GCC & VS weirdness

posted in Odorless Entertainment

Published March 12, 2008

Yesterday while coding the destructor for the TextureManager I noticed some rather odd difference in VS and GCC/G++.

First off the piece of code that VS was having trouble with that G++ didn't mind or complain at all about was this:

const unsigned int size = _vLoadedTextures.size();
GLuint textures[size];

Seems, simple enough right?

G++ doesn't throw any errors or warnings about this but VS gives me at least 3 errors.

Error 1 error C2057: expected constant expression
Error 2 error C2466: cannot allocate an array of constant size 0
Error 3 error C2133: 'textures' : unknown size

The first error makes me laugh.

Why is this an issue in VS? Does it not like sizing arrays at run time? I know I could just use malloc but seriously, wtf?

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extralongpants

I believe this is because array.size() could result in a value of 0, and according to MSDN, arrays of size 0 are typically not allowed:

Quote:
The number of elements in the array is given by the constant expression. The first element in the array is the 0th element, and the last element is the (n-1) element, where n is the number of elements the array can contain. The constant-expression must be of an integral type and must be greater than 0. A zero-sized array is legal only when the array is the last field in a struct or union and when the Microsoft extensions (/Ze) are enabled.

Although your integer is constant, it cannot be verified at compile time that your integer will be assigned a valid value ( > 0).

March 12, 2008 10:10 AM

_swx_

No, the problem is that size is not a constant expression.

From http://msdn2.microsoft.com/en-us/library/tby3xex3(VS.80).aspx

"A constant expression is evaluated at compile time, not run time, ..."

So the issue is that your code isn't valid C++.

March 12, 2008 11:10 AM

bladerunner627

Quote:Original post by _swx_
No, the problem is that size is not a constant expression.

From http://msdn2.microsoft.com/en-us/library/tby3xex3(VS.80).aspx

"A constant expression is evaluated at compile time, not run time, ..."

So the issue is that your code isn't valid C++.

"The operands of a constant expression can be .. sizeof expressions, and other constant expressions."

Still, I think it's more of a VS issue, VS has trouble conforming to C++ standards.

http://developer.apple.com/DOCUMENTATION/DeveloperTools/gcc-4.0.1/gccint/Constants.html
This site shows a few types of constant expressions, vector being one of them.

Meh, well, either way it's rather annoying.

March 12, 2008 11:59 AM

_swx_

sizeof is done at compile time, so it's a constant expression. The issue is not vc++, if anything it's why gcc would allow it...

March 12, 2008 12:11 PM

rip-off

Pass g++ the "pedantic" switch, and you get an error:

Quote:
help.cpp: In function 'void example(unsigned int)':
help.cpp:5: error: ISO C++ forbids variable-size array 'buffer'

Where buffer is a char array whose size is the value of the parameter passed to 'example'.

March 12, 2008 03:16 PM

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bladerunner627

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GCC & VS weirdness

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bladerunner627

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