Returning Function Pointers from Functions

You can return a function pointer from a function just like you would return any other pointer type. The syntax looks like this:

1bool greater(int a, int b) { return a > b; }
2
3bool (*GetComparator())(int, int) {  
4  return greater;               
5}

This function returns a pointer to a function that takes two int parameters, and returns a bool. The returned function checks if the first parameter is greater than the second.

Some use cases for returning function pointers include:

1. Configuring behavior - Returning different function pointers based on configuration or environment allows flexibly changing functionality.
2. Memoization - Capturing data in the closure of the returned function enables caching/memoization of expensive operations.
3. Dependency injection - Caller can provide functions to be called by the callee, enabling inversion of control.

For example, a sorting function could allow specifying the comparison operation:

1#include <algorithm>
2#include <iostream>
3#include <vector>
4
5bool less(int a, int b) { return a < b; }
6bool greater(int a, int b) { return a > b; }
7
8bool (*GetComparator(bool asc))(int, int) {  
9  return asc ? less : greater;               
10}
11
12int main() {
13  std::vector<int> v{5, 2, 3, 4, 1};
14
15  std::sort(
16    v.begin(), v.end(), GetComparator(true));
17
18  for (int i : v) {
19    std::cout << i << ' ';
20  }
21}

11 2 3 4 5