This is an unofficial snapshot of the ISO/IEC JTC1 SC22 WG21 Core Issues List revision 116a. See http://www.open-std.org/jtc1/sc22/wg21/ for the official list.

2024-12-19

1874. Type vs non-type template parameters with class keyword

[Moved to DR at the November, 2014 meeting.]

The Standard is not clear enough that a template parameter like class T is to be interpreted as a type parameter and not an ill-formed non-type parameter of class type T.

Proposed resolution (October, 2014):

1. Change 13.2 [temp.param] paragraph 2 as follows, moving the example from paragraph 3 to paragraph 2:

There is no semantic difference between class and typename in a template-parameter. typename followed by an unqualified-id names a template type parameter. typename followed by a qualified-id denotes the type in a non-type¹³⁷ 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. [Note: A template parameter may be a class template. For example... —end note]

2. Change 13.2 [temp.param] paragraph 3 as follows, moving the example from paragraph 2 to paragraph 3:

A type-parameter whose identifier does not follow an ellipsis defines its identifier to be a typedef-name (if declared with class or typename) or template-name (if declared with template) in the scope of the template declaration. [Note: Because of the name lookup rules, a template-parameter that could be interpreted as either a non-type template-parameter or a type-parameter (because its identifier is the name of an already existing class) is taken as a type-parameter. For example... —end note] [Note: A template parameter may be a class 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]