Question

I am mixing some C and C++ libraries and have only a single pointer available to do some work in a callback function. All I need to do is iterate through a vector. Here’s a simplified, untested example:

bool call_back(void* data){
  done=...
  if (!done) cout << *data++ << endl;
  return done;
}

Note that this function is in an extern "C" block in C++. call_back will be called until true is returned. I want it to cout the next element each time it’s called. data is a pointer to something that I can pass from elsewhere in the code (an iterator in the above example, but can be anything). Something from data will likely be used to calculate done. I see two obvious options to give to data:

1. Have data point to my vector.
2. Have data point to an iterator of my vector.

I can’t use an iterator without having the .end() method available, right? I can’t use a vector alone (unless maybe I start removing its data). I could make a struct with both vector and iterator, but is there a better way? What would you do?

Answer 1

Why not have data point to a structure with all the information you need.

The point about the old “C” style callbacks is that a void* can point to any object. Your callback function knows what the type is, but it can be anything.

typedef struct Plop
{
    std::vector<int>::iterator begin;
    std::vector<int>::iterator end;
} Plop;

bool call_back(void* data)
{
    // Or static_cast<> for the pedantic.
    // I like reinterpret_cast<> because it is a clue to humans that this is dangerious
    // and as long as the object was originally a Plop* pointer it is guaranteed to work.
    Plop*   info = reinterpret_cast<Plop*>(data);

    bool    done= info.begin == info.end;

    if (!done) cout << *data++ << endl;
    return done;
}