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How to Implement Generic C Callbacks Using Class Members and `std::function`?
How to Implement Generic C Callbacks Using Class Members and `std::function`?
Dec 07, 2024 pm 06:56 PM
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
class EventHandler
{
public:
void addHandler(std::function<void(int)> callback)
{
cout << "Handler added..." << endl;
// Let's pretend an event just occured
callback(1);
}
};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.
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;
}Free-Standing Functions and Lambda Functions
If the callback is a free-standing function without a class context, std::bind is not required.
void freeStandingCallback(int x)
{
// ...
}
int main()
{
// ...
handler->addHandler(freeStandingCallback);
}
For anonymous callbacks, lambda functions can be used with the event handler.
handler->addHandler([](int x) { std::cout << "x is " << x << '\n'; });
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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