Non-constant array size-initialization?

Question

From C++ document http://www.cplusplus.com/doc/tutorial/arrays/
To define an array like this int a[b]; the variable b must be a constant.

Here is what I am running under g++ (Ubuntu/Linaro 4.6.3-1ubuntu5) 4.6.3

int main(){
    int a = 10;
    int b[a];

    for(int i = 0; i < 10; i++){
        cout << b[i] << endl;
    }
    return 0;
}

variable a is not a constant and I have no error. May I ask start from what version of g++ will accept this kind of array definition?

Well, in this case I expect the compiler optimizes a out so that you are, in fact, initializing with a constant. Try instead initializing with rand()/100000 and see what happens (#include <cstdlib>) — Matt Phillips
– Matt Phillips, Commented Oct 18, 2013 at 0:09
@MattPhillips doubt that, the optimizer comes in long after that. — Luchian Grigore
– Luchian Grigore, Commented Oct 18, 2013 at 0:10
@MattPhillips: Optimization shouldn't be relevant. The language rules require a constant expression, and that doesn't just mean something that the compiler is able to figure out how to evaluate at compile time. — Keith Thompson
– Keith Thompson, Commented Oct 18, 2013 at 0:11

Keith Thompson · Accepted Answer · 2013-10-18 00:10:42Z

4

The compiler is using a non-standard extension. Your code isn't valid, standard C++. Variable length arrays aren't a feature of C++.

Note that the size has to be a compile-time constant, not merely a constant (i.e. const).

edited Oct 18, 2013 at 0:10

Keith Thompson

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answered Oct 18, 2013 at 0:08

Luchian Grigore

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3 Comments

PeterT Over a year ago

"Variable length arrays aren't a feature of C++" yet

Luchian Grigore Over a year ago

@PeterT hopefully, never.

PeterT Over a year ago

I meant this could be in C++14. I don't particularly like it either, but some people seem to want it badly. Not to mention all the new special restrictions that they have will only confuse beginners

General Grievance · Accepted Answer · 2022-03-10 04:27:06Z

4

Variable length arrays are allowed as an extension in GCC.

See GCC Docs.

edited Mar 10, 2022 at 4:27

General Grievance

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answered Oct 18, 2013 at 0:11

Rami

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Comments

Mr. Smith · Accepted Answer · 2013-10-18 12:26:41Z

1

You can't create dynamic arrays in C++, because your compiler needs to know how big your program is before compiling. But to you can create an array with 'new':

int *b = new int[a];

This will create a new array reserving new storage. You can access this array the normal way.

for(int i=0; i<a; i++)
{
   b[i];
}

answered Oct 18, 2013 at 12:26

Mr. Smith

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Comments

user427390 · Accepted Answer · 2013-10-18 00:13:56Z

0

For a dynamically sized array you can use a std::vector in C++ (not exactly an array, but close enough and the backing store is available to you if you need the raw array). If you insist on creating a dynamic block of data you can simply use 'new type[]'.

int a = 100;
int[] b = new int[a];

answered Oct 18, 2013 at 0:13

user427390

1 Comment

sflee Over a year ago

Thanks for helping but this is not I want.

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