Question

I have a functioning C++ callback function, triggered by a user ‘mouse down’ event. (The IDE is VS2010.)

With each call, I’d like to increment a simple count variable that is local to the callback’s scope. Simply put, what is the ‘best practices’ way to do this?

Thanks in advance for any opinions or directives.

Answer 1

Replace your callback function with a functor – they can store state. An example functor:

#include <iostream>
#include <memory>

class Functor
{
private:
    std::shared_ptr<int> m_count;

public:
    Functor()
    :    m_count(new int(0))
    {}

    void operator()()
    {
        ++(*m_count);
        // do other stuff...
    }

    int count() const
    {
        return *m_count;
    }
};

template <typename F>
void f(F callback)
{
    // do stuff
    callback();
    // do other stuff
}

int main()
{
    Functor callback;
    f(callback);
    f(callback);
    std::cout << callback.count();    // prints 2
    return 0;
}

Note the use of a shared_ptr inside the functor – this is because f has a local copy of the functor (note the pass-by-value) and you want that copy to share its int with the functor to which you have access. Note also that f has to take its argument by value, since you want to support all callables, and not just functors.