• Announcements

    • khawk

      Download the Game Design and Indie Game Marketing Freebook   07/19/17

      GameDev.net and CRC Press have teamed up to bring a free ebook of content curated from top titles published by CRC Press. The freebook, Practices of Game Design & Indie Game Marketing, includes chapters from The Art of Game Design: A Book of Lenses, A Practical Guide to Indie Game Marketing, and An Architectural Approach to Level Design. The GameDev.net FreeBook is relevant to game designers, developers, and those interested in learning more about the challenges in game development. We know game development can be a tough discipline and business, so we picked several chapters from CRC Press titles that we thought would be of interest to you, the GameDev.net audience, in your journey to design, develop, and market your next game. The free ebook is available through CRC Press by clicking here. The Curated Books The Art of Game Design: A Book of Lenses, Second Edition, by Jesse Schell Presents 100+ sets of questions, or different lenses, for viewing a game’s design, encompassing diverse fields such as psychology, architecture, music, film, software engineering, theme park design, mathematics, anthropology, and more. Written by one of the world's top game designers, this book describes the deepest and most fundamental principles of game design, demonstrating how tactics used in board, card, and athletic games also work in video games. It provides practical instruction on creating world-class games that will be played again and again. View it here. A Practical Guide to Indie Game Marketing, by Joel Dreskin Marketing is an essential but too frequently overlooked or minimized component of the release plan for indie games. A Practical Guide to Indie Game Marketing provides you with the tools needed to build visibility and sell your indie games. With special focus on those developers with small budgets and limited staff and resources, this book is packed with tangible recommendations and techniques that you can put to use immediately. As a seasoned professional of the indie game arena, author Joel Dreskin gives you insight into practical, real-world experiences of marketing numerous successful games and also provides stories of the failures. View it here. An Architectural Approach to Level Design This is one of the first books to integrate architectural and spatial design theory with the field of level design. The book presents architectural techniques and theories for level designers to use in their own work. It connects architecture and level design in different ways that address the practical elements of how designers construct space and the experiential elements of how and why humans interact with this space. Throughout the text, readers learn skills for spatial layout, evoking emotion through gamespaces, and creating better levels through architectural theory. View it here. Learn more and download the ebook by clicking here. Did you know? GameDev.net and CRC Press also recently teamed up to bring GDNet+ Members up to a 20% discount on all CRC Press books. Learn more about this and other benefits here.
Sign in to follow this  
Followers 0
lride

Calling member function via nullptr

7 posts in this topic

class A{   
void say(){std::cout<<"hi";}
};
 
int main()
{
    A * a=nullptr;
    a->say();   //no crash, works fine
}
 

Is this undefined behavior?

1

Share this post


Link to post
Share on other sites

Not sure.

 

It's safe because it doesn't use the this pointer and isn't virtual. It would be better off being static though, as soon as you use the this pointer or make it virtual it will crash.

 

Non-static member functions are just name mangled functions which have an implicit this pointer passed as an argument.

 

EDIT: i.e. there is no pointer derefernce going on. It just compiles to CallSomeNameMangledFunction(nullptr); Virtual functions are different they compile to this->vtbl.VirtualFunc(this); so there is a dereference going on.

Edited by Paradigm Shifter
2

Share this post


Link to post
Share on other sites
Although what Paradigm Shifter says is in practice what will happen, the standard says doing this is undefined behavior. Just don't do it.
2

Share this post


Link to post
Share on other sites

It is undefined. The -> operator is defined in terms of a dereference and the . operator, so you are clearly dereferencing a null pointer which is undefined behavior.

 

From a practical implementation point of view, no actual memory access may be performed to actually cause a crash, but your code is nevertheless ill-defined.

2

Share this post


Link to post
Share on other sites

I'll bow to others knowledge about it being undefined.

 

The simple and obvious implementation however shouldn't dereference anything at the point of calling the function. Inside the function you are toast if you do dereference this.

 

I could imagine some implementations doing some extra work (maybe in a debug configuration) which does attempt to use the this pointer for a simple member function call (logging, etc.).

 

Don't do it anyway, it's asking for trouble.

 

EDIT: And the code posted shouldn't compile ;) A::say isn't public ;)

Edited by Paradigm Shifter
0

Share this post


Link to post
Share on other sites

From a practical implementation point of view, no actual memory access may be performed to actually cause a crash, but your code is nevertheless ill-defined.

I think this is why it doesn't crash... If I use a member variable inside the function, it crashes

0

Share this post


Link to post
Share on other sites

If it doesn't crash when calling it with a nullptr, you should question whether it should be a member function at all ;) You may as well write a global SayHi function since it doesn't use any of the functionality of a class A at all.

 

You could make it static but then again it doesn't need any knowledge of the internal representation/implementation of an A either, and it may be useful to non-A's.

1

Share this post


Link to post
Share on other sites

It doesn't crash because the say() method doesn't need to dereference the this pointer. The method might as well be a static method when you think about it. In fact the optimizer probably compiles it as a static method since there's no need to pass the this pointer.

 

Try changing the method so that it loads a member variable though. Or try calling the method through a pointer to an instance method. (The ->* and .* operators.) Now you're probably asking for trouble.

0

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
Sign in to follow this  
Followers 0