How do I implement a callback in C++?

Question

I want to implement a class in c++ that has a callback.

So I think I need a method that has 2 arguments:

• the target object. (let\'s say *myObj)
<

Answer 1

The best solution, use boost::function with boost::bind, or if your compiler supports tr1/c++0x use std::tr1::function and std::tr1::bind.

So it becomes as simple as:

boost::function callback;
Target myTarget;
callback=boost::bind(&Target::doSomething,&myTarget);

callback(); // calls the function

And your set callback becomes:

class MyClassWithCallback{
public:
  void setCallback(boost::function const &cb)
  {
     callback_ = cb;
  }
  void call_it() { callback_(); }
private:
  boost::function callback_;
};

Otherwise you need to implement some abstract class

struct callback { 
 virtual void call() = 0;
 virtual ~callback() {}
};

struct TargetCallback {
 virtual void call() { ((*self).*member)()); }
 void (Target::*member)();
 Target *self;
 TargetCallback(void (Target::*m)(),Target *p) : 
       member(m),
       self(p)
 {}
};

And then use:

myCaller.setCallback(new TargetCallback(&Target::doSomething,&myTarget));

When your class get modified into:

class MyClassWithCallback{
public:
  void setCallback(callback *cb)
  {
     callback_.reset(cb);
  }
  void call_it() { callback_->call(); }
private:
  std::auto_ptr callback_;
};

And of course if the function you want to call does not change you may just implement some interface, i.e. derive Target from some abstract class with this call.