Question

Suppose I have some code like this:

class Visitor {    public:       Visitor(callBackFunction) {}       void visit() {           //do something useful           invokeCallback();       } }  class ClassThatCanBeVisited {     Visitor &visitor;      public:        ClassThatCanBeVisited(Visitor &_visitor) : visitor(_visitor){}        void someUsefulMethod() {           int data= 42;           visitor.visit(data);        } };   void callBackFunction() {     //do something useful in the context of the Main file } int main() {      Visitor visitor;      ClassThatCanBeVisited foo(visitor);      foo.someUsefulMethod(); } 

I need to create a simple callback that will be called whenever the Visitor::visit() is called. I know that I probably should put the code of the callback inside my Visitor, but it is in a different context, so I would like to pass the callBackFunction() to the Visitor so he could invoke my callback function.

I looked for things on the web and saw boost::function, but c++ already has the basic functors.

Which one should I use for better clarity of the code? The callback is going to be a simple void() function, but it might grow, you never know the future 🙂

What is the recommended way to do this?

Answer 1

You can use callback interface and its hierarchy if you don’t want to use boost::function.

class VisitorCallback { public:     virtual void handle( const Visitor& ) = 0; }; 

If you have or can use boost::function – use it, it is a good way to get rid of all those callback classes.

Edit:
@edisongustavo:

boost::function and boost::bind won’t probably make your code more readable. But it will give you an opportunity to pass free functions ( I mean functions out of class and static class functions ) as callback as well as existing functions of any class.

With boost functions you can pass functions with for example 2 parameters as callback which expect only one parameter.

typedef boost::function< void ( int ) > IntExpectingCallback;  void TwoIntFunction( int, int ); { } ... IntExpectingCallback callback = boost::bind( TwoIntFunction, 5, _1 ); 

But again, this won’t make your code more readable unless all your team knows and favor boost.