# C++ Function

## callback function
* In C++, a callback function is a function that is passed as a parameter to another function and then executed inside that function
* allows for customizable operations within the function, as the behavior can be defined by the callback provided by the caller
* c++中使用callback的兩種方式function pointer與std::function
### function pointer
* A function pointer is a variable that stores the address of a function that can later be called through this pointer

    #include <iostream>

    // Function that uses the callback
    void processValue(int x, void (*cb)(int)) {
        std::cout << "Processing value: " << x << std::endl;
        cb(x);  // Calling the callback function
    }

    // Callback implementation
    void myCallback(int x) {
        std::cout << "Callback called with value: " << x << std::endl;
    }

    int main() {
        int a = 5;
        processValue(a, myCallback);  // Passing `myCallback` as a callback function
        return 0;
    }

* ```typedef``` or ```using``` can improve code readability, maintainability, and ease of use

    #include <iostream>

    // Definition of the callback function type
    typedef void (*CallbackFunction)(int);

    // Function that uses the callback
    void processValue(int x, CallbackFunction cb) {
        std::cout << "Processing value: " << x << std::endl;
        cb(x);  // Calling the callback function
    }

    // Callback implementation
    void myCallback(int x) {
        std::cout << "Callback called with value: " << x << std::endl;
    }

    int main() {
        int a = 5;
        processValue(a, myCallback);  // Passing `myCallback` as a callback function
        return 0;
    }

### std::function
* In more complex applications, particularly those involving **class methods as callbacks**, function pointers can be somewhat limiting. To use member functions as callbacks, you typically need to use:
* ```std::function``` and ```std::bind``` from the C++ Standard Library (since C++11), which provide a more flexible way to pass callable objects (like **lambda expressions**, **function pointers**, or **bind expressions**).
* **Lambda Expressions**: These can capture context and be used as inline callback definitions, enhancing readability and usability.

    #include <iostream>
    #include <functional>

    void processValue(int x, std::function<void(int)> cb) {
        std::cout << "Processing value: " << x << std::endl;
        cb(x);  // Calling the callback function
    }

    int main() {
        int a = 10;
        processValue(a, [](int y){ std::cout << "Lambda Callback with value: " << y << std::endl; });
        return 0;
    }