A C++ Callback Example

Imagine that you wish to write a function in C++ that goes through all the values in an array, and prints out all those values that match certain criteria, where the criteria are specified by calling back a supplied function. The callback function is specified by passing in a pointer to that function.

#include <iostream>
using namespace std;

typedef void select(int value);

static void greaterThan10(int value)
{
    if (value > 10)
        cout << " " << value;
}

static void lessThan20(int value)
{
    if (value < 20)
        cout << " " << value;
}

static void forEach(int vals[], size_t elements, select *callMe)
{
    for (int i = 0; i < elements; i++)
    {
        callMe(vals[i]);
    }
}

int main()
{
    int values[] = { 4, -5, 8, 37, 56, 2, 15, 92 };
    cout << "Values > 10 : ";
    forEach(values, sizeof(values)/sizeof(values[0]), greaterThan10);
    cout << endl;

    cout << "Values < 20 : ";
    forEach(values, sizeof(values)/sizeof(values[0]), lessThan20);
    cout << endl;

    return 0;
}

In other words, each of the values in the array that matches the condition enforced in each callback function.