alias TL1 = Filter!(isNarrowString, Types1);
      static assert(is(TL1 == AliasSeq!(string, wstring, char[])));

      alias Types2 = AliasSeq!(int, byte, ubyte, dstring, dchar, uint, ulong);
      alias TL2 = Filter!(isUnsigned, Types2);
      static assert(is(TL2 == AliasSeq!(ubyte, uint, ulong)));

template templateNot(alias pred)
  Negates the passed template predicate.
  Examples:

      import std.traits : isPointer;

      alias isNoPointer = templateNot!isPointer;
      static assert(!isNoPointer!(int*));
      static assert(allSatisfy!(isNoPointer, string, char, float));

template templateAnd(Preds...)
  Combines several template predicates using logical AND, i.e. constructs a new predicate which evaluates to true for a given input T if and only if all of the passed predicates are true for T.
  The predicates are evaluated from left to right, aborting evaluation in a short-cut manner if a false result is encountered, in which case the latter instantiations do not need to compile.
  Examples:

      import std.traits : isNumeric, isUnsigned;

      alias storesNegativeNumbers = templateAnd!(isNumeric, templateNot!isUnsigned);
      static assert(storesNegativeNumbers!int);
      static assert(!storesNegativeNumbers!string && !storesNegativeNumbers!uint);

      // An empty list of predicates always yields true.
      alias alwaysTrue = templateAnd!();
      static assert(alwaysTrue!int);

template templateOr(Preds...)
  Combines several template predicates using logical OR, i.e. constructs a new predicate which evaluates to true for a given input T if and only at least one of the passed predicates is true for T.
  The predicates are evaluated from left to right, aborting evaluation in a short-cut manner if a true result is encountered, in which case the latter instantiations do not need to compile.
  Examples:

      import std.traits : isPointer, isUnsigned;

      alias isPtrOrUnsigned = templateOr!(isPointer, isUnsigned);
      static assert( isPtrOrUnsigned!uint &&  isPtrOrUnsigned!(short*));
      static assert(!isPtrOrUnsigned!int  && !isPtrOrUnsigned!(string));

      // An empty list of predicates never yields true.
      alias alwaysFalse = templateOr!();
      static assert(!alwaysFalse!int);

template aliasSeqOf(alias range)
  Converts an input range range to an alias sequence.
  Examples:

      import std.algorithm : map, sort;
      import std.string : capitalize;

      struct S
      {
          int a;
          int c;
          int b;
      }

      alias capMembers = aliasSeqOf!([__traits(allMembers, S)].sort().map!capitalize());
      static assert(capMembers[0] == "A");
      static assert(capMembers[1] == "B");
      static assert(capMembers[2] == "C");

  Examples:

      enum REF = [0, 1, 2, 3];
      foreach(I, V; aliasSeqOf!([0, 1, 2, 3]))
      {
          static assert(V == I);
          static assert(V == REF[I]);
      }

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