Can anyone explain me this part of the code

Question

It is a callback function but I can't figure out how this part works

if (cb_onPress) { cb_onPress(*this); } //fire the onPress event

class Button;
typedef void (*buttonEventHandler)(Button&);

class Button {
  public:
 //code
   private:
 //code
 buttonEventHandler  cb_onPress;
};

 void Button::process(void)
 {
  //code
    if (cb_onPress) { cb_onPress(*this); }   //fire the onPress event

 }       

void Button::pressHandler(buttonEventHandler handler)
{
  cb_onPress = handler;
}

Answer 1

cb_onPress is a pointer to a function returning void and taking a Button& parameter. It could point to something like this:

void foo(Button&){ std::cout << "Foo Button!\n"; }

This line, inside a Button member function,

if (cb_onPress) { cb_onPress(*this); } 

checks that the pointer to function is not null, and if so, calls it, passing the same instance of Button as parameter (that is what passing *this achieves).

Example of use:

Button b;
b.pressHandler(foo);  // sets cb_onPress to point to foo
....
b.process();          // Prints "Foo Button"

although presumably the call to process happens internally, in response to an event.

Answer 2

if (cb_onPress) { cb_onPress(*this); }

cb_onPress is a pointer to a function. If the pointer's a nullptr you can't call it, so the code checks it's not beforehand.

The overall supported client usage is like this:

void myButtonEventHandler(Button& b) { ...do something when pressed... };

Button button;  // make a button
button.pressHandler(myButtonEventHandler);

Answer 3

if (cb_onPress)

checks if cb_onPress is null pointer. In other words checks if that function was defined before. If it isn't then it calls function

cb_onPress

on that object