D Programming Language
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# Templates

> I think that I can safely say that nobody understands C++ template mechanics. -- Richard Deyman

Templates are D's approach to generic programming. Templates are defined with a TemplateDeclaration:

    TemplateDeclaration:
        template Identifier TemplateParameters Constraintopt { DeclDefsopt }

    TemplateParameters:
        ( TemplateParameterListopt )

    TemplateParameterList:
        TemplateParameter
        TemplateParameter ,
        TemplateParameter , TemplateParameterList

    TemplateParameter:
        TemplateTypeParameter
        TemplateValueParameter
        TemplateAliasParameter
        TemplateTupleParameter
        TemplateThisParameter

The body of the TemplateDeclaration must be syntactically correct even if never instantiated. Semantic analysis is not done until instantiated. A template forms its own scope, and the template body can contain classes, structs, types, enums, variables, functions, and other templates.

Template parameters can be types, values, symbols, or tuples. Types can be any type. Value parameters must be of an integral type, floating point type, or string type and specializations for them must resolve to an integral constant, floating point constant, null, or a string literal. Symbols can be any non-local symbol. Tuples are a sequence of 0 or more types, values or symbols.

Template parameter specializations constrain the values or types the TemplateParameter can accept.

Template parameter defaults are the value or type to use for the TemplateParameter in case one is not supplied.

## Explicit Template Instantiation

Templates are explicitly instantiated with:

    TemplateInstance:
        Identifier TemplateArguments

    TemplateArguments:
        ! ( TemplateArgumentListopt )
        ! TemplateSingleArgument

    TemplateArgumentList:
        TemplateArgument
        TemplateArgument ,
        TemplateArgument , TemplateArgumentList

    TemplateArgument:
        Type
        AssignExpression
        Symbol

    Symbol:
        SymbolTail
        . SymbolTail

    SymbolTail:
        Identifier
        Identifier . SymbolTail
        TemplateInstance
        TemplateInstance . SymbolTail

    TemplateSingleArgument:
        Identifier
        BasicTypeX
        CharacterLiteral
        StringLiteral
        IntegerLiteral
        FloatLiteral
        true
        false
        null
        this
        SpecialKeyword

Once instantiated, the declarations inside the template, called the template members, are in the scope of the TemplateInstance:

    template TFoo(T) { alias t = T*; }
    ...
    TFoo!(int).t x; // declare x to be of type int*

If the TemplateArgument is one token long, the parentheses can be omitted:

    TFoo!int.t x;   // same as TFoo!(int).t x;

A template instantiation can be aliased:

    template TFoo(T) { alias t = T*; }
    alias abc = TFoo!(int);
    abc.t x;        // declare x to be of type int*

Multiple instantiations of a TemplateDeclaration with the same TemplateArgumentList, before implicit conversions, all will refer to the same instantiation. For example:

    template TFoo(T) { T f; }
    alias a = TFoo!(int);
    alias b = TFoo!(int);
    ...
    a.f = 3;
    assert(b.f == 3);  // a and b refer to the same instance of TFoo

This is true even if the TemplateInstances are done in different modules.

Even if template arguments are implicitly converted to the same template parameter type, they still refer to different instances:

    struct TFoo(int x) { }
    static assert(is(TFoo!(3) == TFoo!(2 + 1))); // 3 and 2+1 are both 3 of type int
    static assert(!is(TFoo!(3) == TFoo!(3u)));   // 3u and 3 are different types

If multiple templates with the same Identifier are declared, they are distinct if they have a different number of arguments or are differently specialized.

For example, a simple generic copy template would be:

    template TCopy(T) {
      void copy(out T to, T from) {
        to = from;
      }
    }

To use the template, it must first be instantiated with a specific type:

    int i;
    TCopy!(int).copy(i, 3);

## Instantiation Scope

TemplateInstantances are always performed in the scope of where the TemplateDeclaration is declared, with the addition of the template parameters being declared as aliases for their deduced types.

For example:

module a

    template TFoo(T) { void bar() { func(); } }

module b

    import a;

    void func() { }
    alias f = TFoo!(int); // error: func not defined in module a

and:

module a

    template TFoo(T) { void bar() { func(1); } }
    void func(double d) { }

module b

    import a;

    void func(int i) { }
    alias f = TFoo!(int);
    ...
    f.bar();  // will call a.func(double)

TemplateParameter specializations and default values are evaluated in the scope of the TemplateDeclaration.

