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Templates

Template Declarations

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

TemplateDeclaration:
    template Identifier TemplateParameters Constraintopt { DeclDefsopt }
TemplateParameters: ( TemplateParameterListopt )
TemplateParameterList: TemplateParameter TemplateParameter , TemplateParameter , TemplateParameterList

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

Template parameters can take types, values, symbols, or sequences.

template t(T) // declare type parameter T
{
    T v; // declare a member variable of type T within template t
}

A template parameter can have a specialization which constrains an argument the TemplateParameter can accept.

template t(T : int) // type T must implicitly convert to int
{
    ...
}

If multiple templates with the same Identifier are declared, they are distinct if they have different parameters or are differently specialized.

If a template has a member which has the same identifier as the template, the template is an Eponymous Template. template declarations with one eponymous member are usually written as specific short syntax template declarations instead.

Template Instantiation

A template must be instantiated before use. This means passing an argument list to the template. Those arguments are typically then substituted into the template body, which becomes a new scoped entity.

Function templates can be implicitly instantiated if the compiler can infer the template arguments from a function call. Otherwise the template must be instantiated explicitly.

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 FundamentalType 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 Ptr = T*; }
...
TFoo!(int).Ptr x; // declare x to be of type int*

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

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

A template instantiation can be aliased:

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

Common Instantiation

Multiple instantiations of a TemplateDeclaration with the same TemplateArgumentList will all 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 the same instance. This example uses a TemplateValueParameter and a struct template:

struct TFoo(int x) { }

// Different template parameters create different struct types
static assert(!is(TFoo!(3) == TFoo!(2)));
// 3 and 2+1 are both 3 of type int - same TFoo instance
static assert(is(TFoo!(3) == TFoo!(2 + 1)));

// 3u is implicitly converted to 3 to match int parameter,
// and refers to exactly the same instance as TFoo!(3)
static assert(is(TFoo!(3) == TFoo!(3u)));

Practical Example

A simple generic copy template would be:

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

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

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

See also function templates.

Instantiation Scope

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

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

Example:

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 arguments are evaluated in the scope of the TemplateDeclaration.

Template Parameters

TemplateParameter:
    TemplateTypeParameter
    TemplateValueParameter
    TemplateAliasParameter
    TemplateSequenceParameter
    TemplateThisParameter

Template parameters can take types, values, symbols, or sequences.

A default argument specifies the type, value or symbol to use for the TemplateParameter when a matching argument is not supplied.

Type Parameters

TemplateTypeParameter:
    Identifier
    Identifier TemplateTypeParameterSpecialization
    Identifier TemplateTypeParameterDefault
    Identifier TemplateTypeParameterSpecialization TemplateTypeParameterDefault
TemplateTypeParameterSpecialization: : Type
TemplateTypeParameterDefault: = Type

Specialization and Pattern Matching

Templates may be specialized for particular types of arguments by following the template parameter identifier with a : and the pattern for 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 matching pattern T[] 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. If the result is ambiguous, it is an error.

Type Parameter 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*) { }    // match template argument against T* pattern
alias bar = TBar!(char*);    // (2) T is deduced to be char

template TAbc(D, U : D[]) { }    // D[] is pattern to be matched
alias abc1 = TAbc!(int, int[]);  // (2) D is deduced to be int, U is int[]
alias abc2 = TAbc!(char, int[]); // (4) error, D is both char and int

template TDef(D : E*, E) { }   // E* is pattern to be matched
alias def = 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 foo = TFoo!(B);      // (3) T is B

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

This Parameters

TemplateThisParameter:
    this TemplateTypeParameter

TemplateThisParameters are used in member function templates to pick up the type of the this reference. It also will infer the mutability of the this reference. For example, if this is const, then the function is marked const.

struct S
{
    void foo(this T)() const
    {
        pragma(msg, T);
    }
}

void main()
{
    const(S) s;
    (&s).foo();
    S s2;
    s2.foo();
    immutable(S) s3;
    s3.foo();
}
Prints:
const(S)
S
immutable(S)

