C++ 20 Concepts: Require operator overloading

Question

I am trying to understand how to declare a concept that requires a particular operator is overloaded for a given type. Lets say I have the following function that takes a vector of an arbitrary type and prints it to 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 << '}';
}

This code will work just fine if the type printable has an overloaded << operator, but if it doesn't, then it fails with a very unhelpful compiler error. I feel like I should be able to somehow declare a concept that requires a type has a valid << operator defined, and use that concept in the function declaration, so that I can get a more useful compiler error, but I haven't been able to figure out how to do it.

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 << '}';
}

If you wish you could also make it more generic to operate on basic_ostream.

---

Here is the clang error message:

<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;
       ^

and 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