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

Passing Class Member Functions as Callbacks

When utilizing an API that requires a function pointer as a callback, difficulties may arise when attempting to pass a class member function from within the class itself. This article explores the intricacies of this issue and offers solutions to overcome it effectively.

Problem Statement

Consider a hypothetical class, CLoggersInfra, that contains a member function, RedundancyManagerCallBack. You seek to use this member function as a callback within another API function Init, but you encounter the following compilation error:

Error  8 error C3867: 'CLoggersInfra::RedundancyManagerCallBack': function call missing argument list; use '&CLoggersInfra::RedundancyManagerCallBack' to create a pointer to member

Delving into the Issue

The error stems from a fundamental characteristic of member functions in C . While they appear to belong exclusively to a particular class instance, they inherently receive an extra hidden parameter, which is a reference to the class instance itself. This is commonly referred to as the "this" pointer.

Solutions Explained

1. Using Boost

Boost, a third-party library, provides a powerful tool for resolving this issue: boost::bind. It allows you to create a new function that "locks in" a specific parameter. In this case, the hidden "this" parameter can be locked to a particular class instance, effectively creating a new function with only one visible parameter.

2. Wrapping with a Custom Object

Alternatively, you can create a custom object that accepts a reference to the class instance and provides a "run" or "execute" method that invokes the member function with the supplied parameters. However, this approach requires modifying the API to accept your custom object instead of a raw function pointer.

3. C 11 Lambda Functions

In C 11 and later, lambda functions offer a more elegant solution. They can capture the "this" pointer, eliminating the need for a separate custom object. Lambda functions provide a concise and convenient way to achieve the desired functionality.

Conclusion

Understanding the inherent nature of member functions and their hidden "this" parameter is crucial for successfully passing them as callbacks. By leveraging the capabilities of boost::bind or C 11 lambda functions, you can effectively resolve compilation errors and utilize class member functions as callback functions smoothly.

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