Jump to content
  • Advertisement
BlackDE

C++ C++ saving this ptr offset

Recommended Posts

Hi, I'm creating a component based entity model for my game(not to confuse with an ECS). Because I decided that I won't need dynamic addition and removal of components, I got the idea to implement components with multiple inheritance: I have an Entity Base class which only has a few functions for entity lifetime etc. Every component virtually inherits from that class. At the end an entity inherits from all the components it needs. Now from time to time a component has to access another component like this:

dynamic_cast<OtherComponent*>(this)->doOtherComponentsStuff() 

The Movement component for example has to access the velocity component every frame. This results in a lot of dynamic_casts and poor performance. But because the components of an entity never change I would call dynamic_cast only once(in the ctor), save the offset of the resulting address and use it later to access the other component without the performance penalty of dynamic_cast. Here's what I mean:

class Entity
{
public:
	virtual ~Entity() = default;
};

class Derived2 : public virtual Entity
{
public:
	void doThat()
	{
		std::cout << "Did that\n";
	}
};

class Derived1 : public virtual Entity
{
public:
	Derived1()
	{
		m_derived2Off = reinterpret_cast<intptr_t>(dynamic_cast<Derived2*>(static_cast<Entity*>(this))) - reinterpret_cast<intptr_t>(this);
	}
	void doIt()
	{
		getDerived2()->doThat();
	}
private:
	intptr_t m_derived2Off;
	Derived2 *getDerived2()const
	{
		return reinterpret_cast<Derived2*>(m_derived2Off + reinterpret_cast<intptr_t>(this));
	}
};

class Player final:
public Derived1,
public Derived2
{
};

int main()
{
	Player obj;

	obj.doIt();
}

It worked for me, but I'm not sure if there might be some undefined behavior involved.

Share this post


Link to post
Share on other sites
Advertisement

It looks like you're doing inheritance wrong.  

Done correctly you don't need to know what the concrete type is.  If you find yourself doing a dynamic cast to a derived type, you are likely doing something wrong.

The general principles have the acronym SOLID.   I suggest you start reading from that article.

You should have a well-designed abstract base interface that other code is derived from.  All the derived versions should operate correctly on that interface.  All objects should be replaceable with similar objects and still be correct.  This is called Liskov substitution principle named after a famous person who described it. It is also called the Template Method pattern.  This is also used in ECS systems and is a major reason they are popular, all components follow the well-defined interface.

Then you should always work with the abstract type, not the derived classes.  This is called the dependency inversion principle. This is used in many systems, including ECS systems, to prevent brittleness.  In your example you would need to modify Derived1 any time you added any new functionality. It would quickly go to a long list of redirections detecting if it is Derived2, or Derived3, or Derived4, or Derived103. But if you always work with the abstract types the problem goes away.

After that, I suggest you read this article. That's an example of the right way to implement virtual functions.

And after that, recognize that most code in games is not about individual actions but about bulk actions. Many times people build objects and systems that work on single items, single data structures, single instances. But the code rarely operates on a single thing, instead working on bulk items, bulk data structures and array, collections of instances. Take whatever reasonable steps you can to convert to bulk actions rather than individual actions, which avoids considerable overhead and can be leveraged for cache benefits.

Share this post


Link to post
Share on other sites
15 hours ago, frob said:

And after that, recognize that most code in games is not about individual actions but about bulk actions. Many times people build objects and systems that work on single items, single data structures, single instances. But the code rarely operates on a single thing, instead working on bulk items, bulk data structures and array, collections of instances. Take whatever reasonable steps you can to convert to bulk actions rather than individual actions, which avoids considerable overhead and can be leveraged for cache benefits.

This also means that optimizing access from a component of an entity to other components of the same entity has limited usefulness. Most computations are going to involve components of different entities, and often the same type of component for many entities (e.g. collision detection and response using the respective geometric shapes of entities).

Share this post


Link to post
Share on other sites

This seems like a large misunderstanding of the use of both inheritance and the component entity model. I think it would be beneficial for you to look up and develop your understanding of both paradigms. 

 

From an inheritance standpoint , You should never find yourself in an instance in which you’re deriving from two different classes with the same base class. From an entity component standpoint, the purpose of the entity is to be a container for the components, not a subclass... and there shouldn’t be any manipulation of “self” using reinterpret cast and etc to access a required component, among other things.

