"No instance of overloaded function" error with static friend template function

Question

I have a class A that is trying to call a nonmember function DoTheThing. DoTheThing is a friend of class A so that it can call a private member function TheThing of A. DoTheThing is a template function so that it can call TheThing in multiple user-defined classes. Because the error references an overloaded function, I believe that I am redefining DoTheThing within A, but I cannot figure out how to fix this error.

#include <iostream>
#include <vector>

template<typename Component>
    requires requires (std::vector<double>& vec, int i) {Component::TheThing(vec, i); }
    static void DoTheThing(std::vector<double>& vec, int i) {
        Component::TheThing(vec, i);
    }


class A {
    template<class Component>
    friend void DoTheThing(std::vector<double>& vec, int i);
public:
    A() {
        vec_.resize(10, 5);
        DoTheThing<A>(vec_, 7); // Error: no instance of overloaded function
    }
private:
    static void TheThing(std::vector<double>& vec, int i) {
        vec[i] = vec[i] * i;
    }


    std::vector<double> vec_;
};

Am I redefining DoTheThing? How do I make the non-member DoTheThing a friend of A? How do I call DoTheThing in the constructor of A?

Answer 1

You are not constraining the friend declaration with an ad-hoc requires clause, so you are actually not granting friendship to the same DoTheThing function you want. You need to replicate the requires clause in the friend declaration as well:

class A {
    template<class Component>
    requires requires (std::vector<double>& vec, int i) {Component::TheThing(vec, i); }
    friend void DoTheThing(std::vector<double>& vec, int i);
// ...
};

Here's a demo.

---

However, you should name this concept, and the usage will be simpler:

template<typename Component>
concept CanDoThing = requires (std::vector<double>& vec, int i) { 
  Component::TheThing(vec, i); 
};

template<CanDoThing Component>
static void DoTheThing(std::vector<double>& vec, int i) {
  Component::TheThing(vec, i);
}

class A {
  template<CanDoThing Component>
  friend void DoTheThing(std::vector<double>& vec, int i);
// ...
};