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!
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