13 Templates [temp]

13.10 Function template specializations [temp.fct.spec]

13.10.2 Explicit template argument specification [temp.arg.explicit]

Template arguments can be specified when referring to a function template specialization that is not a specialization of a constructor template by qualifying the function template name with the list of template-arguments in the same way as template-arguments are specified in uses of a class template specialization.

[Example 1:

template<class T> void sort(Array<T>& v); void f(Array<dcomplex>& cv, Array<int>& ci) { sort<dcomplex>(cv); // sort(Array<dcomplex>&) sort<int>(ci); // sort(Array<int>&) } and template<class U, class V> U convert(V v); void g(double d) { int i = convert<int,double>(d); // int convert(double) char c = convert<char,double>(d); // char convert(double) }

— end example]

Template arguments shall not be specified when referring to a specialization of a constructor template ([class.ctor], [class.qual]).

A template argument list may be specified when referring to a specialization of a function template

(3.1)
when a function is called,
(3.2)
when the address of a function is taken, when a function initializes a reference to function, or when a pointer to member function is formed,
(3.3)
in an explicit specialization,
(3.4)
in an explicit instantiation, or
(3.5)
in a friend declaration.

Trailing template arguments that can be deduced or obtained from default template-arguments may be omitted from the list of explicit template-arguments.

A trailing template parameter pack ([temp.variadic]) not otherwise deduced will be deduced as an empty sequence of template arguments.

If all of the template arguments can be deduced, they may all be omitted; in this case, the empty template argument list <> itself may also be omitted.

In contexts where deduction is done and fails, or in contexts where deduction is not done, if a template argument list is specified and it, along with any default template arguments, identifies a single function template specialization, then the template-id is an lvalue for the function template specialization.

[Example 2: template<class X, class Y> X f(Y); template<class X, class Y, class ... Z> X g(Y); void h() { int i = f<int>(5.6); // Y deduced as double int j = f(5.6); // error: X cannot be deduced f<void>(f<int, bool>); // Y for outer f deduced as int (*)(bool) f<void>(f<int>); // error: f<int> does not denote a single function template specialization int k = g<int>(5.6); // Y deduced as double; Z deduced as an empty sequence f<void>(g<int, bool>); // Y for outer f deduced as int (*)(bool), // Z deduced as an empty sequence } — end example]

[Note 1:

An empty template argument list can be used to indicate that a given use refers to a specialization of a function template even when a non-template function ([dcl.fct]) is visible that would otherwise be used.

For example: template <class T> int f(T); // #1 int f(int); // #2 int k = f(1); // uses #2 int l = f<>(1); // uses #1

— end note]

Template arguments that are present shall be specified in the declaration order of their corresponding template-parameters.

The template argument list shall not specify more template-arguments than there are corresponding template-parameters unless one of the template-parameters is a template parameter pack.

[Example 3: template<class X, class Y, class Z> X f(Y,Z); template<class ... Args> void f2(); void g() { f<int,const char*,double>("aa",3.0); f<int,const char*>("aa",3.0); // Z deduced as double f<int>("aa",3.0); // Y deduced as const char*; Z deduced as double f("aa",3.0); // error: X cannot be deduced f2<char, short, int, long>(); // OK } — end example]

Implicit conversions ([conv]) will be performed on a function argument to convert it to the type of the corresponding function parameter if the parameter type contains no template-parameters that participate in template argument deduction.

[Note 2:

Template parameters do not participate in template argument deduction if they are explicitly specified.

For example,

template<class T> void f(T); class Complex { Complex(double); }; void g() { f<Complex>(1); // OK, means f<Complex>(Complex(1)) } — end note]

[Note 3:

Because the explicit template argument list follows the function template name, and because constructor templates ([class.ctor]) are named without using a function name ([class.qual]), there is no way to provide an explicit template argument list for these function templates.

— end note]

Template argument deduction can extend the sequence of template arguments corresponding to a template parameter pack, even when the sequence contains explicitly specified template arguments.

[Example 4: template<class ... Types> void f(Types ... values); void g() { f<int*, float*>(0, 0, 0); // Types deduced as the sequence int*, float*, int } — end example]