How to make work simple button class with callback? - c++

Question

How to make simple button class to call custom function on press. Some code example:
class Button
{
public:
Button();
Draw();
Press();
SetCallback(void(*cback)());
priate:
void (*callback)();
}
void Button::SetCallback(void(*cback)())
{
callback = cback;
}
class GameState
{
...
}
class MenuState : public GameState
{
Button *btn;
}
class Game
{
...
}
bool Game::Init()
{
std::unique_ptr<GameState> menu = std::unique_ptr<MenuState>(new MenuState);
}
void Game::PopState(){
states.pop_back();
}
How to set callback in button to Game::PopState() function in this example. I need Then I press button it call Game::PopState function.

Answer 1

What you're looking for is a tutorial on function pointers, which can be found here.
If you need to pass an argument, your button function pointer should either have a pointer or a reference to your Game object, as member functions require references( to which object owns the function call). If the popstate function in your Game class is private you may need to make it public or a friend function.
You should be able to modify the example below by replacing the string reference with a reference to the Game object. If you need a more generic callback look into templates(better but a bit more complicated) or type punning(generally considered bad practice)
#include <iostream>
#include <string>
class Button
{
public:
void SetCallback(void(*cback)(std::string&));
void Press(std::string str){
//Make sure some sort of check is done in order to prevent undefined behavior from a call
if(callback != nullptr)
callback(str);
}
Button(){
callback = nullptr;
}
private:
void (*callback)(std::string&);
};
void Button::SetCallback(void(*cback)(std::string&))
{
callback = cback;
}
//Method 2 for calling function
void DoStuff(std::string& str){
std::cout << str;
}
int main(void){
Button btn;
//Method 1 for calling function, replace body with Game.PopStates();
auto foo = [](std::string& str){ std::cout<<str.size();};
btn.SetCallback(foo);
btn.Press("A");
//Method 2 for calling function
btn.SetCallback(&DoStuff);
btn.Press("\nPressed Button");
return 0;
}
Your function pointer for Game should be something like this:
void SetCallback(void(*cback)(Game&));
You can call it like this:
auto foo = [](Game& g){ g.DoStuffInClass(); };
btn.SetCallback(foo);
btn.Press(game);
The above examples work fine and are usually all you need to do, but this article here goes more in-depth about member function pointers. The main takeaway from the article is typedef is another way for writing member function pointers like shown:
typedef int (Fred::*FredMemFn)(char x, float y); // Please do this!
All the above does(when implemented with PopStates), however, is lock your button callback into using void functions called by your Game objects, which limits your ability to use the button. You can use a pointer instead of a reference if you want to allow special cases for acting without a Game object, but either way your Game object has to find some way of being put in inside your callback.