13.2 Relaxed typename Rules

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13.2 Relaxed typename Rules

Some programmers and language designers find the rules for the use of typename (see Section 5.1 on page 43 and Section 9.3.2 on page 130) too strict. For example, in the following code, the occurrence of typename in typename Array<T>::ElementT is mandatory, but the one in typename Array<int>::ElementT is prohibited (an error):

 template <typename T>  class Array {    public:      typedef T ElementT;   };  template <typename T>  void clear (typename Array<T>::ElementT& p);  // OK  template<>  void clear (typename Array<int>::ElementT& p);  // ERROR  

Examples such as this can be surprising, and because it is not difficult for a C++ compiler implementation simply to ignore the extra keyword, the language designers are considering allowing the typename keyword in front of any qualified typename that is not already elaborated with one of the keywords struct , class , union , or enum . Such a decision would probably also clarify when the .template , ->template , and ::template constructs (see Section 9.3.3 on page 132) are permissible.

Ignoring extraneous uses of typename and template is relatively straightforward from an implementer's point of view. Interestingly, there are also situations when the language currently requires these keywords but when an implementation could do without them. For example, in the previous function template clear() , a compiler can know that the name Array<T>::ElementT cannot be anything but a type name (no expressions are allowed at that point), and therefore the use of typename could be made optional in that situation. The C++ standardization committee is therefore also examining changes that would reduce the number of situations when typename and template are required.

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