calling C++ class member functions using a functional template

Question

I want to be able to call a member function (eg. doSomething() in class testClass ) outside the class using some sort of functional template. In short, be able to call a non-static member function somewhat in this manner, functionCaller(t.doSomething) from main().

Say I have a C++ class like:

class testClass {
public:
    testClass()
    {
        value = 0;
    }
    int prod(int i)
    {
        value *= i;
        return value;
    }
    int doSomething()
    {
        value = prod(10);
        return value;
    }

private:
    int value;
};

My functional template looks something like this:

template<typename T, typename ret>
ret callFunction(T t, ret (T::*func)()) {

    // checkSomePermissionsHere()
    return t.func()
}

If I try to use this template in the following manner:

int main()
{
    testClass t1;
    callFunction(t1, &testClass::doSomething);
    return 0;
}

I get this error:

error: no member named 'func' in 'testClass'
return t.func(a);
         ^

What is the correct way to call this member function (doSomething) on object t1 through the callFunction method? I want to implement callFunction as part of a general API which does some checks before executing the provided fucntion.

I'm using clang/LLVM 3.5 for my compiler.

You might be interested in [`std::function`](http://en.cppreference.com/w/cpp/utility/functional/function) and [`std::bind`](http://en.cppreference.com/w/cpp/utility/functional/bind), it makes handling these issues much easier. — Some programmer dude, Aug 07 '15 at 09:44
That would be really nice @JoachimPileborg, can you please expand a bit? If I understand nicely, I might be able to improve my answer! :D — gsamaras, Aug 07 '15 at 09:48
@gsamaras See e.g. [this old answer of mine](http://stackoverflow.com/questions/14189440/c-class-member-callback-simple-examples/14189561#14189561). — Some programmer dude, Aug 07 '15 at 09:49

score 3 · Accepted Answer · edited May 23 '17 at 12:29

Change this:

return t.func()

to this:

return (t.*func)();

and it should compile and execute fine.

As Joachim Pileborg stated, you could use std::function and std::bind. In that case, your code could look like this:

#include <functional>
#include <iostream>

class EventHandler
{
    public:
        void addHandler(std::function<void()> callback)
        {
            std::cout << "Handler added..." << std::endl;
            // Let's pretend an event just occured
            callback();
        }
};

class testClass {
public:
    testClass()
    {
        value = 0;
        EventHandler handler;
        handler.addHandler(std::bind(&testClass::doSomething, this));
    }
    int doSomething()
    {
        value = prod(10);
        std::cout << "I did something!\n";
        return value;
    }

private:
    int value;
};

int main()
{
    testClass t1;
    return 0;
}

The code above was based on this answer.

I'm not sure I have access to functional. Do you know if clang 3.5 supports it yet? Otherwise std::functon would have been a good idea. Thank for the suggestion. EDIT: I see it has been supported for some time now — Adwait Dongare, Aug 18 '15 at 18:37

calling C++ class member functions using a functional template

1 Answers1