C++ event handling interface with function pointers - c++

Question

I want to implement a simple event interface with function pointers for a simple game in C++. This will be done to improve the event interface of the allegro lib. Therefore I've written the following code but it doesn't work.
typedef void (*event_handler)(int); //type for the event handler
const int TESTKEY_A = 1; // example Key as event arg
class Game
{
private:
bool is_running ;
protected:
event_handler on_key_down[2];
public:
void run();
void do_events(int e) ;
void stop() {is_running = false;}
};
void Game::run()
{
is_running=true;
while(is_running)
do_events(1);
}
void Game::do_events(int e)
{
if(e==1)
{
for(int i = 0; i < 2 ;i++)
on_key_down[i](TESTKEY_A);
}
}
class Pong_Game : public Game
{
public:
Pong_Game();
void On_Key_Down_Player1(int key) { return;}
void On_Key_Down_Player2(int key) { return;}
};
Pong_Game::Pong_Game()
{
on_key_down[0] = &this->On_Key_Down_Player1;
on_key_down[1] = &this->On_Key_Down_Player2;
}
int main()
{
Game *my_game = new Pong_Game();
my_game->run();
return 0;
}
Compiler log:
Compiler: Default compiler
Executing g++.exe...
g++.exe "U:\Eigene Dateien\eventhandler.cpp" -o "U:\Eigene Dateien\eventhandler.exe" -I"C:\Dev-Cpp\lib\gcc\mingw32\3.4.2\include" -I"C:\Dev-Cpp\include\c++\3.4.2\backward" -I"C:\Dev-Cpp\include\c++\3.4.2\mingw32" -I"C:\Dev-Cpp\include\c++\3.4.2" -I"C:\Dev-Cpp\include" -L"C:\Dev-Cpp\lib"
U:\Eigene Dateien\eventhandler.cpp: In constructor `Pong_Game::Pong_Game()':
U:\Eigene Dateien\eventhandler.cpp:45: error: ISO C++ forbids taking the address of a bound member function to form a pointer to member function. Say `&Pong_Game::On_Key_Down_Player1'
U:\Eigene Dateien\eventhandler.cpp:45: error: cannot convert `void (Pong_Game::*)(int)' to `void (*)(int)' in assignment
U:\Eigene Dateien\eventhandler.cpp:46: error: ISO C++ forbids taking the address of a bound member function to form a pointer to member function. Say `&Pong_Game::On_Key_Down_Player2'
U:\Eigene Dateien\eventhandler.cpp:46: error: cannot convert `void (Pong_Game::*)(int)' to `void (*)(int)' in assignment
Execution terminated
EDIT:
- changed code
- added compiler log
Thank you!

Answer 1

You are using a member function to initialize a "plain" function pointer. Member functions are different from "plain" functions in that they have a hidden T *this passed along with the function.
This seems like something that would be MUCH better solved by having a "event handler" interface, implemented by inheritance, than using function pointers.
Something like this:
class Game
{
private:
bool is_running ;
public:
void run(){is_running=true; while(is_running) do_events(1);}
void do_events(int e) {if(e==1) On_Key_Down(TESTKEY_A);}
void stop() {is_running = false;}
virtual void On_Key_Down(int key) = 0;
... other event handlers here ...
};
class Pong_Game : public Game
{
public:
void Pong_Game() {}
void On_Key_Down(int key) { // handle key event...}
... other event handlers here ...
};
As per comments:
To cover multiple players, you may want to implement a "per player event handling":
class Player
{
public:
enum moves { move_up,
move_down,
move_left,
move_right,
move_jump,
move_shoot, ... };
...
virtual void On_Key_Down(int key) = 0;
...
};
class Player_A
{
public:
...
virtual moves On_Key_Down(int key) { if (key == 'W') return move_up; ... }
}
class Player_B
{
public:
...
virtual moves On_Key_Down(int key) { if (key == 'I') return move_up; ... }
}
class Pong_Game : public Game
{
private:
vector<Player *> players;
public:
void Pong_Game() {}
void On_Key_Down(int key)
{
for(p : players) {
Player::moves m = p->On_key_down();
...
}
}
... other event handlers here ...
};
(This is just quickly hacked up - the "moves" may perhaps be better placed somewhere else, and the exact structure is just "what I could think of right now" - you probably need a class Player_A_Pong : public Player_A or some such to deal with the specifics of "player A's racket is here, player B's racket is here..." - although there are probably other ways to deal with that too).