How Can I Pass a C Class Member Function as a Callback to an API?

Passing Class Member Functions as Callbacks

In attempting to pass a class member function as a callback to an API, you may encounter compilation errors due to the missing argument list. To resolve this issue, it is recommended to use function pointers or lambda expressions that capture the class instance.

Function Pointers

The error message suggests using '&CLoggersInfra::RedundancyManagerCallBack' to create a pointer to the member. This is because class member functions typically take two parameters: the first parameter being a pointer to the class instance, and the second parameter being the function's argument(s).

void (CLoggersInfra::*ptr)(int); // Function pointer type
ptr = &CLoggersInfra::RedundancyManagerCallBack;

However, the function pointer approach requires you to explicitly specify the first parameter, which may not be practical in all scenarios.

Lambda Expressions (C 11 and Later)

Lambda expressions provide a more concise and flexible way to capture the class instance and create a callback function:

auto lambda = [this](int arg) { this->RedundancyManagerCallBack(arg); };

This lambda expression captures the 'this' pointer and defines an anonymous function that takes an integer argument 'arg'. You can then pass the lambda as the callback:

m_cRedundencyManager->Init(lambda);

Conclusion

By using function pointers or lambda expressions, you can efficiently pass class member functions as callbacks while adhering to the API requirements and avoiding compilation errors. Lambda expressions are particularly convenient in C 11 and later versions, as they simplify the capture of class instances.

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