AFAIK this code is not a valid c++ code by standard:
int a = 5;
int b[a];
but it seems many compilers can compile that code (mostly with warning) and it just behaves as expected. Am I wrong is is it compilers being nice to me?
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exact duplicate: Dynamic array In Stack, c++ initialize array with declared size as a value of an integer, ...moooeeeep– moooeeeep2011-11-23 08:24:45 +00:00Commented Nov 23, 2011 at 8:24
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@fmaas: didn't find those questions sorryAli1S232– Ali1S2322011-11-23 08:45:53 +00:00Commented Nov 23, 2011 at 8:45
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Actually I was aware of them just because I stumbled on this a few days ago by pure coincidence...moooeeeep– moooeeeep2011-11-23 09:03:30 +00:00Commented Nov 23, 2011 at 9:03
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3 Answers
It is called variable length array (VLA) which is not allowed by Standard C++, any version of C++, though some GCC supports this as an extension.
If you're using GCC, then
- Compile it with
-pedanticoption, you will see warning. - Compile it with
-pedantic -Werroroption, you will see warning turned into error.
VLA is allowed only by C99, though not by other versions of C.
3 Comments
James Kanze
Won't
-std=c++98 (or -std=c++0x if you're experimenting, rather than writing production code) suffice to suppress support of VLAs in g++?
Sarfaraz Nawaz
@JamesKanze: No. It doesn't. I tried with
-std=c++98, -std=c++03, and -std=c++0x, but none of them suppresses VLA in g++. It is weird, though. I'm using MinGw version 4.6.1 :-)James Kanze
That sort of surprises me. But I'll admit that I've never tried it. (I always compile with
-std=c++98 -pedantic. And in C++, I don't use T[] very often, either.)The compiler is being nice. :)
It's actually part of the C standard, and some compilers (like GCC) extend C++ with this feature.
answered Nov 23, 2011 at 8:20
Some programmer dude
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1 Comment
Sarfaraz Nawaz
More specifically and accurately, it is part of C99 Standard.
This is the C99 standards Variable Length Array (or VLA), many compilers that can compile C99, often allow some of its features to be used in non-standard conforming C++ code.
G++ is one of these compilers, see here.
