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 TemplateParameter: TemplateTypeParameter TemplateValueParameter TemplateAliasParameter TemplateSequenceParameter TemplateThisParameter
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. Type parameters can take any type.
template t(T) // declare type parameter T { T v; // declare a member variable of type T within template t }
Value parameters can take any expression which can be statically evaluated at compile time. Alias parameters can take almost any symbol. Sequence parameters can take zero 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.
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.
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*
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 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)));
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.
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.
For example:
template TFoo(T) { void bar() { func(); } }module b
import a; void func() { } alias f = TFoo!(int); // error: func not defined in module aand:
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.
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:
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 bar = TBar!(char*); // (2) T is deduced to be char template TAbc(D, U : D[]) { } 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) { } 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
TemplateTypeParameter: Identifier Identifier TemplateTypeParameterSpecialization Identifier TemplateTypeParameterDefault Identifier TemplateTypeParameterSpecialization TemplateTypeParameterDefault TemplateTypeParameterSpecialization: : Type TemplateTypeParameterDefault: = Type
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. If the result is ambiguous, it is an error.
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(); }
const(S) S immutable(S)
This 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)); }
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 }
Any specialization or default expression provided must be evaluatable at compile-time.
This example of 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); }
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 user-defined type names, global names, local names, module names, template names, and template instance names.
Symbol examples:
class Foo { static int x; } template Bar(alias a) { alias sym = a.x; } void main() { alias bar = Bar!(Foo); bar.sym = 3; // sets Foo.x to 3 }
shared int x; template Foo(alias var) { auto ptr = &var; } void main() { alias bar = Foo!(x); *bar.ptr = 3; // set x to 3 static shared int y; alias abc = Foo!(y); *abc.ptr = 3; // set y to 3 }
template Foo(alias var) { void inc() { var++; } } void main() { int v = 4; alias foo = Foo!v; foo.inc(); assert(v == 5); }See also Implicit Template Nesting.
import std.conv; template Foo(alias a) { alias sym = a.text; } void main() { alias bar = Foo!(std.conv); bar.sym(3); // calls std.conv.text(3) }
shared int x; template Foo(alias var) { auto ptr = &var; } template Bar(alias Tem) { alias instance = Tem!(x); } void main() { alias bar = Bar!(Foo); *bar.instance.ptr = 3; // sets x to 3 }
shared int x; template Foo(alias var) { auto ptr = &var; } template Bar(alias sym) { alias p = sym.ptr; } void main() { alias foo = Foo!(x); alias bar = Bar!(foo); *bar.p = 3; // sets x to 3 }
Value examples:
template Foo(alias x, alias y) { static int i = x; static string s = y; } void main() { import std.stdio; alias foo = Foo!(3, "bar"); writeln(foo.i, foo.s); // prints 3bar }
template Foo(alias x) { static int i = x; } void main() { // compile-time argument evaluation enum two = 1 + 1; alias foo = Foo!(5 * two); assert(foo.i == 10); static assert(foo.stringof == "Foo!10"); // compile-time function evaluation int get10() { return 10; } alias bar = Foo!(get10()); // bar is the same template instance as foo assert(&bar.i is &foo.i); }
template Foo(alias fun) { enum val = fun(2); } alias foo = Foo!((int x) => x * x); static assert(foo.val == 4);
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
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
TemplateSequenceParameter: Identifier ...
If the last template parameter in the TemplateParameterList is declared as a TemplateSequenceParameter, it is a match with any trailing template arguments. Such a sequence of arguments can be defined using the template std.meta.AliasSeq and 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.
An AliasSeq whose elements consist entirely of types is called a type sequence or TypeSeq. An AliasSeq whose elements consist entirely of values is called a value sequence or ValueSeq.
An AliasSeq can be used as an argument list to instantiate another template, or as the list of parameters for a function.
import std.stdio; template print(args...) { void f() { writeln("args are ", args); // args is a ValueSeq } } template write(Args...) // Args is a TypeSeq { void f(Args args) // args is a ValueSeq { writeln("args are ", args); } } void main() { print!(1,'a',6.8).f(); // prints: args are 1a6.8 write!(int, char, double).f(1, 'a', 6.8); // prints: args are 1a6.8 }
The number of elements in an AliasSeq can be retrieved with the .length property. The nth element can be retrieved by indexing the AliasSeq with [n], and sub-sequences are denoted by the slicing syntax.
AliasSeq-s are static compile-time entities, there is no way to dynamically change, add, or remove elements either at compile-time or run-time.
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); }
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 }
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
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*
See also: Function Template Default Arguments.
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) }
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: Template This Parameters.
Analogously to class templates, struct, union and interfaces can be transformed into templates by supplying a template parameter list.
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 a !(TemplateArgumentList):
writefln("The square of %s is %s", 3, square!(int)(3));
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.
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
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 }
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)*
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));
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
ConstructorTemplate: this TemplateParameters Parameters MemberFunctionAttributesopt Constraintopt : this TemplateParameters Parameters MemberFunctionAttributesopt Constraintopt FunctionBody
Templates can be used to form constructors for classes and structs.
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; }
AliasDeclaration can also have optional template parameters:
alias Sequence(TL...) = TL;
It is lowered to:
template Sequence(TL...) { alias Sequence = TL; }
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)); }
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 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); }
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.
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.
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
Templates cannot be used to add non-static fields or virtual functions to classes or interfaces. For example:
class Foo { template TBar(T) { T xx; // becomes a static field of Foo int func(T) { ... } // non-virtual static T yy; // Ok static int func(T t, int y) { ... } // Ok } }