17 Templates [temp]

17.1 Template parameters [temp.param]

1
#
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
 ]
2
#
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.
[ 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.
3
#
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
 ]
4
#
A non-type template-parameter shall have one of the following (optionally cv-qualified) types:
  • (4.1)
    integral or enumeration type,
  • (4.2)
    pointer to object or pointer to function,
  • (4.3)
    lvalue reference to object or lvalue reference to function,
  • (4.4)
    pointer to member,
  • (4.5)
    std​::​nullptr_­t, or
  • (4.6)
    a type that contains a placeholder type ([dcl.spec.auto]).
5
#
[ 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.
6
#
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
 ]
7
#
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
 ]
8
#
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
 ]
9
#
A default template-argument may be specified for any kind of template-parameter (type, non-type, template) that is not a template parameter pack ([temp.variadic]).
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.
10
#
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
 ]
11
#
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
 ]
12
#
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
 ]
13
#
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
 ]
14
#
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
 ]
15
#
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 ([temp.variadic]).
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
 ]
135)
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.