Question

To overcome the impossibility of giving C library a callback to C++ member function, wanted to implement something like this:

SomeObject* findSomeObjectByHandlePointer(datahandle *dh) { }..

by using a map, which contains addresses of *datahandle as an index, and addresses of *SomeObject’s as value.

When SomeObject is created, it produces a group of datahandle’s, which are unique for the object. Then, it passes a pointer to *datahandle and static callback function to C library, then C library calls back and returns a pointer to datahandle, that in turn can be associated back with a SomeObject.

Which types can you recommend for storing pointer values in a map besides safe but slow <string, SomeObject*>?

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This answer tells me to avoid using auto_ptr too.

Answer 1

Normally, C-like callbacks take a void* user_data parameter, which allows you to pass in anything you want:

void c_func(void (*fptr)(void*), void* user_data){
  // do some stuff
  fptr(user_data);
}

Now, simply make the following static member function:

class A{
public:
  static void c_callback(void* my_data){
    A* my_this = static_cast<A*>(my_data);
    // do stuff with my_this
  }
};

Edit: According to @Martin’s comment, you may get unlucky with a static member function. Better use an extern "C" function:
extern “C” void c_callback(void* my_data){
// same as static method
}
And pass that + your A instance to that c_func:

int main(){
  A a;
  c_func(&A::c_callback,&a);
}

Or if that A instance needs to outlive the current scope, you need to somehow save the heap-allocated pointer somewhere and delete it manually later on. A shared_ptr or the likes won’t work here, sadly. 🙁

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On your problem of storing pointer in a map, that’s not a problem at all, see this little example on Ideone.