Constant expressions in array declarations

Question

Array dimension must be known at compile time, which means that the dimension must be a constant expression

A separate point is made that

unsigned count = 42;           // not a constant expression
constexpr unsigned size = 42;  // a constant expression

I would, then expect for the following declaration to fail

a[count];                      // Is an error according to Primer

And yet, it does not. Compiles and runs fine.

What is also kind of strange is that ++count; subsequent to array declaration also causes no issues.

Program compiled with -std=c++11 flag on g++4.71

Why is that?

Are you compiling with all warnings? `-W -Wall -Wextra -pedantic` — Kerrek SB, Aug 23 '12 at 23:00
http://stackoverflow.com/questions/2863347/declaring-the-array-size-with-a-non-constant-variable — P.P, Aug 23 '12 at 23:01
Indeed `ISO C++ forbids variable length array` warning comes up. Not an error though. Please answer so i can accept — James Raitsev, Aug 23 '12 at 23:01
Here's another one: http://stackoverflow.com/questions/737240/c-c-array-size-at-run-time-w-o-dynamic-allocation-is-allowed — P.P, Aug 23 '12 at 23:03

score 8 · Accepted Answer · answered Aug 23 '12 at 23:11

8

Your code is not actually legal C++. Some compilers allow variable-length arrays as an extension, but it's not standard C++. To make GCC complain about this, pass -pedantic. In general, you should always pass at least these warning flags go GCC:

-W -Wall -Wextra -pedantic

answered Aug 23 '12 at 23:11

Kerrek SB

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Isn't `-W` and `-Wextra` the same thing? – Jesse Good Aug 23 '12 at 23:28
@JesseGood: Hm, maybe, I might not be up-to-date. Feel free to post something more appropriate. – Kerrek SB Aug 23 '12 at 23:29

score 3 · Answer 2 · answered Aug 23 '12 at 22:58

3

According to this link: http://gcc.gnu.org/onlinedocs/gcc/Variable-Length.html, GCC supports variable-length C arrays in C90 mode and in C++. Since it's not standard C++, you should treat this as a compiler extension and thus assume it's not portable.

answered Aug 23 '12 at 22:58

bstamour

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would you know which gcc 4.x support VLAs ? i686-apple-darwin11-llvm-g++-4.2 (GCC) 4.2.1 seems not to – denis Jan 13 '13 at 17:45
I'm not really sure when the feature was added. – bstamour Jan 14 '13 at 12:53

score 1 · Answer 3 · answered Aug 23 '12 at 23:47

The other answers already provide the solution, g++ allows variables length arrays (VLAs) as an extension in C++ (technically VLAs are a C feature from C90). To make sure that you are using standard conforming C++, pass -pedantic to get a warning and -pedantic -Werror to make the warning into a hard error.

I recommend the following when compiling in debug mode:

g++ -std=c++11 -O0 -g3 -pedantic -pedantic-errors -Wall -Wextra -Werror -Wconversion

O0 is an optimization flag and -g3 is used for debugging. These need to be changed when you want to use optimization and don't need debugging. However, -Werror -Wconversion sometimes might need to be removed as you might not be able to change code for certain reasons, such as when using third party libraries. For a description of what each one does, refer to here.

Constant expressions in array declarations

3 Answers3