Avoiding Runtime Type Checks

TemplateThisParameter is especially useful when used with inheritance. For example, consider the implementation of a final base method which returns a derived class type. Typically this would return a base type, but that would prohibit calling or accessing 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;
    static assert(is(typeof(list.add(1)) == List!int));
    list.add(1).remove(1);  // ok, List.add

    Addable!int a = list;
    // a.add calls Addable.add
    static assert(is(typeof(a.add(1)) == Addable!int));
}

Value Parameters

TemplateValueParameter:
    BasicType Declarator
    BasicType Declarator TemplateValueParameterSpecialization
    BasicType Declarator TemplateValueParameterDefault
    BasicType Declarator TemplateValueParameterSpecialization TemplateValueParameterDefault
TemplateValueParameterSpecialization: : ConditionalExpression
TemplateValueParameterDefault: = AssignExpression = SpecialKeyword

A template value parameter can take an argument of any expression which can be statically evaluated 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()
{
    import std.stdio;
    writeln(foo!("hello").bar()); // prints: hello betty
}

Specialization

Any specialization or default expression provided must be evaluatable at compile-time.

In this example, template foo has a value parameter that is specialized for 10:

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

void main()
{
    assert(foo!(int, 10).x == 10);
    static assert(!__traits(compiles, foo!(int, 11)));
}

This can be useful when a different template body is required for a specific value. Another template overload would be defined to take other integer literal values.

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 symbol names or values computed at compile-time. Almost any kind of D symbol can be used, including type names, global names, local names, module names, template names, and template instances.

Symbol Aliases

Value Aliases

Typed Alias Parameters

Alias parameters can also be typed. These parameters will accept symbols of that type:

template Foo(alias int x) { }
int x;
float f;

Foo!x;  // ok
Foo!f;  // fails to instantiate

Specialization

Alias parameters can accept both literals and user-defined type symbols, but they are less specialized than the matches to type parameters and value parameters:

template Foo(T)         { ... }  // #1
template Foo(int n)     { ... }  // #2
template Foo(alias sym) { ... }  // #3

struct S {}
int var;

alias foo1  = Foo!(S);      // instantiates #1
alias foo2  = Foo!(1);      // instantiates #2
alias foo3a = Foo!([1,2]);  // instantiates #3
alias foo3b = Foo!(var);    // instantiates #3
template Bar(alias A) { ... }                 // #4
template Bar(T : U!V, alias U, V...) { ... }  // #5

class C(T) {}
alias bar = Bar!(C!int);    // instantiates #5

Sequence Parameters

TemplateSequenceParameter:
    Identifier ...

If the last template parameter in the TemplateParameterList is declared as a TemplateSequenceParameter, it is a match with zero or more trailing template arguments. Any argument that can be passed to a TemplateAliasParameter can be passed to a sequence parameter.

Such a sequence of arguments can itself be aliased for use outside a template. The std.meta.AliasSeq template simply aliases its sequence parameter:

alias AliasSeq(Args...) = Args;

A TemplateSequenceParameter will thus henceforth be referred to by that name for clarity. An AliasSeq is not itself a type, value, or symbol. It is a compile-time sequence of any mix of types, values or symbols, or none.

The elements of an AliasSeq are automatically expanded when it is referenced in a declaration or expression. An AliasSeq can be used as arguments to instantiate a template.

Homogeneous Sequences

A ValueSeq can be used as arguments to call a function:

import std.stdio : writeln;

template print(args...) // args must be a ValueSeq
{
    void print()
    {
        writeln("args are ", args);
    }
}

void main()
{
    print!(1, 'a', 6.8)(); // prints: args are 1a6.8
}

A TypeSeq can be used to declare parameters for a function:

import std.stdio : writeln;

template print(Types...) // Types must be a TypeSeq
{
    void print(Types args) // args is a ValueSeq
    {
        writeln("args are ", args);
    }
}

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

A TypeSeq can similarly be used to declare variables. Parameters or variables declared with a TypeSeq are called an lvalue sequence.

