c++ - Why does not a template template parameter allow 'typename' after the parameter list

Question

Template template typename?

When using template template syntax as in template <template <typename> class T>, it is required to use the keyword class, as using typename gives an error along the lines of:

error: template template parameter requires 'class' after the parameter list

Everywhere else the keywords typename and class are interchangeable in the basic case of declaring a template parameter.

You could argue that the requirement when using template template is a hint that you are expected to pass a class type, but this is not always the case (especially not after C++11 introduced templated type aliases).

template <template <typename> class T> // 'class' keyword required.
struct Foo {
    using type = T<int>;
};

template <typename T>
using type = T (*)();

using func_ptr_t = Foo<type>::type;

What is the reasoning behind this?

• Is there any specific reason as to why typename is not allowed in template template declarations?
• Does the C++ standard say anything about this?

Answer 1

Short answer: because the Standard says so.

Longer answer: prior to Standardization, C++ templates required the class keyword for all template parameters. However, to stress the fact that templates could also be of non-class (i.e. builtin) type, an alternative keyword typename was introduced. However, in C++98, template-template parameters could only be of class-type, and this was the reason that the typename keyword was not added in that context.

Enter C++11 and its new feature template aliases, that now also introduced non-class templates, and hence non-class template-template parameters:

template<typename T> struct A {};
template<typename T> using B = int;

template<template<typename> class X> struct C;
C<A> ca; // ok
C<B> cb; // ok, not a class template
template<template<typename> typename X> struct D; // error, cannot use typename here

The above example was taken from the current C++1z proposal N4051 titled Allow typename in a template template parameter, and proposes to allow precisely that.

Clang 3.5 SVN now supports this with the -std=c++1z flag.