Share this post


Link to post
Share on other sites

Even if you did need to do something like that, why jump through hoops to calculate offsets and adjust the "this" pointer? It looks like you could just store the value of the dynamic_cast<>ed pointer as a member instead of the offset.

Share this post


Link to post
Share on other sites

Sorry for replying so late. My goal is not to create a class hierarchy. I'm trying to create entities from components with the help of mixins. As for just storing the value from dynamic_cast: the pointer would invalidate after copying an entity.

Share this post


Link to post
Share on other sites
42 minutes ago, BlackDE said:

My goal is not to create a class hierarchy. I'm trying to create entities from components with the help of mixins.

Mixin classes imply inheritance, and inheritance defines a class hierarchy whether you want it or not. You don't have the privilege of "avoiding" inheritance by hacking around with reinterpret_cast and the like instead of using polymorphic pointers properly.

Share this post


Link to post
Share on other sites
6 hours ago, BlackDE said:

Sorry for replying so late. My goal is not to create a class hierarchy. I'm trying to create entities from components with the help of mixins. As for just storing the value from dynamic_cast: the pointer would invalidate after copying an entity.

This does not look like any ECS I've ever seen and I've read a lot of them and reviewed a lot of code.  This looks like a God class in disguise.

Share this post


Link to post
Share on other sites

Create an account or sign in to comment

You need to be a member in order to leave a comment

Create an account

Sign up for a new account in our community. It's easy!

Register a new account

Sign in

Already have an account? Sign in here.

Sign In Now

  • Advertisement
  • Advertisement
  • Popular Tags

  • Popular Now

  • Advertisement
  • Similar Content

    • By Joydeep Mani
      I am trying to build a particle system for unity based on "Destiny particle architecture " released in Siggraph.
      I encounter a problem in trying to understand how they iterated this:

      Converted to spline function and the result is

      i wanted to know how did they converted the function in the editor to represent the graph ??
       
    • By sausagejohnson
      Overview
      Welcome to the 2D UFO game guide using the Orx Portable Game Engine. My aim for this tutorial is to take you through all the steps to build a UFO game from scratch.
      The aim of our game is to allow the player to control a UFO by applying physical forces to move it around. The player must collect pickups to increase their score to win.
      I should openly acknowledge that this series is cheekily inspired by the 2D UFO tutorial written for Unity.
      It makes an excellent comparison of the approaches between Orx and Unity. It is also a perfect way to highlight one of the major parts that makes Orx unique among other game engines, its Data Driven Configuration System.
      You'll get very familiar with this system very soon. It's at the very heart of just about every game written using Orx.
      If you are very new to game development, don't worry. We'll take it nice and slow and try to explain everything in very simple terms. The only knowledge you will need is some simple C++.
      I'd like say a huge thank you to FullyBugged for providing the graphics for this series of articles.
       
