如何在 C++ 中的类体之外定义一个专门的类方法？

Question

I have a template class A<T> and its specialization for integral arguments. And both the class and its specialization declare the method foo() , which I would like to define outside of class bodies:

#include <concepts>

template<class T> 
struct A { static void foo(); };

template<std::integral T>
struct A<T> { static void foo(); };

template<class T>
void A<T>::foo() {}

template<std::integral T>
void A<T>::foo() {}
 
int main() { A<int>::foo(); }

GCC accepts this code.

Clang prints the error https://gcc.godbolt.org/z/hYfYGPfMh :

error: type constraint differs in template redeclaration
template<std::integral T>

And MSVC prints errors on both method definitions:

error C3855: 'A<T>': template parameter 'T' is incompatible with the declaration
error C2447: '{': missing function header (old-style formal list?)
error C2065: 'foo': undeclared identifier

Please suggest how to define methods outside class bodies and make all the compilers happy?

Answer 1

I am not a C++ template expert, I tried something like below

template<class T, bool = std::is_integral_v<T>>
struct A
{};

template<class T>
struct A<T, false>
{ 
   static void foo(); 
};

template<class T>
struct A<T, true> 
{ 
   static void foo(); 
};

template<class T>
void A<T,false>::foo() 
{
  std::cout << "I am working on a non-integral type" << std::endl;
}

template<class T>
void A<T, true>::foo() 
{
  std::cout << "I am working on an integral type" << std::endl;
}

int main()
{
  A<int>::foo();
  A<float> ::foo();

  return 0;
}

and the code gave me the results on MS C++ compiler

I am working on an integral type
I am working on a non-integral type