C++ Cookbook

15.1. Using Function Pointers for Callbacks

Problem

You plan to call some function func1, and at runtime you need it to invoke another function func2. For one reason or another, however, you cannot simply hardcode the name of func2 within func1. func2 may not be known definitively at compile time, or perhaps func1 belongs to a third-party API that you can’t change and recompile. In either case, you need a callback function.

Solution

In the case of the functions above, declare func1 to take a pointer to a function, and pass it the address of func2 at runtime. Use a typedef to make the messy syntax easier to read and debug. Example 15-1 shows how to implement a callback function with a function pointer.

Example 15-1. A callback function

#include <iostream>

// An example of a callback function
bool updateProgress(int pct) {
  
   std::cout << pct << "% complete...\n";
   return(true);
}

// A typedef to make for easier reading
typedef bool (*FuncPtrBoolInt)(int);

// A function that runs for a while
void longOperation(FuncPtrBoolInt f) {

   for (long l = 0; l < 100000000; l++)
      if (l % 10000000 == 0)
         f(l / 1000000);
}

int main() {

   longOperation(updateProgress); // ok
}

Discussion

In a situation such as that shown in Example 15-1, a function pointer is a good idea if updateProgress and longOperation shouldn’t know anything about each other. For example, a function that updates the progress by displaying it to the user—either in a user interface (UI) dialog box, in a console window, or somewhere else—does ...