member function pointer in constructor

Question

I tried to use std::shared_pointer with deleter. I tried to use a member function as the deleter. However it could not compiled. Compiler gave me a error message but I could not understand why it did not work. Does someone knows why it did not work? Thank you very much.

Simplified code is following,

#include <memory>

class MemberFunctionPointerInConstructor {
public:
    MemberFunctionPointerInConstructor(void) {
        std::shared_ptr<int> a = std::shared_ptr<int>(new int(1), deleter);  // this line makes a compiler error message
    }

    void deleter(int* value) {
        delete value;
    }
};

The error message from compiler is following,

error: invalid use of non-static member function
std::shared_ptr<int> a = std::shared_ptr<int>(new int(1), deleter);
                                                                 ^

Thank you very much.

Answer 1

To use a member function that isn't bound to an instance of your class you'd have to declare the method static

static void deleter(int* value) {
    delete value;
}

Answer 2

If you want to use a non-static member function as a deleter, you have to bind it to an instance—but note that the instance would need to still be alive when the deleter is invoked. For example,

class ShorterName {
public:
    ShorterName(void) {
        using namespace std::placeholders;
        auto a = std::shared_ptr<int>(new int(1),
                   std::bind(&A::deleter, this, _1));
    }

    void deleter(int* value) {
        delete value;
    }
};

If you don't need a particular instance, you can make the function static, so it doesn't require an instance.

class ShorterName {
public:
    ShorterName(void) {
        auto a = std::shared_ptr<int>(new int(1), deleter);
    }

    static void deleter(int* value) {
        delete value;
    }
};

Answer 3

There are several ways to solve this. If you actually mean a non-static member function, one way of doing so (not the only one) would be through a lambda function:

class MemberFunctionPointerInConstructor {
public:
    MemberFunctionPointerInConstructor() {
        std::shared_ptr<int> a = std::shared_ptr<int>(
            new int(1), 
            [this](int *p){this->deleter(p);});  
    }   

    void deleter(int* value) {
        delete value;
    }   
};