Sequence Operations

import std.meta : AliasSeq;

int v = 4;
alias nums = AliasSeq!(1, 2, 3, v);
static assert(nums.length == 4);
static assert(nums[1] == 2);

// nums[3] is bound to v, an lvalue
nums[3]++;
assert(v == 5);

// slice first 3 elements
alias trio = nums[0 .. $-1];
// expand into an array literal
static assert([trio] == [1, 2, 3]);

AliasSeqs are static compile-time entities, there is no way to dynamically change, add, or remove elements either at compile-time or run-time. Instead, either:

Sequences can 'unroll' code for each element using a foreach statement.

Type Sequence Deduction

Type sequences can be deduced from the trailing parameters of an implicitly instantiated function template:

import std.stdio;

template print(T, Args...)
{
    void print(T first, Args args)
    {
        writeln(first);
        static if (args.length) // if more arguments
            print(args);        // recurse for remaining arguments
    }
}

void main()
{
    print(1, 'a', 6.8);
}
prints:
1
a
6.8

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

import std.stdio;

/* Partially applies a delegate by tying its first argument to a particular value.
 * R = return type
 * T = first argument type
 * Args = TypeSeq of remaining argument types
 */
R delegate(Args) partial(R, T, Args...)(R delegate(T, Args) dg, T first)
{
    // return a closure
    return (Args args) => dg(first, args);
}

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

    import std.stdio;
    auto plus_two = partial(&plus, 2);
    writeln(plus_two(6, 8)); // prints 16
}
See also: std.functional.partial

Specialization

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

template Foo(T)         { pragma(msg, "1"); }   // #1
template Foo(int n)     { pragma(msg, "2"); }   // #2
template Foo(alias sym) { pragma(msg, "3"); }   // #3
template Foo(Args...)   { pragma(msg, "4"); }   // #4

import std.stdio;

// Any sole template argument will never match to #4
alias foo1 = Foo!(int);          // instantiates #1
alias foo2 = Foo!(3);            // instantiates #2
alias foo3 = Foo!(std);          // instantiates #3

alias foo4 = Foo!(int, 3, std);  // instantiates #4

Default Arguments

Trailing template parameters can be given default arguments:

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*

See also: Function Template Default Arguments.

Eponymous Templates

If a template contains members whose name is the same as the template identifier then these members are assumed to be referred to in a template instantiation:

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

void main()
{
    foo!(int) = 6; // instead of foo!(int).foo
}

The following example has more than one eponymous member and uses Implicit Function Template Instantiation:

template foo(S, T)
{
    // each member contains all the template parameters
    void foo(S s, T t) {}
    void foo(S s, T t, string) {}
}

void main()
{
    foo(1, 2, "test"); // foo!(int, int).foo(1, 2, "test")
    foo(1, 2); // foo!(int, int).foo(1, 2)
}

Aggregate Type Templates

ClassTemplateDeclaration:
    class Identifier TemplateParameters ;
    class Identifier TemplateParameters Constraintopt BaseClassListopt AggregateBody
    class Identifier TemplateParameters BaseClassListopt Constraintopt AggregateBody
InterfaceTemplateDeclaration: interface Identifier TemplateParameters ; interface Identifier TemplateParameters Constraintopt BaseInterfaceListopt AggregateBody interface Identifier TemplateParameters BaseInterfaceList Constraint AggregateBody
StructTemplateDeclaration: struct Identifier TemplateParameters ; struct Identifier TemplateParameters Constraintopt AggregateBody
UnionTemplateDeclaration: union Identifier TemplateParameters ; 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 (see Eponymous Templates:)

template Bar(T)
{
    class Bar
    {
        T member;
    }
}
then the semantic equivalent, called a ClassTemplateDeclaration can be written as:
class Bar(T)
{
    T member;
}

See also: This Parameters.

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;
}

It is lowered to:

template square(T)
{
    T square(T t)
    {
        return t * t;
    }
}

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

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

Implicit Function Template Instantiation (IFTI)

Function templates can be implicitly instantiated if the TemplateArgumentList is deducible from the types of the function arguments:

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

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

Type parameter deduction is not influenced by the order of function arguments.

