Registering callback using std::function in C++

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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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