C++ 20 概念：需要运算符重载

Question

我试图理解如何声明一个需要特定运算符重载的概念。假设我有以下函数，它采用任意类型的向量并将其打印到std::cout：

template<typename printable>
void print_vector(const std::vector<printable>& vec) 
{
  std::cout << '{';
  for (const printable &item : vec) {
    std::cout << item << ',';
  }
  std::cout << '}';
}

如果该类型printable具有重载<<运算符，则此代码将正常工作，但如果没有，则它会因非常无用的编译器错误而失败。我觉得我应该能够以某种方式声明一个需要<<定义了有效运算符的类型的概念，并在函数声明中使用该概念，以便我可以获得更有用的编译器错误，但我一直无法弄清楚如何去做。

Answer 1

template <class T>
concept Printable = requires(std::ostream& os, T a)
{
    os << a;
};


template<Printable T>
void print_vector(const std::vector<T>& vec) {
  std::cout << '{';
  for (const auto &item : vec) {
    std::cout << item << ',';
  }
  std::cout << '}';
}

如果您愿意，还可以使其更通用地操作basic_ostream.

这是铛错误消息：

<source>:30:5: error: no matching function for call to 'print_vector'
    print_vector(x);
    ^~~~~~~~~~~~
<source>:19:6: note: candidate template ignored: constraints not satisfied [with T = X]
void print_vector(std::vector<T> vec) {
     ^
<source>:18:10: note: because 'X' does not satisfy 'Printable'
template<Printable T>
         ^
<source>:10:9: note: because 'os << a' would be invalid: invalid operands to binary expression ('std::ostream' (aka 'basic_ostream<char>') and 'X')
    os << a;
       ^

和 gcc：

<source>: In function 'auto test()':
<source>:30:19: error: use of function 'void print_vector(std::vector<T>) [with T = X]' with unsatisfied constraints
   30 |     print_vector(x);
      |                   ^
<source>:19:6: note: declared here
   19 | void print_vector(std::vector<T> vec) {
      |      ^~~~~~~~~~~~
<source>:19:6: note: constraints not satisfied
<source>: In instantiation of 'void print_vector(std::vector<T>) [with T = X]':
<source>:30:19:   required from here  
<source>:8:9:   required for the satisfaction of 'Printable<T>' [with T = X]
<source>:8:21:   in requirements with 'std::ostream& os', 'T a' [with T = X]    
<source>:10:9: note: the required expression '(os << a)' is invalid    
   10 |     os << a;
      |     ~~~^~~~
cc1plus: note: set '-fconcepts-diagnostics-depth=' to at least 2 for more detail