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

Restrictions

Function template type parameters that are to be implicitly deduced must appear in the type of at least one function parameter:

void foo(T : U*, U)(U t) { ... }

int x;
foo!(int*)(x);   // ok, U is deduced and T is specified explicitly
foo(x);         // error, only U can be deduced, not T

When the template parameters must be deduced, the eponymous members can't rely on a static if condition since the deduction relies on how the members are used:

template foo(T)
{
    static if (is(T)) // T is not yet known...
        void foo(T t) {} // T is deduced from the member usage
}

void main()
{
    foo(0); // Error: cannot deduce function from argument types
    foo!int(0); // Ok since no deduction necessary
}

Type Conversions

If template type parameters match the literal expressions on function arguments, the deduced types may consider narrowing conversions of them.

void foo(T)(T v)        { pragma(msg, "in foo, T = ", T); }
void bar(T)(T v, T[] a) { pragma(msg, "in bar, T = ", T); }

foo(1);
// an integer literal type is analyzed as int by default
// then T is deduced to int

short[] arr;
bar(1, arr);
// arr is short[], and the integer literal 1 is
// implicitly convertible to short.
// then T will be deduced to short.

bar(1, [2.0, 3.0]);
// the array literal is analyzed as double[],
// and the integer literal 1 is implicitly convertible to double.
// then T will be deduced to double.

The deduced type parameter for dynamic array and pointer arguments has an unqualified head:

void foo(T)(T arg) { pragma(msg, T); }

int[] marr;
const(int[]) carr;
immutable(int[]) iarr;
foo(marr);  // T == int[]
foo(carr);  // T == const(int)[]
foo(iarr);  // T == immutable(int)[]

int* mptr;
const(int*) cptr;
immutable(int*) iptr;
foo(mptr);  // T == int*
foo(cptr);  // T == const(int)*
foo(iptr);  // T == immutable(int)*

Return Type Deduction

Function templates can have their return types deduced based on the ReturnStatements in the function, just as with normal functions. See Auto Functions.

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

auto i = square(2);
static assert(is(typeof(i) == int));

Auto Ref Parameters

Template functions can have auto ref parameters. An auto ref parameter becomes a ref parameter if its corresponding argument is an lvalue, otherwise it becomes a value parameter:

int countRefs(Args...)(auto ref Args args)
{
    int result;

    foreach (i, _; args)
    {
        if (__traits(isRef, args[i]))
            result++;
    }
    return result;
}

void main()
{
    int y;
    assert(countRefs(3, 4) == 0);
    assert(countRefs(3, y, 4) == 1);
    assert(countRefs(y, 6, y) == 2);
}

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 i;
    i = min(4, 3);
    assert(i == 3);

    int x = 7, y = 8;
    i = min(x, y);
    assert(i == 7);
    // result is an lvalue
    min(x, y) = 10;    // sets x to 10
    assert(x == 10 && y == 8);

    static assert(!__traits(compiles, min(3, y) = 10));
    static assert(!__traits(compiles, min(y, 3) = 10));
}

Default Arguments

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*

Variadic Function Templates can have parameters with default values. These parameters are always set to their default value in case of IFTI.

size_t fun(T...)(T t, string file = __FILE__)
{
    import std.stdio;
    writeln(file, " ", t);
    return T.length;
}

assert(fun(1, "foo") == 2);  // uses IFTI
assert(fun!int(1, "filename") == 1);  // no IFTI

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.

Enum & Variable Templates

Like aggregates and functions, manifest constant and variable declarations can have template parameters, providing there is an Initializer:

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;
}

Nested Templates

If a template is declared in aggregate or function local scope, the instantiated functions will implicitly capture the context of the enclosing scope.

class C
{
    int num;

    this(int n) { num = n; }

    template Foo()
    {
        // 'foo' can access 'this' reference of class C object.
        void foo(int n) { this.num = n; }
    }
}

void main()
{
    auto c = new C(1);
    assert(c.num == 1);

    c.Foo!().foo(5);
    assert(c.num == 5);

    template Bar()
    {
        // 'bar' can access local variable of 'main' function.
        void bar(int n) { c.num = n; }
    }
    Bar!().bar(10);
    assert(c.num == 10);
}

Above, Foo!().foo will work just the same as a final member function of class C, and Bar!().bar will work just the same as a nested function within function main().