      What are we making?
      Visit the video below to see the look and gameplay of the final game:
      Getting Orx
      The latest up to date version of Orx can be cloned from github and set up with:
      git clone https://github.com/orx/orx.git Once cloning has completed, the setup script in the root of the files will start automatically for you. This script creates an $ORX environment variable for your system. The variable will point to the code subfolder where you cloned Orx.
      Why? I'll get to the in a moment, but it'll make your life easier.
      The setup script also creates several projects for various IDEs and operating system: Visual Studio, Codelite, Code::Blocks, and gmake. You can pick one of these projects to build the Orx library.
      Building the Orx Library
      While the Orx headers are provided, you need to compile the Orx library so that your own games can link to it. Because the setup script has already created a suitable a project for you (using premake), you can simply open one for your chosen OS/IDE and compile the Orx library yourself.
      There are three configurations to compile: Debug, Profile and Release. You will need to compile all three.
      For more details on compiling the Orx lbrary at: http://orx-project.org/wiki/en/tutorials/cloning_orx_from_github at the Orx learning wiki.
      The $ORX Environment Variable
      I promised I would explain what this is for. Once you have compiled all three orx library files, you will find them in the code/lib/dynamic folder:
      orx.dll orxd.dll orxp.dll Also, link libraries will be available in the same folder:
      orx.lib orxd.lib orxp.lib When it comes time to create our own game project, we would normally be forced to copy these library files and includes into every project.
      A better way is to have our projects point to the libraries and includes located at the folder that the $ORX environment variable points to (for example: C:\Dev\orx\code).
      This means that your projects will always know where to find the Orx library. And should you ever clone and re-compile a new version of Orx, your game projects can make immediate use of the newer version.
      Setting up a 2D UFO Project
      Now the you have the Orx libraries cloned and compiled, you will need a blank project for your game. Supported options are: Visual Studio, CodeLite, Code::Blocks, XCode or gmake, depending on your operating system.
      Once you have a game project, you can use it to work through the steps in this tutorial.
      Orx provides a very nice system for auto creating game projects for you. In the root of the Orx repo, you will find either the init.bat (for Windows) or init.sh (Mac/Linux) command.
      Create a project for our 2D game from the command line in the Orx folder and running:
      init c:\temp\ufo or
      init.sh ~/ufo Orx will create a project for each IDE supported by your OS at the specified location. You can copy this folder anywhere, and your project will always compile and link due to the $ORX environment variable. It knows where the libraries and includes are for Orx.
      Open your project using your favourite IDE from within the ufo/build folder.
      When the blank template loads, there are two main folders to note in your solution:
      config src Firstly, the src folder contains a single source file, ufo.cpp. This is where we will add the c++ code for the game. The config folder contains configuration files for our game.
        What is config?
      Orx is a data driven 2D game engine. Many of the elements in your game, like objects, spawners, music etc, do not need to be defined in code. They can be defined (or configured) using config files.
      You can make a range of complex multi-part objects with special behaviours and effects in Orx, and bring them into your game with a single line of code. You'll see this in the following chapters of this guide.
      There are three ufo config files in the config folder but for this guide, only one will actually be used in our game. This is:
      ufo.ini All our game configuration will be done there.
      Over in the Orx library repo folder under orx/code/bin, there are two other config files:
      CreationTemplate.ini SettingsTemplate.ini These are example configs and they list all the properties and values that are available to you. We will mainly concentrate on referring to the CreationTemplate.ini, which is for objects, sounds, etc. It's good idea to include these two files into your project for easy reference.
      Alternatively you can view these online at https://github.com/orx/orx/blob/master/code/bin/CreationTemplate.ini and here: https://github.com/orx/orx/blob/master/code/bin/SettingsTemplate.ini
       
      The code template
      Now to take a look at the basic ufo.cpp and see what is contained there.
      The first function is the Init() function.
      This function will execute when the game starts up. Here you can create objects have been defined in the config, or perform other set up tasks like handlers. We'll do both of these soon.
      The Run() function is executed every main clock cycle. This is a good place to continually perform a task. Though there are better alternatives for this, and we will cover those later. This is mainly used to check for the quit key.
      The Exit() function is where memory is cleaned up when your game quits. Orx cleans up nicely after itself. We won't use this function as part of this guide.
      The Bootstrap() function is an optional function to use. This is used to tell Orx where to find the first config file for use in our game (ufo.ini). There is another way to do this, but for now, we'll use this function to inform Orx of the config.
      Then of course, the main() function. We do not need to use this function in this guide.
      Now that we have everything we need to get start, you should be able to compile successfully. Run the program and an Orx logo will appear slowly rotating.

      Great. So now you have everything you need to start building the UFO game.
      If you experience an issue compiling, check the troubleshooting article for Orx projects    for help.
       
      Setting up the game assets
      Our game will have a background, a UFO which the player will control, and some pickups that the player can collect.
      The UFO will be controlled by the player using the cursor keys.
      First you'll need the assets to make the game. You can download the file  assets-for-orx-ufo-game.zip which contains:
      The background file (background.png😞

      The UFO and Pickup sprite images (ufo.png and pickup.png😞
       
      And a pickup sound effect (pickup.ogg😞
      pickup.ogg
      Copy the .png files into your data/texture folder
      Copy the .ogg file into your data/sound folder.
      Now these files can be accessed by your project and included in the game.
       
