[Tutorial] C++ Callbacks

[Kitchen_]

[Tutorial] C++ Callbacks

I'll be explaining how to create and use callbacks.

First, I'll start by explaining what a callback is. A callback is basically a pointer to a function that executes when a certain event

happens. This could be a button click, a window resize, or anything else. For this example, I'll be creating a button class that has an OnCreate callback.

The first thing to do is create the button, here is what I have:

Button.h

#pragma once

class Button
{
public:
    //
    // Fields
    //
    int X;
    int Y;
    int Width;
    int Height;
    
    //
    // Functions
    //
    Button(int x, int y, int w, int h);
};

Button.cpp

#include "Button.h"

Button::Button(int x, int y, int w, int h)
{
    X = x;
    Y = y;
    Width = w;
    Height = h;
}

This is the basic outline for a button. Now, lets create a new instance of it:

Main.cpp

#include <stdlib.h>
#include <stdio.h>
#include "Button.h"

int main(int argc, char* argv[])
{
    Button* button1 = new Button(5, 5, 100, 26);

    printf("button1 created\n");

    // So we can see the output. This isn't needed in Visual 
    // Studio when you start without debugging.
    system("PAUSE");

    return 0;
}

This is pretty basic so far, nothing special happens. Now lets add an event, for when it's created:

Button.h

#pragma once

class Button
{
public:
    //
    // Fields
    //
    int X;
    int Y;
    int Width;
    int Height;
    // Our callback
    void (*OnCreateCallback)(void);
    
    //
    // Functions
    //
    Button(int x, int y, int w, int h, void (*callback)(void));
    void OnCreate();
};

Button.cpp

#include "Button.h"

Button::Button(int x, int y, int w, int h, void (*callback)(void))
{
    X = x;
    Y = y;
    Width = w;
    Height = h;
    OnCreateCallback = callback;

    // Trigger the callback
    OnCreate();
}

void Button::OnCreate()
{
    // Make sure the callback isn't null
    if(OnCreateCallback != 0)
    {
        OnCreateCallback();
    }    
}

Main.cpp

#include <stdlib.h>
#include <stdio.h>
#include "Button.h"

Button* button1;

void Button1Created()
{
    printf("button1 created\n");
}

int main(int argc, char* argv[])
{
    // When we call the constructor, we're passing the
    // name of the function that handles the OnCreate
    // event. If you pass 0, the event won't get called
    button1 = new Button(5, 5, 100, 26, Button1Created);

    // So we can see the output. This isn't needed in Visual 
    // Studio when you start without debugging.
    system("PAUSE");

    return 0;
}

And that is basically how you add a callback in C++. I hope this was understandable and that it makes sense!

Attached is the Visual Studio project.

 

[gilgamash]

And for those further interested in this, I recommend having a look into for instance the command pattern, which is the pattern variant of callbacks.

G.