Aggregate Type Limitations

A nested template cannot add non-static fields to an aggregate type. Fields declared in a nested template will be implicitly static.

A nested template cannot add virtual functions to a class or interface. Methods inside a nested template will be implicitly final.

class Foo
{
    template TBar(T)
    {
        T xx;           // becomes a static field of Foo
        void func(T) {} // implicitly final
        //abstract void baz(); // error, final functions cannot be abstract

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

void main()
{
    alias bar = Foo.TBar!int;
    bar.xx++;
    //bar.func(1); // error, no this

    auto o = new Foo;
    o.TBar!int.func(1); // OK
}

Implicit Nesting

If a template has a template alias parameter, and is instantiated with a local symbol, the instantiated function will implicitly become nested in order to access runtime data of the given local symbol.

template Foo(alias sym)
{
    void foo() { sym = 10; }
}

class C
{
    int num;

    this(int n) { num = n; }

    void main()
    {
        assert(this.num == 1);

        alias fooX = Foo!(C.num).foo;

        // fooX will become member function implicitly, so &fooX
        //     returns a delegate object.
        static assert(is(typeof(&fooX) == delegate));

        fooX(); // called by using valid 'this' reference.
        assert(this.num == 10);  // OK
    }
}

void main()
{
    new C(1).main();

    int num;
    alias fooX = Foo!num.foo;

    // fooX will become nested function implicitly, so &fooX
    //     returns a delegate object.
    static assert(is(typeof(&fooX) == delegate));

    fooX();
    assert(num == 10);  // OK
}

Not only functions, but also instantiated class and struct types can become nested via implicitly captured context.

class C
{
    int num;
    this(int n) { num = n; }

    class N(T)
    {
        // instantiated class N!T can become nested in C
        T foo() { return num * 2; }
    }
}

void main()
{
    auto c = new C(10);
    auto n = c.new N!int();
    assert(n.foo() == 20);
}

See also: Nested Class Instantiation.

void main()
{
    int num = 10;
    struct S(T)
    {
        // instantiated struct S!T can become nested in main()
        T foo() { return num * 2; }
    }
    S!int s;
    assert(s.foo() == 20);
}

A templated struct can become a nested struct if it is instantiated with a local symbol passed as an aliased argument:

struct A(alias F)
{
    int fun(int i) { return F(i); }
}

A!F makeA(alias F)() { return A!F(); }

void main()
{
    int x = 40;
    int fun(int i) { return x + i; }
    A!fun a = makeA!fun();
    assert(a.fun(2) == 42);
}

Context Limitation

Currently nested templates can capture at most one context. As a typical example, non-static template member functions cannot take local symbol by using template alias parameter.

class C
{
    int num;
    void foo(alias sym)() { num = sym * 2; }
}

void main()
{
    auto c = new C();
    int var = 10;
    c.foo!var();    // NG, foo!var requires two contexts, 'this' and 'main()'
}

But, if one context is indirectly accessible from other context, it is allowed.

int sum(alias x, alias y)() { return x + y; }

void main()
{
    int a = 10;
    void nested()
    {
        int b = 20;
        assert(sum!(a, b)() == 30);
    }
    nested();
}

Two local variables a and b are in different contexts, but outer context is indirectly accessible from innter context, so nested template instance sum!(a, b) will capture only inner context.

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)
{
    static if (n == 1)
        enum factorial = 1;
    else
        enum factorial = n * factorial!(n - 1);
}

static assert(factorial!(4) == 24);

For more information and a CTFE (Compile-time Function Execution) factorial alternative, see: Template Recursion.

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

Template constraints can be used with aggregate types (structs, classes, unions). Constraints are effectively used with library module std.traits:

import std.traits;

struct Bar(T)
    if (isIntegral!T)
{
    ...
}
...
auto x = Bar!int;       // OK, int is an integral type
auto y = Bar!double;    // Error, double does not satisfy constraint
Operator Overloading
Template Mixins