Creating a signal from within a class to call an external function? - c++

Question

I was wandering if you could offer any guidance. I am currently writing a sprite class for a simple game engine. Previous engines I have used allow functionality to "Connect" an external function to a signal, emitted when an animation finishes.
E.g.
create the sprite object
create the external function
connect the external function to the sprites completion signal
When the signal is emitted the external function is called.
This function does not necessarily share any data with the sprite, its purely game logic timing functionality. E.g player scores a goal, "Congrats" sprite animation is triggered, then the on completion function will add an amount to the players score.
I have looked into it and it looks like I need to use a callback function/ function ptr but I don't have any experience using them as of yet.
Any help would be greatly appreciated.

Answer 1

In c++ a function pointer can be used like this:
#include <iostream>
//defining a type that is a 'function that returns void and takes an int'
typedef void FnPtr( int i );
//this is a 'function that returns void and takes an int'
void AFunction( int i )
{
std::cout << "AFunction Called!";
}
//a function that accepts a pointer to a 'function that returns void and takes an int'
void AFunctionThatTakesAFnPtrAsAnArg( FnPtr* fn )
{
//store the pointer to use "later".
FnPtr* local = fn;
.
.
.
//use it.
local( 3 );
}
int main( int, char** )
{
AFunctionThatTakesAFnPtrAsAnArg( AFunction );
}
Note the function can be global, or a static class member. If you want to call into a function in an object instance then see this - particularly my answer! :-)
What is a C++ delegate?
EDIT: To better fit questions asked in comments:
#include <iostream>
typedef void FnPtr();
void AFunction()
{
std::cout << "Animation done";
}
class Sprite
{
public:
void SetFnPointer( FnPtr* fn )
{
m_Fn = fn;
}
void DoAnimation()
{
m_Fn();
}
private:
FnPtr* m_Fn;
};
int main( int, char** )
{
Sprite s;
s.SetFnPointer( AFunction );
s.DoAnimation();
}

Answer 2

Take a look at http://www.boost.org/doc/libs/1_49_0/doc/html/signals.html
or http://libsigc.sourceforge.net/
The boost::signals library is really easy to use:
boost::signal<void(float)> update_sig;
Then somewhere in your game logic (some_client_class):
update_sig.connect( boost::bind(&some_client_class::callback, this, _1) );
And finally in your core update (main run loop of your game for example):
float delta = cur_time - last_update_time;
update_sig(delta); // executes all connected slots
Good luck :)