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Imagine you're building an application that processes some data and once it's done, the user needs to be notified about the result. Since you don’t want to hard-code the notification logic, it’s better to allow the user to register a custom function that will be called automatically after the data processing completes.
Using std::function for callback registration
One of the most flexible ways to implement callbacks in C++ is by using std::function. std::function can store and invoke any callable object (functions, lambdas, or function objects) that matches a specific signature. The overall syntax is:
std::function<function_return_type(function_argument_types)> name; Let’s define a ProcessData unction that accepts a callback and notifies the user once processing is complete.
#include <iostream> #include <functional> // Type alias for the callback type using CallbackType = std::function<void(const bool)>; // Function accepting callback as an argument void ProcessData(const CallbackType& callback) { // Simulate data processing std::cout << "Processing data" << std::endl; // Notify user via callback callback(true); } // Callback function definition void Callback(const bool result) { std::cout << "Callback called! Result: " << result << std::endl; } int main() { // Register a callback using an existing function ProcessData(Callback); // Register a callback using lambda function ProcessData([](const bool result) { std::cout << "Lambda called! Result: " << result << std::endl; }); // Register using callable object CallbackType cb = [](const bool result){ std::cout << "Callable called! Result: " << result << std::endl; }; ProcessData(cb); } 
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