Help Required - compilation error.

Help Required - compilation error.

[Jalaiah Murari]

Hi,

I used GCC 2.7.2.3 in the past to compile the following code:
class sample
{
private:
unsigned char wk_tmp[20000 - ((int)(long)&(((DATASTRUCTURE*)0)->mydata))];

};


Now, When i try to compile using GCC 4.1.2, I get the following error for the above declaration.
error: array bound is not an integer constant


I see that  the array size has to be mentioned directly, instead of dynamic calculation.
Is there any workaround for this. Do I need to turn on any flag in gcc/g++ when compiling this file.


Thanks in advance.

Regards
Jalaiah. M

Re: Help Required - compilation error.

[Jonathan Wakely]

On 11 August 2011 14:50, Jalaiah Murari wrote:
> Hi,
>
> I used GCC 2.7.2.3 in the past to compile the following code:
> class sample
> {
> private:
> unsigned char wk_tmp[20000 - ((int)(long)&(((DATASTRUCTURE*)0)->mydata))];
>
> };
>
>
> Now, When i try to compile using GCC 4.1.2, I get the following error for the above declaration.
> error: array bound is not an integer constant
>
>
> I see that  the array size has to be mentioned directly, instead of dynamic calculation.
> Is there any workaround for this. Do I need to turn on any flag in gcc/g++ when compiling this file.

No, the code is invalid and can't be compiled.

It looks as though you probably want to use offsetof(DATASTRUCTURE, mydata)

Re: Help Required - compilation error.

[david.hagood]

> Hi,
>
> I used GCC 2.7.2.3 in the past to compile the following code:
> class sample
> {
> private:
> unsigned char wk_tmp[20000 - ((int)(long)&(((DATASTRUCTURE*)0)->mydata))];
>
> };

Your code is broken.

You REALLY want to use a smarter type here:

#include <vector>
class sample
{
   private:
     std::vector<unsigned char> wk_temp;
   public:
     sample();
};

sample::sample()
:wk_temp((int)(long)&(((DATASTRUCTURE*)0)->mydata)))
{
}

Using a vector will let the language handle the allocation of the space,
check for overflows, and so on.

(now, the wisdom of what you are using this for - why the allocation type,
etc. are another issue you may want to think about, but you didn't give us
enough information to help you there.)

Bottom line: you REALLY shouldn't be using dumb types like C arrays or
pointers in C++ if you can avoid it.

Re: Help Required - compilation error.

[Jonathan Wakely]

On 11 August 2011 17:25,  <david.hagood@gmail.com> wrote:
>> Hi,
>>
>> I used GCC 2.7.2.3 in the past to compile the following code:
>> class sample
>> {
>> private:
>> unsigned char wk_tmp[20000 - ((int)(long)&(((DATASTRUCTURE*)0)->mydata))];
>>
>> };
>
> Your code is broken.
>
> You REALLY want to use a smarter type here:
>
> #include <vector>
> class sample
> {
>   private:
>     std::vector<unsigned char> wk_temp;
>   public:
>     sample();
> };
>
> sample::sample()
> :wk_temp((int)(long)&(((DATASTRUCTURE*)0)->mydata)))

That will dereference a null pointer at runtime, probably not what's intended!

I agree the code is broken, but as it appears to be an attempt to find
the address of &DATASTRUCTURE::mydata relative to a DATASTRUCTURE
object at address 0, I think it's equivalent to
offsetof(DATASTRUCTURE, mydata), which is a constant, so the original
declaration can be replaced with:

unsigned char wk_tmp[20000 - offsetof(DATASTRUCTURE, mydata)];

Re: Help Required - compilation error.

[Jalaiah Murari]

Hi All,

Thanks for your responses.

 offsetof(DATASTRUCTURE, mydata) worked for me.

Regards
Jalaiah. M


----- Original Message -----
From: Jonathan Wakely <jwakely.gcc@gmail.com>
To: david.hagood@gmail.com
Cc: Jalaiah Murari <m_jalaiah@yahoo.com>; "gcc-help@gcc.gnu.org" <gcc-help@gcc.gnu.org>
Sent: Thursday, August 11, 2011 10:37 PM
Subject: Re: Help Required - compilation error.

On 11 August 2011 17:25,  <david.hagood@gmail.com> wrote:
>> Hi,
>>
>> I used GCC 2.7.2.3 in the past to compile the following code:
>> class sample
>> {
>> private:
>> unsigned char wk_tmp[20000 - ((int)(long)&(((DATASTRUCTURE*)0)->mydata))];
>>
>> };
>
> Your code is broken.
>
> You REALLY want to use a smarter type here:
>
> #include <vector>
> class sample
> {
>   private:
>     std::vector<unsigned char> wk_temp;
>   public:
>     sample();
> };
>
> sample::sample()
> :wk_temp((int)(long)&(((DATASTRUCTURE*)0)->mydata)))

That will dereference a null pointer at runtime, probably not what's intended!

I agree the code is broken, but as it appears to be an attempt to find
the address of &DATASTRUCTURE::mydata relative to a DATASTRUCTURE
object at address 0, I think it's equivalent to
offsetof(DATASTRUCTURE, mydata), which is a constant, so the original
declaration can be replaced with:

unsigned char wk_tmp[20000 - offsetof(DATASTRUCTURE, mydata)];