将成员函数分配给函数指针

Question

If I have two classes like this :

class A
{
    public:
        int *(*fun)( const int &t );
        A( int *( *f )( const int &t ) ) : fun( f ) {}
};

class B
{
    private:
        float r;

        int *f(const int &t)
        {
            return new int( int( r ) + t );
        }
        A a;
        B() : a( A( f ) ) {}
};

This results in compiler error.

I want to assign f to a 's function pointer.

The thing is that A can be used by many classes not just B so I can't simply define fun as B::*fun .

None of the posts I've found on the internet and here on stackoverflow address the issue of using the function pointers with many classes each having its own member function but the same prototype.

So what to do?

Answer 1

this result in compiler error

Because f is a A 's non-static member function returning int* , that accepts single const reference to int . That means, that its type is not:

int* (*)(const int &);

but:

int* (A::*)(const int&);

Also, your code signalizes very bad design - I think you need simple virtual function. But if you want to keep writing things this way, you may want to read: ISOCPP: Pointers to members .

Remember, that non-static member functions of type C always accepts additional implicit argument of type C* (or const C* , if function is declared with const qualifier), that points to instance of C this function was called on.

Answer 2

Your code looks confusing and, personally, I believe that C function pointers look ugly on C++'s OO implementation . So I would advise you to use the std::function . It only has been available since C++11 . If you cannot use it, try looking on Boost's Implementation .

I can give you an example of how to use the std::function :

bool MyFunction(int i)
{
    return i > 0;
}

std::function<bool(int)> funcPointer = MyFunction;

Using this you will drastically improve your code reliability. As of your problem, specifically:

class A
{
public:
    std::function<int*(const int&)> fun;
    A(std::function<int*(const int&)> f) : fun(f) {}
};

class B
{
private:
    float r;
    int *f(const int &t)
    {
        return new int(int(r) + t);
    }
    A *a;
    B()
    {

        std::function<int*(const int&)> tempFun = std::bind(&B::f, this, _1);
        a = new A(tempFun);
    }
};

You have to add the following namespace:

using namespace std::placeholders;

Answer 3

So what to do

Not much. Other than templating A on the type of objects for which it will hold a pointer to a member function taking a reference to a const int and returning a pointer to int.

What you're trying to do is to mix a pointer to a free function with a pointer to member function. Whilst it sounds like they're both function pointers they're different enough to not be able to pass through the same type definition.