How to define concept of a function that returns bool

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I want to define a concept of a function that takes in a single argument and returns bool. Here is what I came up with:

template <typename T>
concept ConditionFunc = requires(T t) {
    { ConditionFunc(t) } -> std::same_as<bool>;
};

I want to use it like this

#include <concepts>
#include <vector>

bool IsEven(int n)
{
    return n % 2 == 0;
}

template <typename T>
void Foo(std::vector<T>& v, ConditionFunc auto func)
{
    // stuff
}

int main()
{
    std::vector<int> vec = {1, 2, 3, 4, 5};
    Foo(v, IsEven);
}

But I am getting an error because the concept requirements are not met. GCC reports that template type T that is used to define the concept ConditionFunc is deduced to be bool (*)(int) but I expected it to be int.

How can I define this concept correctly?

c++-concepts c++20 c++
1 Answers

Your concept should be based on 2 types, the argument type T, and the function type:

template <typename Func, typename T>
concept ConditionFunc = requires(T t, Func f) {
    { f(t) } -> std::same_as<bool>;
};

Then you can constrain Foo to accept a function with the signature bool(T);, like this:

template <typename T>
void Foo(std::vector<T>& v, ConditionFunc<T> auto &func) 
{
    // stuff
}

Here's a demo.

user_83728530
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