function pointer to overloaded static member - to use as custom deleter in unique_ptr

Question

I have a class with static and overloaded member function. I want to use one them as a custom deleter in a unique_ptr there are lots of questions on this topic, none of them worked for me.

#include <iostream>
#include <memory>
#include <functional>

class A {
    public:
    static void release() {
        std::cout << "void released\n";
    }
    static void release(int*i) {
        std::cout << *i << " released\n";
    }
};

int main()
{
    int i = 10;
    std::unique_ptr<int, decltype(&A::release(int*))> ptr(&i, &A::release); // compiler error
    std::unique_ptr<int, std::function<void(int*)>> ptr(&i, &A::release); // compiler error
    return 0;
}

try it out here: https://onlinegdb.com/H14txk3sL

Piotr Skotnicki · Accepted Answer · 2020-05-27T13:11:34.170

std::unique_ptr<int, void(*)(int*)> ptr(&i, &A::release); 
//                   ~~~~~~~~~~~~^

This way, std::unique_ptr's constructor will expect a specific type of a pointer, which will help the compiler resolve ambiguity.

This:

decltype(&A::release(int*))

is not a valid syntax. In order yo use decltype(e), you'd have to write decltype(&A::release), but this again would raise an ambiguity error, and so it would have to become:

decltype(static_cast<void(*)(int*)>(&A::release))

but that's a long-winded way of saying void(*)(int*).

This:

std::function<void(int*)>

doesn't help in resolving ambiguity, becuase std::functions's constructor is a template as well, which means the compiler again misses a context that would help it to choose one of the overloaded functions.

function pointer to overloaded static member - to use as custom deleter in unique_ptr

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