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Understanding Callback Functions in C++

This article provides an in-depth explanation of callback functions in C++, including what they are and how to use them.
It also includes a simple example that demonstrates how to use a callback function to perform an action in response to an event.

A callback function, also known as a "hook" function, is a function that is passed as an argument to another function.
The purpose of a callback function is to be
called by the function it was passed to, usually
in response to an event or some other trigger.

In C++, you can pass a function as an argument to another function by using function pointers.
A function pointer is a pointer that points to
a function rather than to an object or data.

Here's an example that demonstrates how to use a callback function in C++:

#include <iostream>

// Define the callback function
void printNumber(int num) {
  std::cout << "The number is: " << num << std::endl;
}

// Define the function that takes the callback function as an argument
void performAction(void (*callback)(int)) {
  callback(42);
}

int main() {
  performAction(printNumber);
  return 0;
}

In this example, the printNumber function is the callback function, and the performAction function is the function that takes the callback function as an argument.

The performAction function calls the callback function by invoking the function pointer callback and passing 42 as the argument.

You can also use lambda functions as callback functions in C++. Lambda functions, also known as anonymous functions, provide a convenient way to define small, throw-away functions right in the place where they are used. Here's an example that demonstrates how to use a lambda function as a callback function in C++:

#include <iostream>

// Define the function that takes the callback function as an argument
void performAction(void (*callback)(int)) {
  callback(42);
}

int main() {
  performAction([](int num) {
    std::cout << "The number is: " << num << std::endl;
  });
  return 0;
}

In this example, the performAction function is the same as in the previous example, but instead of passing a function pointer, we pass a lambda function as the argument. The lambda function is defined right in the place where it is used,
and it has the same signature as the printNumber function from the previous example.

Summary

Callback functions provide a powerful mechanism for executing code in response to events or other triggers. They are especially useful in situations where you need to pass a function as an argument to another function. Whether you use function pointers or lambda functions, callback functions are a valuable tool in the C++ programmer's toolbox.