Question 37356 in Ubuntu Question posted in
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Concept defining method and concrete class defining static method – Ubuntu

Continuing my studies on concepts, in the following example:

template <typename t>
concept c = requires(t p_t) {
  { p_t.f() } -> std::same_as<int>;
};

template <c t_c> int m1(t_c p_c) { return p_c.f(); }

template <c t_c> int m2() { return t_c::f(); }

struct a {
  int f() { return 3; }
};

struct b {
  static int f() { return 4; }
};

int main() {
   std::cout << m1(a{}) << std::endl;
   std::cout << m2<b>() << std::endl;
}

Shouldn’t std::cout << m2<b>() << std::endl; to cause a compiler error, since b does not satisfy c?

I am using gcc (Ubuntu 11.3.0-1ubuntu1~22.04) 11.3.0, and there was no compile or runtime error.

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2

Answers


  1. Chosen as BEST ANSWER
    0 
    template <typename t>
concept c = requires(t p_t) {
  { t::f() } -> std::same_as<int>;
};

template <c t_c> int m1(t_c p_c) { return p_c.f(); }

template <c t_c> int m2() { return t_c::f(); }

struct a {
  int f() { return 3; }
};

struct b {
  static int f() { return 4; }
};

int main() {
  std::cout << m1(a{}) << std::endl;
  std::cout << m2<b>() << std::endl;
}

    This would be the correct concept, which cause the 'a' does not satisfy 'c' Patrick Roberts


  2. 0

    Concepts are not base classes. You do not (or at least, should not) define concepts on the basis of a function having a specific signature. Concepts verify the validity of expressions (or constant expression-y things).

    And if f is a static member function of type b, and you have an instance of b called t, it is 100% legal C++ syntax to invoke b::f via the expression t.f().

    Therefore, the concept is satisfied.

    Also, your template function m2 is a liar. It claims that it will use a type in accord with the concept c, but then it does something that isn’t part of c.

    C++20 concepts cannot tell if a function constrained by a concept is lying. All it can do is check if the expressions are available.

    And if the user wants to satisfy the concept via a static member function, you shouldn’t care. As long as it does what it’s supposed to do, let the user have the freedom to fulfill the condition as they see fit.

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