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Raptisoft

Is this a VS compiler bug with references?

4 posts in this topic

Hi all,

 

I ran into a funny problem today... using visual studio 2010, I did this:

Quad& aQuad=aSprite1.GetTextureQuad();
printf("Quad Center: %f,%f",aQuad.Center().mX,aQuad.Center().mY);

//
// Re-use the reference...
//
aQuad=aSprite2.GetTextureQuad();
printf("Quad Center (2): %f,%f",aQuad.Center().mX,aQuad.Center().mY);


....and I get the same value for both printfs-- the value that came from aSprite2.  It's like it completely discarded the referenced data to aSprite1.

 

If I change it to this:

Quad& aQuad=aSprite1.GetTextureQuad();
printf("Quad Center: %f,%f",aQuad.Center().mX,aQuad.Center().mY);

//
// Use a whole new reference!
//
Quad& aQuad2=aSprite2.GetTextureQuad();
printf("Quad Center (2): %f,%f",aQuad2.Center().mX,aQuad2.Center().mY);

...Then it works as expecting, printing out two different center points for two different quads.

 

So my question is, am I running into a compiler bug here, or is this a known shortcoming of references that I've just never heard of until this point?

 

Thanks!

Edited by Raptisoft
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Thanks!  So it was just something I didn't know.  I think this is the first time I ever even tried to initialize one twice (I tend to treat references a pointers that can never be null).

 

However, just to be clear-- I WAS trying to overwrite the contents-- after I was done with them.  I was just re-using a variable.  I expected the contents of the Quad& reference to be filled in the same way that *Quad's contents would have been filled-- procedurally as the program went along.

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Thanks!  So it was just something I didn't know.  I think this is the first time I ever even tried to initialize one twice (I tend to treat references a pointers that can never be null).


While I do recommend understanding that C++ references are in every real-world implementation basically just pointers that can't be assigned null (though you can get a null reference with a small bit of trickery), I also highly recommend understanding that conceptually that's not what a reference is. It's an alias for another object. There's a much bigger and better reason why C++ has references besides being non-nullable pointers.
 

However, just to be clear-- I WAS trying to overwrite the contents-- after I was done with them.  I was just re-using a variable.  I expected the contents of the Quad& reference to be filled in the same way that *Quad's contents would have been filled-- procedurally as the program went along.


That is how they should behave. The code as you posted it (if I make a few assumptions about the rest of the code) should output what you expected. Get the first quad, print those values, _then_ overwrite it with the second quad's values, then write those new values.

But that all depends on what the return type of GetTextureQuad is. Because you're using non-const references to store the return value you might be running into undefined behavior if GetTextureQuad does not also return a reference. The compiler may be reordering the code in ways it's totally allowed to (it's allowed to do anything it damn well wants with undefined behavior) if you're writing "illegal" code. I can't say for sure that's what is happening without seeing a complete _simplified_ code example (few people are interested in reading your entire codebase) that exhibits the problem. Certainly however I've never once seen any compiler erroneously reorder code just because of a reference (in valid code), and we've been using VS2010 for years now on some _very_ big and complex game codebases.

Compile with the highest warning level and "warnings as errors" turned on. Always. It'll help catch language violations early. Even seasoned professionals (or _especially_ seasoned professionals) turn those on.

Edited by SeanMiddleditch
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