## Argument Deduction

The types of template parameters are deduced for a particular template instantiation by comparing the template argument with the corresponding template parameter.

For each template parameter, the following rules are applied in order until a type is deduced for each parameter:

1. If there is no type specialization for the parameter, the type of the parameter is set to the template argument.
2. If the type specialization is dependent on a type parameter, the type of that parameter is set to be the corresponding part of the type argument.
3. If after all the type arguments are examined there are any type parameters left with no type assigned, they are assigned types corresponding to the template argument in the same position in the TemplateArgumentList.
4. If applying the above rules does not result in exactly one type for each template parameter, then it is an error.

For example:

    template TFoo(T) { }
    alias Foo1 = TFoo!(int);     // (1) T is deduced to be int
    alias Foo2 = TFoo!(char*);   // (1) T is deduced to be char*

    template TBar(T : T*) { }
    alias Foo3 = TBar!(char*);   // (2) T is deduced to be char

    template TAbc(D, U : D[]) { }
    alias Bar1 = TAbc!(int, int[]);  // (2) D is deduced to be int, U is int[]
    alias Bar2 = TAbc!(char, int[]); // (4) error, D is both char and int

    template TDef(D : E*, E) { }
    alias Bar3 = TDef!(int*, int); // (1) E is int
                                 // (3) D is int*

Deduction from a specialization can provide values for more than one parameter:

    template Foo(T: T[U], U) {
      ...
    }

    Foo!(int[long])  // instantiates Foo with T set to int, U set to long

When considering matches, a class is considered to be a match for any super classes or interfaces:

    class A { }
    class B : A { }

    template TFoo(T : A) { }
    alias Foo4 = TFoo!(B);    // (3) T is B

    template TBar(T : U*, U : A) { }
    alias Foo5 = TBar!(B*, B); // (2) T is B*
                               // (3) U is B

## Template Type Parameters

    TemplateTypeParameter:
        Identifier
        Identifier TemplateTypeParameterSpecialization
        Identifier TemplateTypeParameterDefault
        Identifier TemplateTypeParameterSpecialization TemplateTypeParameterDefault

    TemplateTypeParameterSpecialization:
        : Type

    TemplateTypeParameterDefault:
        = Type

### Specialization

Templates may be specialized for particular types of arguments by following the template parameter identifier with a : and the specialized type. For example:

    template TFoo(T)        { ... } // #1
    template TFoo(T : T[])  { ... } // #2
    template TFoo(T : char) { ... } // #3
    template TFoo(T,U,V)    { ... } // #4

    alias foo1 = TFoo!(int) ;           // instantiates #1
    alias foo2 = TFoo!(double[]);       // instantiates #2 with T being double
    alias foo3 = TFoo!(char);           // instantiates #3
    alias fooe = TFoo!(char, int);      // error, number of arguments mismatch
    alias foo4 = TFoo!(char, int, int); // instantiates #4

The template picked to instantiate is the one that is most specialized that fits the types of the TemplateArgumentList. Determine which is more specialized is done the same way as the C++ partial ordering rules. If the result is ambiguous, it is an error.

## Template This Parameters

    TemplateThisParameter:
        this TemplateTypeParameter

TemplateThisParameters are used in member function templates to pick up the type of the this reference.

    import std.stdio;

    struct S {
      const void foo(this T)(int i) {
        writeln(typeid(T));
      }
    }

    void main() {
      const(S) s;
      (&s).foo(1);
      S s2;
      s2.foo(2);
      immutable(S) s3;
      s3.foo(3);
    }

Prints:

    const(S)
    S
    immutable(S)

This is especially useful when used with inheritance. For example, you might want to implement a final base method which returns a derived class type. Typically you would return a base type, but this won't allow you to call or access derived properties of the type:

    interface Addable(T)
    {
        final auto add(T t)
        {
            return this;
        }
    }

    class List(T) : Addable!T
    {
        List remove(T t)
        {
            return this;
        }
    }

    void main()
    {
        auto list = new List!int;
        list.add(1).remove(1);  // error: no 'remove' method for Addable!int
    }