      Setting up the Playfield
      We will start by setting up the background object. This is done using config.
      Open the ufo.ini config file in your editor and add the following:
       
      [BackgroundGraphic] Texture = background.png Pivot = center  
      The BackgroundGraphic defined here is called a Graphic Section. It has two properties defined. The first is Texture which has been set as background.png.
      The Orx library knows where to find this image, due to the properties set in the Resource section:
       
      [Resource] Texture = ../../data/texture  
      So any texture files that are required (just like in our BackgroundGraphic section) will be located in the ../../data/texture folder.
      The second parameter is Pivot. A pivot is the handle (or sometimes “hotspot” in other frameworks). This is set to be center. The position is 0,0 by default, just like the camera. The effect is to ensure the background sits in the center of our game window.
      There are other values available for Pivot. To see the list of values, open the CreationTemplate.ini file in your editor. Scroll to the GraphicTemplate section and find Pivot in the list. There you can see all the possible values that could be used.
      top left is also a typical value.
      We need to define an object that will make use of this graphic. This will be the actual entity that is used in the game:
       
      [BackgroundObject] Graphic = BackgroundGraphic Position = (0, 0, 0)  
      The Graphic property is the section BackgroundGraphic that we defined earlier. Our object will use that graphic.
      The second property is the Position. In our world, this object will be created at (0, 0, 0). In Orx, the coordinates are (x, y, z). It may seem strange that Orx, being a 2D game engine has a Z axis. Actually Orx is 2.5D. It respects the Z axis for objects, and can use this for layering above or below other objects in the game.
      To make the object appear in our game, we will add a line of code in our source file to create it.
      In the Init() function of ufo.cpp, remove the default line:
      orxObject_CreateFromConfig("Object"); and replace it with:
      orxObject_CreateFromConfig("BackgroundObject"); Compile and run.
      The old spinning logo is now replaced with a nice tiled background object.

      Next, the ufo object is required. This is what the player will control. This will be covered in Part 2.
    • By BearishSun
      bs::framework is a newly released, free and open-source C++ game development framework. It aims to provide a modern C++14 API & codebase, focus on high-end technologies comparable to commercial engine offerings and a highly optimized core capable of running demanding projects. Additionally it aims to offer a clean, simple architecture with lightweight implementations that allow the framework to be easily enhanced with new features and therefore be ready for future growth.
      Some of the currently available features include a physically based renderer based on Vulkan, DirectX and OpenGL, unified shading language, systems for animation, audio, GUI, physics, scripting, heavily multi-threaded core, full API documentation + user manuals, support for Windows, Linux and macOS and more.
      The next few updates are focusing on adding support for scripting languages like C#, Python and Lua, further enhancing the rendering fidelity and adding sub-systems for particle and terrain rendering.
      A complete editor based on the framework is also in development, currently available in pre-alpha stage.
      You can find out more information on www.bsframework.io.

      View full story
    • By BearishSun
      bs::framework is a newly released, free and open-source C++ game development framework. It aims to provide a modern C++14 API & codebase, focus on high-end technologies comparable to commercial engine offerings and a highly optimized core capable of running demanding projects. Additionally it aims to offer a clean, simple architecture with lightweight implementations that allow the framework to be easily enhanced with new features and therefore be ready for future growth.
      Some of the currently available features include a physically based renderer based on Vulkan, DirectX and OpenGL, unified shading language, systems for animation, audio, GUI, physics, scripting, heavily multi-threaded core, full API documentation + user manuals, support for Windows, Linux and macOS and more.
      The next few updates are focusing on adding support for scripting languages like C#, Python and Lua, further enhancing the rendering fidelity and adding sub-systems for particle and terrain rendering.
      A complete editor based on the framework is also in development, currently available in pre-alpha stage.
      You can find out more information on www.bsframework.io.
    • By Gnollrunner
      Hi again,  After some looking around I have decided to base my game directly on Direct X rather than using an existing game engine.  Because of the nature of the stuff I'm doing it just didn't seem to fit very well and I kept running into road blocks.  At this point I have a big blob of code for doing fractal world generation and some collision code,  and I'm trying to put it into some form that resembles a game engine.  Since I've never used one before It's a bit alien to me ..... so can someone direct me to a book, website, article, whatever... that covers this?  I'm mainly looking for stuff that covers C++ library design. I'm not adverse to using 3rd party tools for stuff I can used them for.
  • Advertisement
×

Important Information

By using GameDev.net, you agree to our community Guidelines, Terms of Use, and Privacy Policy.

Participate in the game development conversation and more when you create an account on GameDev.net!

Sign me up!