6 Basics [basic]

6.4 Scope [basic.scope]

6.4.9 Template parameter scope [basic.scope.temp]

The declarative region of the name of a template parameter of a template template-parameter is the smallest template-parameter-list in which the name was introduced.

The declarative region of the name of a template parameter of a template is the smallest template-declaration in which the name was introduced.

Only template parameter names belong to this declarative region; any other kind of name introduced by the declaration of a template-declaration is instead introduced into the same declarative region where it would be introduced as a result of a non-template declaration of the same name.

[Example 1: namespace N { template<class T> struct A { }; // #1 template<class U> void f(U) { } // #2 struct B { template<class V> friend int g(struct C*); // #3 }; }

The declarative regions of T, U and V are the template-declarations on lines #1, #2, and #3, respectively.

But the names A, f, g and C all belong to the same declarative region — namely, the namespace-body of N.

(g is still considered to belong to this declarative region in spite of its being hidden during qualified and unqualified name lookup.)

— end example]

The potential scope of a template parameter name begins at its point of declaration and ends at the end of its declarative region.

[Note 1:

This implies that a template-parameter can be used in the declaration of subsequent template-parameters and their default arguments but cannot be used in preceding template-parameters or their default arguments.

For example, template<class T, T* p, class U = T> class X { /* ... */ }; template<class T> void f(T* p = new T);

This also implies that a template-parameter can be used in the specification of base classes.

For example, template<class T> class X : public Array<T> { /* ... */ }; template<class T> class Y : public T { /* ... */ };

The use of a template parameter as a base class implies that a class used as a template argument must be defined and not just declared when the class template is instantiated.

— end note]

The declarative region of the name of a template parameter is nested within the immediately-enclosing declarative region.

[Note 2:

As a result, a template-parameter hides any entity with the same name in an enclosing scope ([basic.scope.hiding]).

[Example 2: typedef int N; template<N X, typename N, template<N Y> class T> struct A;

Here, X is a non-type template parameter of type int and Y is a non-type template parameter of the same type as the second template parameter of A.

— end example]

— end note]

[Note 3:

Because the name of a template parameter cannot be redeclared within its potential scope ([temp.local]), a template parameter's scope is often its potential scope.

However, it is still possible for a template parameter name to be hidden; see [temp.local].

— end note]