Here the method add returns the base type, which doesn't implement the remove method. The template this parameter can be used for this purpose:

    interface Addable(T)
    {
        final R add(this R)(T t)
        {
            return cast(R)this;  // cast is necessary, but safe
        }
    }

    class List(T) : Addable!T
    {
        List remove(T t)
        {
            return this;
        }
    }

    void main()
    {
        auto list = new List!int;
        list.add(1).remove(1);  // ok
    }

## Template Value Parameters

    TemplateValueParameter:
        BasicType Declarator
        BasicType Declarator TemplateValueParameterSpecialization
        BasicType Declarator TemplateValueParameterDefault
        BasicType Declarator TemplateValueParameterSpecialization TemplateValueParameterDefault

    TemplateValueParameterSpecialization:
        : ConditionalExpression

    TemplateValueParameterDefault:
        = AssignExpression
        = SpecialKeyword

Template value parameter types can be any type which can be statically initialized at compile time. Template value arguments can be integer values, floating point values, nulls, string values, array literals of template value arguments, associative array literals of template value arguments, or struct literals of template value arguments.

    template foo(string s) {
      string bar() { return s ~ " betty"; }
    }

    void main() {
      writefln("%s", foo!("hello").bar()); // prints: hello betty
    }

This example of template foo has a value parameter that is specialized for 10:

    template foo(U : int, int T : 10) {
      U x = T;
    }

    void main() {
      assert(foo!(int, 10).x == 10);
    }

## Template Alias Parameters

    TemplateAliasParameter:
        alias Identifier TemplateAliasParameterSpecializationopt TemplateAliasParameterDefaultopt
        alias BasicType Declarator TemplateAliasParameterSpecializationopt TemplateAliasParameterDefaultopt

    TemplateAliasParameterSpecialization:
        : Type
        : ConditionalExpression

    TemplateAliasParameterDefault:
        = Type
        = ConditionalExpression

Alias parameters enable templates to be parameterized with any type of D symbol, including global names, local names, typedef names, module names, template names, and template instance names. Literals can also be used as arguments to alias parameters.

- Global names

      int x;

      template Foo(alias X) {
        static int* p = &X;
      }

      void test() {
        alias bar = Foo!(x);
        *bar.p = 3;       // set x to 3
        static int y;
        alias abc = Foo!(y);
        *abc.p = 3;       // set y to 3
      }

- Type names

      class Foo {
        static p = int;
      }

      template Bar(alias T) {
        alias q = T.p;
      }

      void test() {
        alias bar = Bar!(Foo);
        bar.q = 3;  // sets Foo.p to 3
      }

- Module names

      import std.string;

      template Foo(alias X) {
        alias y = X.toString;
      }

      void test() {
        alias bar = Foo!(std.string);
        bar.y(3);   // calls std.string.toString(3)
      }

- Template names

      int x;

      template Foo(alias X) {
        static int* p = &X;
      }

      template Bar(alias T) {
        alias abc = T!(x);
      }

      void test() {
        alias bar = Bar!(Foo);
        *bar.abc.p = 3;  // sets x to 3
      }

- Template alias names

      int x;

      template Foo(alias X) {
        static int* p = &X;
      }

      template Bar(alias T) {
        alias q = T.p;
      }

      void test() {
        alias foo = Foo!(x);
        alias bar = Bar!(foo);
        *bar.q = 3;  // sets x to 3
      }

- Literals

      template Foo(alias X, alias Y) {
        static int i = X;
        static string s = Y;
      }

      void test() {
        alias foo = Foo!(3, "bar");
        writeln(foo.i, foo.s);  // prints 3bar
      }

## Template Tuple Parameters

    TemplateTupleParameter:
        Identifier ...

If the last template parameter in the TemplateParameterList is declared as a TemplateTupleParameter, it is a match with any trailing template arguments. The sequence of arguments form a Tuple. A Tuple is not a type, an expression, or a symbol. It is a sequence of any mix of types, expressions or symbols.

A Tuple whose elements consist entirely of types is called a TypeTuple. A Tuple whose elements consist entirely of expressions is called an ExpressionTuple.

