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 TemplateSequenceParameter 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 sequences. 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. Sequences can contain 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.
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 the 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 to exactly the same instance as 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
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:
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:
- If there is no type specialization for the parameter, the type of the parameter is set to the template argument.
- 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.
- 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.
- 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. Determining which is more specialized is done in 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 almost any kind of D symbol, including user-defined type names, global names, local names, module names, template names, and template instance names. Literals can also be used as arguments to alias parameters.
Examples:
- User-defined type names
class Foo { static int p; } template Bar(alias T) { alias q = T.p; } void main() { alias bar = Bar!(Foo); bar.q = 3; // sets Foo.p to 3 }
- Global names
shared int x; template Foo(alias X) { auto p = &X; } void main() { alias bar = Foo!(x); *bar.p = 3; // set x to 3 static shared int y; alias abc = Foo!(y); *abc.p = 3; // set y to 3 }
- Module names
import std.string; template Foo(alias X) { alias y = X.toString; } void main() { alias bar = Foo!(std.string); bar.y(3); // calls std.string.toString(3) }
- Template names
shared int x; template Foo(alias X) { auto p = &X; } template Bar(alias T) { alias abc = T!(x); } void main() { alias bar = Bar!(Foo); *bar.abc.p = 3; // sets x to 3 }
- Template instance names
shared int x; template Foo(alias X) { auto p = &X; } template Bar(alias T) { alias q = T.p; } void main() { 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 main() { alias foo = Foo!(3, "bar"); writeln(foo.i, foo.s); // prints 3bar }
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 Sequence Parameters
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.
template print(args...) { void print() { writeln("args are ", args); // args is a ValueSeq } } template write(Args...) { void write(Args args) // Args is a TypeSeq // args is a ValueSeq { 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 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:
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; } 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 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
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*
Eponymous Templates
If a template contains members whose name is the same as the template identifier and if the type or the parameters type of these members include at least all the template parameters 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 }Using functions and more types than the template:
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) }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 in 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 }
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
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
Limitations
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 } }