Using class attribute as array-bound

Question

I would like to do this:

Class Obj{
private:
int a;
int b[a][3];

public:
Obj(int a);
}

So that i can specify the size of my array when creating my object.But I get this compilation error: error: array bound is not an integer constant

I don't want to use vectors, dos anyone know how I can do this? Thanks.

If you're not using vectors, welcome to the painful world of dynamic memory allocation. — Qaz
– Qaz, Commented Apr 4, 2013 at 9:49
As @chris says .. you need to do it your self with the new operator or malloc. — Rob
– Rob, Commented Apr 4, 2013 at 9:50
It's even worse in a class because you have to fill in three or five new members. — Qaz
– Qaz, Commented Apr 4, 2013 at 9:51
Have to mention it: blogs.msdn.com/b/oldnewthing/archive/2013/02/06/10391383.aspx — Qaz
– Qaz, Commented Apr 4, 2013 at 9:56
Well the reason I dont wan't to use vectors is that I don't now how to deal with vectors of array. In this i would like an array of 'a' arrays of size 3... — Cehem
– Cehem, Commented Apr 4, 2013 at 10:10

ForEveR · Accepted Answer · 2013-04-04 10:23:07Z

1

Array size should be constant. There is only one way, without dynamic allocation - use predefined constant for all arrays of this class.

class Obj{
private:
static constexr int a = 5;
int b[a][3];

public:
Obj();
};

If you want different sizes, then you should allocate memory dynamically, if you don't use vectors.

edited Apr 4, 2013 at 10:23

answered Apr 4, 2013 at 9:50

ForEveR

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1 Comment

Cehem Over a year ago

What's the point of indicating 'a' in ;y constructor if it's constant ?

Turgal · Accepted Answer · 2013-04-04 09:52:27Z

0

You should use a dynamic allocation:

b = new int *[a] ;
//memory allocated for  elements of each column.
for( int i = 0 ; i < a ; i++ )
   b[i] = new int[3];

answered Apr 4, 2013 at 9:52

Turgal

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1 Comment

Qaz Over a year ago

Might as well keep it at int (*b)[3].

Saqlain · Accepted Answer · 2013-04-04 10:05:48Z

0

If you want to depend on extension then ISO C99 allows variable length arrays through extension, have a look at it http://gcc.gnu.org/onlinedocs/gcc/Variable-Length.html, with this extension arrays are declared like any other automatic arrays, but with a length that is not a constant expression.

Side effect is your code will compile with compile which got this extension.

answered Apr 4, 2013 at 10:05

Saqlain

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