1 
   class manip{
public:
    int t;
    manip(int tom){
        t = tom;
    }
    int sub(int val){
        return t - val;
    }
    int add(int val){
        return t + val;
    }
    int perform(int(manip::*key)(int), int val){
        return (this->*key)(val);
    }
};

int main() {
    manip g(5);
    cout << g.perform(&manip::add, 9) << ":" << g.perform(&manip::sub, 9);

(This is just a simplified version of a problem im trying to solve in a larger piece of code)

the problem lies here

    int(b::*func)(int) = b::add;

    int c = func(2);

this gives me a syntax error on the second line (because i have no reference to the "this" data). How do i change it so that the function being called isnt b::add but rather inst.add.

Edit : Posted a working version of the code. Thanks Speed8ump

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5
  • Possible duplicate stackoverflow.com/questions/14189440/… Commented Jan 5, 2014 at 2:39
  • Suggestion to use the std::mem_fn helper to get a std::function pointing to the member function. As we're in C++ we should avoid C style function pointers. Commented Jan 5, 2014 at 2:51
  • I'm just learning CPP so I'm not terribly experienced, but std::mem_fn looks like it might be what I'm looking for, ill play with it. The only issue is that it looks ugly to use std::mem_fn every time performOperation is called, is there a way to use std::mem_fn inside performOperation? Commented Jan 5, 2014 at 3:18
  • You have a syntax error, you need to use ... = &b::add;. Commented Jan 5, 2014 at 5:32
  • no the issue is b::add is nonstatic Commented Jan 5, 2014 at 5:34

3 Answers 3

Reset to default
2

in your example 'func' is a member function pointer. It must be used on an instance of the data type it is a member of:

int c = (inst.*func)(2);
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1 Comment

This... seemed to work just fine... Not sure why people keep barking about callbacks, passer functions, etc... I'm going to bed ATM but if noone posts a good reason not to use it (I assume there is one if people are going through so much effort not to use this) i'll best answer it.
1

You need to give a parameter pointing to the struct (or aA).

int main(){
    b inst(5)
    int(b::*func)(int, struct b*) = b::add;
    int c = func(2, &inst);
}

int add(int a, struct b* ptr){
    return a + ptr->aA;
}
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4 Comments

In the actual code, there will be alot of decently long functions that I want to swap out and making nonstatic versions of each function isn't viable
For what purpose do you try to avoid memberfunctions ? If its performance you should know that using function pointers will likely make it even worse performancewise than using a member function. This is because processors are designed to support fast member functions.
As stated, runtime there will be alot of functions with a decent amount of head so I'd rather stay away from it, it's also just to keep the code alot neater for my own eyes (I come from java and C++ to me is just so... bleh)
For me, using classes with member function would look smoothest. Its a hell easier to understand whats going on, when u need to debug something one day.
1

If your compiler is new enough you can avoid function pointers and use std::function instead

 int(b::*func)(int) = b::add;

becomes

#include <functional>
#include <iostream>
 class manip{
public:
    int t;
    manip(int tom){
        t = tom;
    }
    int sub(int val){
        return t - val;
    }
    int add(int val){
        return t + val;
    }
    int perform(std::function<int(manip*,int)>f, int val){
        return f(this, val);
    }
};

int main() {
    manip g(5);
    std::cout << g.perform(&manip::add, 9) << ":" << g.perform(&manip::sub, 9);
}
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5 Comments

see my comment on user1610743 about the second one, first one's a bit too confusing for me
I think we really need a bit more context on how do you plan to use the code in real app.
I guess a key componet of the question I didn't clearly state is that the function using the function argument will use itself as the instance parameter
I have updated my answer to match your edits. The only change necessary is the signature and body of perform() operation. Although to be honest your code still looks strange. I have a feeling you are doing something very wrong here
I suspect it is the result of you trying to write java style code in C++. We would be able to help better if you give us a more specific example of a real problem you are trying to solve

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