How to Pass a C Class Member Function as a Callback?

Passing Class Member Functions as Callbacks

In object-oriented programming, it is occasionally necessary to pass a class member function as a callback to an external API. However, attempting this straightforwardly may encounter compilation errors, as demonstrated in the following scenario:

m_cRedundencyManager->Init(this->RedundancyManagerCallBack);

The compiler complains about a missing argument list for the function pointer, suggesting the use of &CLoggersInfra::RedundancyManagerCallBack instead. However, this still fails to compile.

Understanding Member Functions

Before delving into the solution, it is crucial to understand the true nature of member functions. In C , member functions are essentially regular functions that possess an additional hidden parameter: this. This hidden parameter represents the object instance the function belongs to.

Consider the following example:

class A {
public:
    int data;
    void foo(int addToData) { data += addToData; }
};

The function A::foo takes one parameter, but internally, it operates with two parameters: addToData and this. The latter points to the A object for which foo is called. This behavior is implicit when using the member function syntax an_a_object.foo(5). Syntactically, the compiler translates this to A::foo(&an_a_object, 5).

The Problem with Callbacks

Returning to the original problem, the API's Init function expects a function pointer that takes a single parameter. However, class member functions like CLoggersInfra::RedundancyManagerCallBack inherently require two parameters, including the hidden this parameter. This incompatibility leads to the compilation error.

Solution: Binding with Boost or Lambda Functions

The conventional solution involves binding the member function to a specific class instance using Boost's boost::bind library or C 11's lambda functions.

With boost::bind, one can create a new function that takes the required parameters by "locking in" the hidden this parameter to a specific object instance. This new function can then be passed as the callback.

#include <boost/bind.hpp>
auto bound_callback = boost::bind(&CLoggersInfra::RedundencyManagerCallBack, this);
Init(boost::function<void()>(bound_callback));

C 11 offers lambda functions as a simpler alternative to boost::bind. Lambda functions can also capture the this pointer, allowing for concise callback binding:

auto lambda_callback = [this]() { RedundancyManagerCallBack(); };
Init(std::function<void()>(lambda_callback));

The above is the detailed content of How to Pass a C Class Member Function as a Callback?. For more information, please follow other related articles on the PHP Chinese website!