A Tuple can be used as an argument list to instantiate another template, or as the list of parameters for a function.

    template Print(A ...) {
      void print() {
        writefln("args are ", A);
      }
    }

    template Write(A ...) {
      void write(A a) // A is a TypeTuple
                      // a is an ExpressionTuple
      {
        writefln("args are ", a);
      }
    }

    void main() {
      Print!(1,'a',6.8).print();                    // prints: args are 1a6.8
      Write!(int, char, double).write(1, 'a', 6.8); // prints: args are 1a6.8
    }

Template tuples can be deduced from the types of the trailing parameters of an implicitly instantiated function template:

    template Foo(T, R...) {
      void Foo(T t, R r) {
        writeln(t);
        static if (r.length) // if more arguments
          Foo(r);            // do the rest of the arguments
      }
    }

    void main() {
      Foo(1, 'a', 6.8);
    }

prints:

    1
    a
    6.8

The tuple can also be deduced from the type of a delegate or function parameter list passed as a function argument:

    import std.stdio;

    /* R is return type
     * A is first argument type
     * U is TypeTuple of rest of argument types
     */
    R delegate(U) Curry(R, A, U...)(R delegate(A, U) dg, A arg)
    {
      struct Foo
      {
        typeof(dg) dg_m;
        typeof(arg) arg_m;

        R bar(U u)
        {
          return dg_m(arg_m, u);
        }
      }

      Foo* f = new Foo;
      f.dg_m = dg;
      f.arg_m = arg;
      return &f.bar;
    }

    void main()
    {
      int plus(int x, int y, int z)
      {
        return x + y + z;
      }

      auto plus_two = Curry(&plus, 2);
      writefln("%d", plus_two(6, 8)); // prints 16
    }

The number of elements in a Tuple can be retrieved with the .length property. The nth element can be retrieved by indexing the Tuple with [n], and sub tuples can be created with the slicing syntax.

Tuples are static compile time entities, there is no way to dynamically change, add, or remove elements.

If both a template with a tuple parameter and a template without a tuple parameter exactly match a template instantiation, the template without a TemplateTupleParameter is selected.

## Template Parameter Default Values

Trailing template parameters can be given default values:

    template Foo(T, U = int) { ... }
    Foo!(uint,long); // instantiate Foo with T as uint, and U as long
    Foo!(uint);      // instantiate Foo with T as uint, and U as int

    template Foo(T, U = T*) { ... }
    Foo!(uint);      // instantiate Foo with T as uint, and U as uint*

## Implicit Template Properties

If a template has exactly one member in it, and the name of that member is the same as the template name, that member is assumed to be referred to in a template instantiation:

    template Foo(T) {
      T Foo; // declare variable Foo of type T
    }

    void test() {
      Foo!(int) = 6; // instead of Foo!(int).Foo
    }

## Template Constructors

    ConstructorTemplate:
        this TemplateParameters Parameters MemberFunctionAttributesopt Constraintopt :
        this TemplateParameters Parameters MemberFunctionAttributesopt Constraintopt FunctionBody

Templates can be used to form constructors for classes and structs.

## Aggregate Templates

    ClassTemplateDeclaration:
        class Identifier TemplateParameters Constraintopt BaseClassListopt AggregateBody
        class Identifier TemplateParameters BaseClassListopt Constraintopt AggregateBody

    InterfaceTemplateDeclaration:
        interface Identifier TemplateParameters Constraintopt BaseInterfaceListopt AggregateBody
        interface Identifier TemplateParameters BaseInterfaceList Constraint AggregateBody

    StructTemplateDeclaration:
        struct Identifier TemplateParameters Constraintopt AggregateBody

    UnionTemplateDeclaration:
        union Identifier TemplateParameters Constraintopt AggregateBody

If a template declares exactly one member, and that member is a class with the same name as the template:

    template Bar(T) {
      class Bar {
        T member;
      }
    }

then the semantic equivalent, called a ClassTemplateDeclaration can be written as:

    class Bar(T) {
      T member;
    }

Analogously to class templates, struct, union and interfaces can be transformed into templates by supplying a template parameter list.

## Function Templates

If a template declares exactly one member, and that member is a function with the same name as the template, it is a function template declaration. Alternatively, a function template declaration is a function declaration with a TemplateParameterList immediately preceding the Parameters.

