Question

Please see the code:

#include <iostream>
#include <typeinfo>

template<int N>
struct C
{
  static constexpr int n = N;
  using this_type_1 = C<n>;
  using this_type_2 = C<N>;
  static this_type_1* p_1;
  static this_type_2* p_2;
};

template<int N>
//C<N>* C<N>::p_1; // <--- error pattern
typename C<N>::this_type_1* C<N>::p_1; // <--- ok pattern

template<int N>
C<N>* C<N>::p_2; // ok

int main(){
  std::cerr
    << typeid(C<0>).name() << "\n"
    << typeid(C<0>::this_type_1).name() << "\n"
    << typeid(C<0>::this_type_2).name() << "\n"
  ;
}

It can compile with g++-4.7.1 and clang++-3.1. But it cannot compile with the commented-out error pattern.

g++ error message is:

test.cpp:15:13: error: conflicting declaration ‘C<N>* C<N>::p_1’
test.cpp:10:23: error: ‘C<N>::p_1’ has a previous declaration as ‘C<N>::this_type_1* C<N>::p_1’
test.cpp:15:13: error: declaration of ‘C<N>::this_type_1* C<N>::p_1’ outside of class is not definition [-fpermissive]

clang++ error message is:

test.cpp:15:13: error: redefinition of 'p_1' with a different type
C<N>* C<N>::p_1; // error
            ^
test.cpp:10:23: note: previous definition is here
  static this_type_1* p_1;
                      ^
1 error generated.

Luckily, I found out a working pattern. But I don’t know why The error pattern cannot be compiled. Please tell me the reason on the basis of the C++ language specification.

Answer 1

The two possible definitions for C<N>::p_1 are not as equivalent as they appear, because C<N>::n may be explicitly specialized at any time before the first instantiation for a given N.

template<int N>
struct C
{
  static constexpr int n = N;
  using this_type_1 = C<n>;
  static this_type_1* p_1;
};

template<int N>
C<N>* C<N>::p_1; // ERROR

template<>
constexpr int C<5>::n = 6;

int main()
{
    C<6>* p = C<5>::p_1;
}

If the compiler had accepted that definition of C<N>::p_1, it would be possible for its declared type to be incorrect.