将对象函数转换为函数指针

Question

I'm trying convert a object function to function pointer but can't get it, i've done something like this, simple example:

typedef struct
{
    int v1;

    int DoSome(int a)
    {
        return v1 * a;
    }
} strx;


int main()
{
    strx a; // instance...
    a.v1 = 2;


    std::function<int(strx* instance, int value)> DoSome = std::mem_fn(&strx::DoSome);

    cout << DoSome(&a, 4) << endl; // 16 ok 

    int(*pDoSome)(strx* instance, int value) = (int(*)(strx*, int))std::mem_fn(&strx::DoSome); // syntax error

    // ptr method...
    pDoSome(&a ,4);

    return 0;
} 

and i have obtained something like:

main.cpp [Error] invalid cast from type 'std::_Mem_fn' to type 'int ( )(strx , int)'

How i can do correctly the casting?

Answer 1

You can't. This is why std::function is more flexible than pointers to functions.

Answer 2

How i can do correctly the casting?

You cannot. Object pointers and function pointers are totally different concepts. Only nullptr can be used to initialize both types of pointers. Otherwise, they are not guaranteed to be compatible.

I suggest sticking with the std::function .

If you must have a function pointer, you have to use a non-member function or a static member function of a class.

Eg

int doSomeFunction(strx* instance, int value)
{
   // Use instance any way you please.
   // ...
   //
   return 0;
}

int main()
{
    strx a; // instance...
    a.v1 = 2;

    int(*pDoSome)(strx* instance, int value) = doSomeFunction;

    // ptr method...
    pDoSome(&a ,4);

    return 0;
}