17 Templates [temp]

17.2 Names of template specializations [temp.names]

A template specialization can be referred to by a template-id:

simple-template-id:
	template-name < template-argument-list >
template-id:
	simple-template-id
	operator-function-id < template-argument-list >
	literal-operator-id < template-argument-list >
template-name:
	identifier
template-argument-list:
	template-argument ...
	template-argument-list , template-argument ...
template-argument:
	constant-expression
	type-id
	id-expression

[Note: The name lookup rules are used to associate the use of a name with a template declaration; that is, to identify a name as a template-name. end note]

For a template-name to be explicitly qualified by the template arguments, the name must be known to refer to a template.

After name lookup finds that a name is a template-name or that an operator-function-id or a literal-operator-id refers to a set of overloaded functions any member of which is a function template, if this is followed by a <, the < is always taken as the delimiter of a template-argument-list and never as the less-than operator. When parsing a template-argument-list, the first non-nested >136 is taken as the ending delimiter rather than a greater-than operator. Similarly, the first non-nested >> is treated as two consecutive but distinct > tokens, the first of which is taken as the end of the template-argument-list and completes the template-id. [Note: The second > token produced by this replacement rule may terminate an enclosing template-id construct or it may be part of a different construct (e.g. a cast).end note] [Example:

template<int i> class X { /* ... */ };

X< 1>2 > x1;                            // syntax error
X<(1>2)> x2;                            // OK

template<class T> class Y { /* ... */ };
Y<X<1>> x3;                             // OK, same as Y<X<1> > x3;
Y<X<6>>1>> x4;                          // syntax error
Y<X<(6>>1)>> x5;                        // OK

end example]

The keyword template is said to appear at the top level in a qualified-id if it appears outside of a template-argument-list or decltype-specifier. In a qualified-id of a declarator-id or in a qualified-id formed by a class-head-name or enum-head-name, the keyword template shall not appear at the top level. In a qualified-id used as the name in a typename-specifier, elaborated-type-specifier, using-declaration, or class-or-decltype, an optional keyword template appearing at the top level is ignored. In these contexts, a < token is always assumed to introduce a template-argument-list. In all other contexts, when naming a template specialization of a member of an unknown specialization ([temp.dep.type]), the member template name shall be prefixed by the keyword template. [Example:

struct X {
  template<std::size_t> X* alloc();
  template<std::size_t> static X* adjust();
};
template<class T> void f(T* p) {
  T* p1 = p->alloc<200>();              // ill-formed: < means less than
  T* p2 = p->template alloc<200>();     // OK: < starts template argument list
  T::adjust<100>();                     // ill-formed: < means less than
  T::template adjust<100>();            // OK: < starts template argument list
}

end example]

A name prefixed by the keyword template shall be a template-id or the name shall refer to a class template or an alias template. [Note: The keyword template may not be applied to non-template members of class templates. end note] [Note: As is the case with the typename prefix, the template prefix is allowed in cases where it is not strictly necessary; i.e., when the nested-name-specifier or the expression on the left of the -> or . is not dependent on a template-parameter, or the use does not appear in the scope of a template. end note] [Example:

template <class T> struct A {
  void f(int);
  template <class U> void f(U);
};

template <class T> void f(T t) {
  A<T> a;
  a.template f<>(t);                    // OK: calls template
  a.template f(t);                      // error: not a template-id
}

template <class T> struct B {
  template <class T2> struct C { };
};

// OK: T​::​template C names a class template:
template <class T, template <class X> class TT = T::template C> struct D { };
D<B<int> > db;

end example]

A simple-template-id that names a class template specialization is a class-name.

A template-id that names an alias template specialization is a type-name.

A > that encloses the type-id of a dynamic_­cast, static_­cast, reinterpret_­cast or const_­cast, or which encloses the template-arguments of a subsequent template-id, is considered nested for the purpose of this description.