How can I access private member variable with a friend function that has a requires expression? Is there a way to get the code to compile without removing requires std::is_default_constructible_v<T>?

#include <iostream>
#include <type_traits>

template<typename T>
class Y
{
    static constexpr int data = 10;

    template <typename U>
    friend U pp();
};

template<typename T>
requires std::is_default_constructible_v<T>
T pp()
{
    std::cout << Y<T>::data << '\n';
    return T();
}

int main() {
    pp<int>();
    return 0;
}

Gives error error: 'constexpr const int Y<int>::data' is private within this context

How can I access private member variable with a friend function that has a requires expression? Is there a way to get the code to compile without removing requires std::is_default_constructible_v<T>?

#include <iostream>
#include <type_traits>

template<typename T>
class Y
{
    static constexpr int data = 10;

    template <typename U>
    friend U pp();
};

template<typename T>
requires std::is_default_constructible_v<T>
T pp()
{
    std::cout << Y<T>::data << '\n';
    return T();
}

int main() {
    pp<int>();
    return 0;
}

Gives error error: 'constexpr const int Y<int>::data' is private within this context

• c++

• 1 How would you distinguish that pp function from the entirely different template<typename T> requires (!std::is_default_constructible_v<T>) T pp() ? – MSalters Commented Feb 10 at 15:34
• 4 You need to move definition in-class or repeat the constraint, no other way AFAIK – Yksisarvinen Commented Feb 10 at 15:54

1 Answer 1

Note you can have mutiple versions of pp() template with different requriements.

For example this is valid code:

template<typename T>
T pp()
{
    std::cout << "I do not need friendships\n";
    return T();
}

template<typename T>
requires std::is_default_constructible_v<T>
T pp()
{
    std::cout << Y<T>::data << '\n';
    return T();
}

When overload resolution is performed and there are multiple choices with concepts, most specialized version is selected.

So in your code you have declared two versions of pp() like in my example above. One is declared when friendship is defined and other outside a Y class.

As a result Y is friended with a function template which has missing implementation.

So to fix this you must use same concept to funtion tempalte as in function template definition:

template<typename T>
class Y
{
    static constexpr int data = 10;

    template <typename U>
    requires std::is_default_constructible_v<U>
    friend U pp();
};

template<typename T>
requires std::is_default_constructible_v<T>
T pp()
{
    std::cout << Y<T>::data << '\n';
    return T();
}

After that it works as expected: https://godbolt./z/jGEqzhdsW

Moving definition of template inside a class fixes this issue by coincidence, sine at same time you are moving a concept there.