g++ allows variable array creation

g++ allows variable array creation

[Omar Shaikh]

Dear All

I found something in the GNU compiler that I am not
sure is a bug or the latest change to the C++
standard. However, the same did not work in Microsoft
C++ (v. 13.10.3077), the C++ compiler that comes with
Microsoft .NET product. I ran this code on both Win32
and Linux platforms with full warnings on (-Wall).
Surprisingly, there were no warnings either.

The compiler that I used on Windows (XP):
MinGW: c++ (GCC) 3.2.3 (mingw special 20030504-1)

Compiler used on (Mandrake) Linux:
g++ (GCC) 3.2.2 (Mandrake Linux 9.1 3.2.2-3mdk)

#include <iostream.h>
class A
{
	int v;
public:
	A () { }
	void SetV (int nv) { v = nv; }
	int GetV () { return v; }
};

int main ()
{
	A x;
	int i;

	cout << "Enter array size: "; cin >> i;
	x.SetV(i);

	const int m = x.GetV();

	int arr[m]; // Works fine!

	cout << "Array size = " << sizeof(arr) << " bytes" <<
endl;

	return 0;
}

Here are two sample runs of the program. (Underscored
text is user i/p.)

Enter array size: _10_
Array size = 40 bytes

Next run

Enter array size: _25_
Array size = 100 bytes

The code above works fine, though the array size can
be decided during runtime. Please let me know if I am
missing something.

Thank you.
Omar

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Re: g++ allows variable array creation

[Andrew Pinski]

On Jan 19, 2004, at 03:31, Omar Shaikh wrote:

> Dear All
>
> I found something in the GNU compiler that I am not
> sure is a bug or the latest change to the C++
> standard. However, the same did not work in Microsoft
> C++ (v. 13.10.3077), the C++ compiler that comes with
> Microsoft .NET product. I ran this code on both Win32
> and Linux platforms with full warnings on (-Wall).
> Surprisingly, there were no warnings either.


That is because VLA is part of GNU C, C99, and GNU C++ and will most 
likely part of a future
C++ standard (if not already part of a draft).  To get a warning from 
GCC use -pedantic.

Thanks,
Andrew Pinski

Re: g++ allows variable array creation

[Gabriel Dos Reis]

Andrew Pinski <pinskia@physics.uc.edu> writes:

| On Jan 19, 2004, at 03:31, Omar Shaikh wrote:
| 
| > Dear All
| >
| > I found something in the GNU compiler that I am not
| > sure is a bug or the latest change to the C++
| > standard. However, the same did not work in Microsoft
| > C++ (v. 13.10.3077), the C++ compiler that comes with
| > Microsoft .NET product. I ran this code on both Win32
| > and Linux platforms with full warnings on (-Wall).
| > Surprisingly, there were no warnings either.
| 
| 
| That is because VLA is part of GNU C, C99, and GNU C++ and will most
| likely part of a future
| C++ standard (if not already part of a draft).  To get a warning from
| GCC use -pedantic.

VLA is part of no C++ standard draft I know of.  Variably modified
types come with their hosts of problems.  How they should interact
with the C++ type system, especially templates, is yet not determined.
And I have seen no proposal to enlighten that point.  
IMHO, it is unlikely that such beasts would be made part of C++ if they
can't play with templates .
And of course, we do have PRs about VLAs in C++ (either with classes
or templates) :-) 

-- Gaby