17 Templates [temp]

17.1 Template parameters [temp.param]

The syntax for template-parameters is:

template-parameter:
	type-parameter
	parameter-declaration
type-parameter:
	type-parameter-key ... identifier
	type-parameter-key identifier = type-id
	template < template-parameter-list > type-parameter-key ... identifier
	template < template-parameter-list > type-parameter-key identifier = id-expression
type-parameter-key:
	class
	typename

[Note: The > token following the template-parameter-list of a type-parameter may be the product of replacing a >> token by two consecutive > tokens ([temp.names]).end note]

There is no semantic difference between class and typename in a type-parameter-key. typename followed by an unqualified-id names a template type parameter. typename followed by a qualified-id denotes the type in a non-type135 parameter-declaration. A template-parameter of the form class identifier is a type-parameter. [Example:

class T { /* ... */ };
int i;

template<class T, T i> void f(T t) {
  T t1 = i;         // template-parameters T and i
  ::T t2 = ::i;     // global namespace members T and i
}

Here, the template f has a type-parameter called T, rather than an unnamed non-type template-parameter of class T. end example] A storage class shall not be specified in a template-parameter declaration. Types shall not be defined in a template-parameter declaration.

A type-parameter whose identifier does not follow an ellipsis defines its identifier to be a typedef-name (if declared without template) or template-name (if declared with template) in the scope of the template declaration. [Note: A template argument may be a class template or alias template. For example,

template<class T> class myarray { /* ... */ };

template<class K, class V, template<class T> class C = myarray>
class Map {
  C<K> key;
  C<V> value;
};

end note]

A non-type template-parameter shall have one of the following (optionally cv-qualified) types:

[Note: Other types are disallowed either explicitly below or implicitly by the rules governing the form of template-arguments ([temp.arg]). end note] The top-level cv-qualifiers on the template-parameter are ignored when determining its type.

A non-type non-reference template-parameter is a prvalue. It shall not be assigned to or in any other way have its value changed. A non-type non-reference template-parameter cannot have its address taken. When a non-type non-reference template-parameter is used as an initializer for a reference, a temporary is always used. [Example:

template<const X& x, int i> void f() {
  i++;                          // error: change of template-parameter value

  &x;                           // OK
  &i;                           // error: address of non-reference template-parameter

  int& ri = i;                  // error: non-const reference bound to temporary
  const int& cri = i;           // OK: const reference bound to temporary
}

end example]

A non-type template-parameter shall not be declared to have floating-point, class, or void type. [Example:

template<double d> class X;     // error
template<double* pd> class Y;   // OK
template<double& rd> class Z;   // OK

end example]

A non-type template-parameter of type “array of T” or of function type T is adjusted to be of type “pointer to T”. [Example:

template<int* a>   struct R { /* ... */ };
template<int b[5]> struct S { /* ... */ };
int p;
R<&p> w;                        // OK
S<&p> x;                        // OK due to parameter adjustment
int v[5];
R<v> y;                         // OK due to implicit argument conversion
S<v> z;                         // OK due to both adjustment and conversion

end example]

A default template-argument is a template-argument ([temp.arg]) specified after = in a template-parameter. A default template-argument may be specified for any kind of template-parameter (type, non-type, template) that is not a template parameter pack. A default template-argument may be specified in a template declaration. A default template-argument shall not be specified in the template-parameter-lists of the definition of a member of a class template that appears outside of the member's class. A default template-argument shall not be specified in a friend class template declaration. If a friend function template declaration specifies a default template-argument, that declaration shall be a definition and shall be the only declaration of the function template in the translation unit.

The set of default template-arguments available for use is obtained by merging the default arguments from all prior declarations of the template in the same way default function arguments are ([dcl.fct.default]). [Example:

template<class T1, class T2 = int> class A;
template<class T1 = int, class T2> class A;

is equivalent to

template<class T1 = int, class T2 = int> class A;

end example]

If a template-parameter of a class template, variable template, or alias template has a default template-argument, each subsequent template-parameter shall either have a default template-argument supplied or be a template parameter pack. If a template-parameter of a primary class template, primary variable template, or alias template is a template parameter pack, it shall be the last template-parameter. A template parameter pack of a function template shall not be followed by another template parameter unless that template parameter can be deduced from the parameter-type-list ([dcl.fct]) of the function template or has a default argument ([temp.deduct]). A template parameter of a deduction guide template ([temp.deduct.guide]) that does not have a default argument shall be deducible from the parameter-type-list of the deduction guide template. [Example:

template<class T1 = int, class T2> class B;     // error

// U can be neither deduced from the parameter-type-list nor specified
template<class... T, class... U> void f() { }   // error
template<class... T, class U> void g() { }      // error

end example]

A template-parameter shall not be given default arguments by two different declarations in the same scope. [Example:

template<class T = int> class X;
template<class T = int> class X { /* ... */ };  // error

end example]

When parsing a default template-argument for a non-type template-parameter, the first non-nested > is taken as the end of the template-parameter-list rather than a greater-than operator. [Example:

template<int i = 3 > 4 >        // syntax error
class X { /* ... */ };

template<int i = (3 > 4) >      // OK
class Y { /* ... */ };

end example]

A template-parameter of a template template-parameter is permitted to have a default template-argument. When such default arguments are specified, they apply to the template template-parameter in the scope of the template template-parameter. [Example:

template <class T = float> struct B {};
template <template <class TT = float> class T> struct A {
  inline void f();
  inline void g();
};
template <template <class TT> class T> void A<T>::f() {
  T<> t;            // error - TT has no default template argument
}
template <template <class TT = char> class T> void A<T>::g() {
    T<> t;          // OK - T<char>
}

end example]

If a template-parameter is a type-parameter with an ellipsis prior to its optional identifier or is a parameter-declaration that declares a parameter pack ([dcl.fct]), then the template-parameter is a template parameter pack. A template parameter pack that is a parameter-declaration whose type contains one or more unexpanded parameter packs is a pack expansion. Similarly, a template parameter pack that is a type-parameter with a template-parameter-list containing one or more unexpanded parameter packs is a pack expansion. A template parameter pack that is a pack expansion shall not expand a parameter pack declared in the same template-parameter-list. [Example:

template <class... Types> class Tuple;                // Types is a template type parameter pack
                                                      // but not a pack expansion
template <class T, int... Dims> struct multi_array;   // Dims is a non-type template parameter pack
                                                      // but not a pack expansion
template<class... T> struct value_holder {
  template<T... Values> struct apply { };             // Values is a non-type template parameter pack
                                                      // and a pack expansion
};
template<class... T, T... Values> struct static_array;// error: Values expands template type parameter
                                                      // pack T within the same template parameter list

end example]

Since template template-parameters and template template-arguments are treated as types for descriptive purposes, the terms non-type parameter and non-type argument are used to refer to non-type, non-template parameters and arguments.