To implement your own callback for IloCplex, first select the callback class corresponding to the callback you want implemented. From it derive your own implementation class and overwrite the virtual method main. This is where you implement the callback actions, using the protected methods of the callback class from which you derived your callback or one of its base classes.

Next write a function that creates a new object of your implementation class using the environment operator new and returning it as an IloCplex::Callback handle object. Here is an example implementation of such a function:

IloCplex::Callback MyCallback(IloEnv env, IloInt num) {
    return (new (env) MyCallbackI(num));
}

It is not customary to write such a function for Java, but new is called explicitly for creating a callback object when needed. Once an implementation object of your callback is created (either with the constructor function in C++ or by directly calling the new operator for Java), use it with IloCplex by calling cplex.use with the callback object as parameter. In C++, to remove a callback that is being used by a cplex object, call callback.end on the IloCplex::Callback handle callback. In java there is no way of removing individual callbacks from your IloCplex object. Instead, you can remove all callbacks by calling cplex.clearCallbacks. Since Java uses garbage collection for memory management, there is nothing equivalent to the end method for callbacks in the Java API.

One object of a callback implementation class can be used with only one IloCplex object at a time. Thus, when you use a callback with more than one cplex object, a copy of the implementation object is created every time cplex.use is called except for the first time. In C++, the method IloCplex::use returns a handle to the callback object that has actually been installed to enable calling end on it.

To construct the copies of the callback objects in C++, class IloCplex::CallbackI defines another pure virtual method:

which must be implemented for your callback class. This method will be called to create the copies needed for using a callback on different cplex objects or on one cplex object with a parallel optimizer.

In most cases you can avoid writing callback classes by hand, using supplied macros that make the process as easy as implementing a function. You must implement a callback by hand only if the callback manages internal data not passed as arguments, or if the callback requires eight or more parameters.