Declaring local arrays using non-const variables to set the length, and it works?

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why a[n] is accepted in c during runtime?
Declaring the array size with a non-constant variable

I just wrote some code to test some other code and I needed an array as input data. As the size of the input data may differ, I declared the variable as follows:

float input[num_pixels_row][num_pixels_col][3];

where num_pixels_row and num_pixels_col are non-const variables which are set using input from the command line. I ran the code and it worked.

Then after a little while I noticed what I had just done and thought "Hey, wait a minute! This shouldn't work!!" But the strange thing is that it does. Since input is declared inside a function it should be allocated on the stack, but how can the compiler determine the stack frame if the size of the array isn't known?

I asked two colleagues and they were just as puzzled. By the way, I compiled the code using g++ 4.6.1

Try compiling it with something other than g++ and see if that works. — Paul R, Dec 05 '11 at 10:28
Please check here [1] [1]: http://stackoverflow.com/questions/2863347/declaring-the-array-size-with-a-non-constant-variable — Prabhu, Dec 05 '11 at 10:32
See also: http://stackoverflow.com/questions/737240/c-c-array-size-at-run-time-w-o-dynamic-allocation-is-allowed — Paul R, Dec 05 '11 at 10:38
Could also check http://stackoverflow.com/questions/1864840/why-an-is-accepted-in-c-during-runtime — edA-qa mort-ora-y, Dec 05 '11 at 10:38

score 4 · Answer 1 · answered Dec 05 '11 at 10:29

4

That's a gcc-specific compiler extension which makes your code sub-standard and nonportable. For example, this won't compile in Visual C++.

answered Dec 05 '11 at 10:29

sharptooth

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score 3 · Accepted Answer · answered Dec 05 '11 at 10:34

3

Variable length arrays are a part of the C99 specification, which gcc also allows in C++ programs.

I don't think this has been added to C++11 though, unfortunately. Though I'd suspect that since many C++ compilers also strive for C compliance that they'll end up supporting this as well.

answered Dec 05 '11 at 10:34

edA-qa mort-ora-y

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Well, VLAs have been made optional in C1x, so it's possible that some C++ compilers (and even C compilers) will ignore VLAs. – R. Martinho Fernandes Dec 05 '11 at 10:42
Which section states they are optional? I couldn't find it when I looked. – edA-qa mort-ora-y Dec 05 '11 at 12:33
I have last April's draft. It's in §6.10.8.3. `#define __STDC_NO_VLA__ 1` is a conformant "implementation" of VLAs :) At the end of §6.7.6.2p4 it's explicitly stated implementations need not support them. – R. Martinho Fernandes Dec 05 '11 at 12:51
I think we need somebody with a final copy, since my draft from Feb 2011 doesn't contain reference to that define or VLAs. – edA-qa mort-ora-y Dec 05 '11 at 12:56
This draft from November http://www.open-std.org/jtc1/sc22/wg14/www/docs/n1539.pdf is the draft that is going for CD ballot. It may take a while until there is a final copy :) – R. Martinho Fernandes Dec 05 '11 at 13:12

Declaring local arrays using non-const variables to set the length, and it works?

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