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How to Implement Generic C Callbacks Using Class Members and `std::function`?

Patricia Arquette
Release: 2024-12-07 18:56:18
Original
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How to Implement Generic C   Callbacks Using Class Members and `std::function`?

C Callback Using Class Member with Generic Implementation

The original question aimed to create a generic event-handling mechanism that would work consistently across different classes. Instead of relying on static methods and passing around class instance pointers, a more modern C approach can be employed using std::function and std::bind.

Event Handler with std::function

The event handler class now accepts a std::function object as an argument. A function object represents a callable entity that can be passed around like a regular function pointer. The event handler method addHandler takes a std::function as an argument, where the passed function has no return value and takes an integer as an argument.

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class EventHandler

{

public:

    void addHandler(std::function<void(int)> callback)

    {

        cout << "Handler added..." << endl;

        // Let's pretend an event just occured

        callback(1);

    }

};

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Binding Specific Functions

To bind a specific class method to the event handler, std::bind is used. std::bind specifies the this pointer and the function to be called when the event is triggered.

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class MyClass

{

public:

    MyClass();

 

    // Note: No longer marked `static`, and only takes the actual argument

    void Callback(int x);

private:

    int private_x;

};

 

MyClass::MyClass()

{

    using namespace std::placeholders; // for `_1`

 

    private_x = 5;

    handler->addHandler(std::bind(&MyClass::Callback, this, _1));

}

 

void MyClass::Callback(int x)

{

    // No longer needs an explicit `instance` argument,

    // as `this` is set up properly

    cout << x + private_x << endl;

}

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Free-Standing Functions and Lambda Functions

If the callback is a free-standing function without a class context, std::bind is not required.

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void freeStandingCallback(int x)

{

    // ...

}

 

int main()

{

    // ...

    handler->addHandler(freeStandingCallback);

}

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For anonymous callbacks, lambda functions can be used with the event handler.

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handler->addHandler([](int x) { std::cout << "x is " << x << '\n'; });

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In this way, using std::function and std::bind provides a flexible and generic solution for callbacks that can be applied to different classes and functions.

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