A function template to compute the square of type T is:

    T Square(T)(T t) {
      return t * t;
    }

Function templates can be explicitly instantiated with a !(TemplateArgumentList):

    writefln("The square of %s is %s", 3, Square!(int)(3));

or implicitly, where the TemplateArgumentList is deduced from the types of the function arguments:

    writefln("The square of %s is %s", 3, Square(3));  // T is deduced to be int

If there are fewer arguments supplied in the TemplateArgumentList than parameters in the TemplateParameterList, the arguments fulfill parameters from left to right, and the rest of the parameters are then deduced from the function arguments.

Function template type parameters that are to be implicitly deduced may not have specializations:

    void Foo(T : T*)(T t) { ... }

    int x,y;
    Foo!(int*)(x);   // ok, T is not deduced from function argument
    Foo(&y);         // error, T has specialization

Template arguments not implicitly deduced can have default values:

    void Foo(T, U=T*)(T t) { U p; ... }

    int x;
    Foo(&x);    // T is int, U is int*

Function templates can have their return types deduced based on the first ReturnStatement in the function:

    auto Square(T)(T t) {
      return t * t;
    }

If there is more than one return statement, then the types of the return statement expressions must match. If there are no return statements, then the return type of the function template is void.

## Variable Templates

Same as aggregates and functions, variable declarations with Initializer can have optional template parameters:

    enum string constant(TL...) = TL.stringof;
    ubyte[T.sizeof] storage(T) = 0;
    auto array(alias a) = a;

These declarations are transformed into templates:

    template constant(TL...) {
      enum string constant = TL.stringof;
    }
    template storage(T) {
      ubyte[T.sizeof] storage = 0;
    }
    template array(alias a) {
      auto array = a;
    }

## Alias Templates

AliasDeclaration can also have optional template parameters:

    alias Sequence(TL...) = TL;

It is lowered to:

    template Sequence(TL...) {
      alias Sequence = TL;
    }

### Function Templates with Auto Ref Parameters

An auto ref function template parameter becomes a ref parameter if its corresponding argument is an lvalue, otherwise it becomes a value parameter:

    int foo(T...)(auto ref T x) {
      int result;

      foreach (i, v; x)
      {
        if (v == 10)
          assert(__traits(isRef, x[i]));
        else
          assert(!__traits(isRef, x[i]));
        result += v;
      }
      return result;
    }

    void main() {
      int y = 10;
      int r;
      r = foo(8);       // returns 8
      r = foo(y);       // returns 10
      r = foo(3, 4, y); // returns 17
      r = foo(4, 5, y); // returns 19
      r = foo(y, 6, y); // returns 26
    }

Auto ref parameters can be combined with auto ref return attributes:

    auto ref min(T, U)(auto ref T lhs, auto ref U rhs)
    {
      return lhs > rhs ? rhs : lhs;
    }

    void main()
    {
      int x = 7, y = 8;
      int i;

      i = min(4, 3);     // returns 3
      i = min(x, y);     // returns 7
      min(x, y) = 10;    // sets x to 10
      static assert(!__traits(compiles, min(3, y) = 10));
      static assert(!__traits(compiles, min(y, 3) = 10));
    }

## Recursive Templates

Template features can be combined to produce some interesting effects, such as compile time evaluation of non-trivial functions. For example, a factorial template can be written:

    template factorial(int n : 1) {
      enum { factorial = 1 }
    }

    template factorial(int n) {
      enum { factorial = n* factorial!(n-1) }
    }

    void test() {
      writefln("%s", factorial!(4)); // prints 24
    }

## Template Constraints

    Constraint:
        if ( Expression )

Constraints are used to impose additional constraints on matching arguments to a template beyond what is possible in the TemplateParameterList. The Expression is computed at compile time and returns a result that is converted to a boolean value. If that value is true, then the template is matched, otherwise the template is not matched.

For example, the following function template only matches with odd values of N:

    void foo(int N)()
            if (N & 1)
    {
      ...
    }
    ...
    foo!(3)();  // ok, matches
    foo!(4)();  // error, no match

## Limitations

Templates cannot be used to add non-static members or virtual functions to classes. For example:

    class Foo {
      template TBar(T) {
        T xx;               // becomes a static member of Foo
        int func(T) { ... } // non-virtual

        static T yy;                        // Ok
        static int func(T t, int y) { ... } // Ok
      }
    }

Templates cannot be declared inside functions.

Templates cannot add functions to interfaces:

    interface TestInterface { void tpl(T)(); }   // error

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