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