提供适当的 operator<< 和可变模板特化

Question

当为模板class的专业化提供operator<<时，clang喜欢内联朋友形式：

#include <iostream>

template <typename A, typename... Bs>
struct Hello;

template <typename A>
struct Hello<A> {
  template <typename A2>
  friend std::ostream & operator<<(std::ostream & s, const Hello<A2> & h) {
    return s << "specialized\n";
  }
};

template <typename A, typename... Bs>
struct Hello {
  template <typename A2, typename... B2s>
  friend std::ostream & operator<<(std::ostream & s, const Hello<A2,B2s...> & h) {
    return s << "generic\n";
  }
};


int main()
{
  std::cout << Hello<int>() 
            << Hello<float>()
            << Hello<int,int>()
            << Hello<int,float>();
}

http://coliru.stacked-crooked.com/a/47743db96c0f3a02

但是 gcc 失败了，它更喜欢非内联版本：

#include <iostream>

template <typename A, typename... Bs>
struct Hello;

template <typename A>
struct Hello<A> {
  template <typename A2>
  friend std::ostream & operator<<(std::ostream & s, const Hello<A2> & h);
};

template <typename A, typename... Bs>
struct Hello {
  template <typename A2, typename... B2s>
  friend std::ostream & operator<<(std::ostream & s, const Hello<A2,B2s...> & h);
};


template <typename A2>
std::ostream & operator<<(std::ostream & s, const Hello<A2> & h) {
  return s << "specialized\n";
}

template <typename A2, typename... B2s>
std::ostream & operator<<(std::ostream & s, const Hello<A2,B2s...> & h) {
  return s << "generic\n";
}


int main()
{
  std::cout << Hello<int>() 
            << Hello<float>() 
            << Hello<int,int>() 
            << Hello<int,float>();
}

http://coliru.stacked-crooked.com/a/45328f7bbdb36598

这反过来又不被 clang 接受。

所以我的问题是：

这两种形式都是标准的 C++ 吗？
是否有两种编译器都接受的形式？

Answer 1

你不需要模板朋友。

template <typename A, typename... Bs>
struct Hello {
  friend std::ostream & operator<<(std::ostream & s, const Hello<A,Bs...> & h) {
    return s << "generic\n";
  }
};

template <typename A>
struct Hello<A> {
  friend std::ostream & operator<<(std::ostream & s, const Hello<A> & h) {
    return s << "specialized\n";
  }
};

http://coliru.stacked-crooked.com/a/22c439bc920f948c

提供适当的 operator<< 和可变模板特化

Provide a proper operator<< with variadic template specialization

c++

templates

operators

friend