Question

I have the following classes:

 class A
{
   public:
        virtual void myfunc(unsigned char c, std::string* dest) = 0;
};

   class B : public class A
{
    public:
        virtual void myfunc(unsigned char c, std::string* dest);
};

void someOtherFunc(const std::string& str,A *pointerFunc)
{
    std::string tmp;
        for_each(str.begin(),
                 str.end(),                 
                 std::bind2nd(std::mem_fun(pointerFunc->myfunc), &tmp));
}

I get the following compilation error:
error: no matching function for call to \u2018mem_fun()\u2019

Do you know why?

Answer 1

You’re looking for std::mem_fun(&A::myfunc).

EDIT: You can’t use mem_fun at all here — no overload of mem_fun allows you to make a two argument member function into a functor. You’re going to have to use something like boost::bind/std::tr1::bind (If you have TR1)/std::bind (If you have C++0x) or you’re going to have to write your own functor.

Note that even if mem_fun was able to do this sort of binding, then std::bind2nd would fail, because bind2nd expects a functor taking two arguments, and binding a member function pointer like this is going to produce a functor with three arguments.

You have a few ways around this:

1. Write your own functor that does what you want.
2. Write an explicit loop instead of std::for_each.
3. One of the not-yet-standard binder functions I mentioned above (and demonstrated in @David’s answer)
4. Don’t bother with the virtual function in the first place — make your method accept a plain function pointer and implement things in terms of the function pointer. Of course this only works if myfunc doesn’t depend on members of the class to which it belongs (in which case it shouldn’t have ever been put into a class in the first place)