How to properly use concepts?

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Currently, I am learning C++ and decided just start with C++20. However, these codes are driving me crazy, since I don't think the result makes any sense.

The following code will have the sentence Valid array. printed. What I meant above is that this is not right. It shouldn't print the sentence at all, since the type I inserted in the parameter doesn't match the concept.

Tested on VS2022 Preview 3 and an online compiler with newest GCC and C++2A(GNU) arguments, generated the same results.

#include <array>
#include <cstdio>
#include <iostream>
#include <type_traits>

using namespace std;

template<typename A> concept ProperArray = requires(A array)
{
    {array.max_size() >= 2U};
    {std::is_arithmetic_v<typename A::value_type> == true};
};

int main()
{
    constexpr bool b = ProperArray<array<std::string, 1>>;
    if constexpr (b)
        cout << "Valid array." << endl;
        
    std::cout << "Hello, Wandbox!" << std::endl;
}
if-constexpr c++-concepts c++20 c++
1 Answers

So, two things.

  1. You are using simple requirements (the extra {} make those compound requirements technically, but since you don't use any optional feature of those, it's equivalent to a simple requirement). Those mostly verify that an expression is syntactically valid. Its value is immaterial to them. What you want is nested requirements:

    template<typename A> concept ProperArray = requires(A array)
{
    requires array.max_size() >= 2U;
    requires std::is_arithmetic<typename A::value_type>::value;
};

    These requirements (whose expression must be a constant expression), check the value indeed. Mind however, that if max_size is not a constexpr or consteval member function, or if the evaluation would not produce a constant expression, then it's a hard error. The concept won't simply be "false".

  2. if constexpr outside of a template is moot. It doesn't do the same discarding magic it does when its condition is value-dependent on a template parameter. You could have used a regular if to the same effect.

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