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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 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 same instance:

struct TFoo(int x) { }

// 3 and 2+1 are both 3 of type int
static assert(is(TFoo!(3) == TFoo!(2 + 1)));

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

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, module names, template names, and template instance names. Literals can also be used as arguments to alias parameters.

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

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(args...)
{
    void print()
    {
        writeln("args are ", args); // args is an ExpressionTuple
    }
}

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

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
}

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.

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

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

Template tuples 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 = TypeTuple 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;
    }

    auto plus_two = partial(&plus, 2);
    writefln("%d", plus_two(6, 8)); // prints 16
}
See also: std.functional.partial

Specialization

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 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

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*

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)*

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

    foreach (i, v; args)
    {
        if (v == 10)
            assert(__traits(isRef, args[i]));
        else
            assert(!__traits(isRef, args[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));
}

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

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 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 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);
}
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);
}

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 : 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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