c++ - 用户定义的文字作为可变参数模板

Question

GCC 4.7.2 似乎具有仅针对数字实现的可变字符模板文字运算符:

template<char... chars>
constexpr size_t operator "" _size() { return sizeof...(chars); }

int main()
{
    std::cout << 42_size;    // (1) works
    std::cout << "foo"_size; // (2) does not
}

• 是否有支持此代码的 GCC 版本？
• (2) 是标准的一部分吗？

Answer 1

C++11标准的2.14.8.5声明

If L is a user-defined-string-literal, let str be the literal without its ud-suffix and let len be the number of code units in str (i.e., its length excluding the terminating null character). The literal L is treated as a call of the form operator "" X (str, len)

因此将您的代码重写为:

#include <iostream>

// (1)
template<char... chars>
constexpr size_t operator "" _size() { return sizeof...(chars); }

// (2)
constexpr size_t operator "" _size( const char* str, size_t sz ) { return sz; }

int
main(void)
{
  std::cout << 42_size << std::endl;    // (1)
  std::cout << "foo"_size << std::endl; // (2)

  return 0;
}

明确指定 (2) 的正确形式