assign a member function to a function pointer

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?

_"I found many results on the net and on SO but none of them address the issue of"_ Really? I answered one of those just yesterday, if memory serves. — Lightness Races in Orbit, Jun 02 '15 at 19:57
@LightnessRacesinOrbit hmm well "just yesterday" is after my search :) — niceman, Jun 02 '15 at 20:01
@LightnessRacesinOrbit And a few days before that...Regardless, SOP as far as I can recall is a scolding and a link to the [C++FAQ](https://isocpp.org/wiki/faq/pointers-to-members#fnptr-vs-memfnptr-types), which answers it quite nicely, though it requires reading. — jaggedSpire, Jun 02 '15 at 20:03
Just an advice: function pointers are a **C Language feature** and not **C++**. You might want to consider looking on `std::function` or **Boost's** implementation. — Alexandre Severino, Jun 02 '15 at 20:04
@LightnessRacesinOrbit, but is it considered a good practice to use raw function pointers in C++? I mean... it feels too much like C. It is like using `memcpy` on a C++ code when you have `std::copy`. — Alexandre Severino, Jun 02 '15 at 20:16

score 3 · Answer 1 · answered Jun 02 '15 at 20:15

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.

score 1 · Answer 2 · answered Jun 02 '15 at 20:03

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.

Alexandre Severino · Accepted Answer · 2015-06-02T20:45:42.110

1

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;

edited Jun 02 '15 at 20:45

answered Jun 02 '15 at 20:12

Alexandre Severino

1,563
1
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I edited it. Try it now. (Code not tested... might need tweaks. But ask me if you have problems.) – Alexandre Severino Jun 02 '15 at 20:29
it resulted in compiler error "no match for operator = of operand types" – niceman Jun 02 '15 at 20:36
The trick that does it is the `std::bind()` - transforming a pointer-to-member function into a free function (sort of), nice! – emvee Jun 02 '15 at 22:34

assign a member function to a function pointer

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