'array bound is not an integer constant' when defining size of array in class, using an element of a const array

Question

#ifndef QWERT_H
#define QWERT_H

const int x [] = {1, 2,};
const int z = 3;
#endif


#include <iostream>
#include "qwert.h"
class Class   
{  
    int y [x[0]];  //error:array bound is not an integer constant
    int g [z];     //no problem  
};


int main ()  
{  

    int y [x[0]];      //no problem
    Class a_class;

}

I can't figure out why this doesn't work. Other people with this problem seem to be trying to dynamically allocate arrays. Any help is much appreciated.

Unrelated, but #ifndef QWERT_H should appear only in qwert.h, and #include "qwert.h" should not appear in qwert.h. — MSalters
– MSalters, Commented Jan 14, 2011 at 9:45

Ed Swangren · Accepted Answer · 2011-01-13 19:51:27Z

4

x is const (as is z obviously), but x[0] is not a constant expression. Array declarations in a class definition must have constant size specifiers.

Consider this for a moment; how would you expect the sizeof operator to evaluate the size of your class if it contains an array of unknown size at compile time?

edited Jan 13, 2011 at 19:51

answered Jan 13, 2011 at 19:17

Ed Swangren

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5 Comments

Yakov Galka Over a year ago

x[0] is const. it's not constant expression though.

Matt K Over a year ago

x[0] is a read-only location in this case. Try to assign a value to x[0] and you'll get a compile-time "assignment of read-only location"

Matt Munson Over a year ago

Yeah i see. I didn't realize there was a potential distinction between constant expressions and mere read-only-locations/consts, or that x[0] would constitute the latter. I guess the problem is that array access takes place too late in compilation for it to be allowed as a constant expression? PS. Thanks for the help guys.

Fred Nurk Over a year ago

@user574733: It's an "that's just how it is" issue rather than there being any fundamental technical reason (such as you hint at with "takes place too late").

Fred Nurk Over a year ago

@user574733: For another example that may help, consider: "extern int const x; int a[x];". Even though x is an int const, it cannot (yet) be used in an integer constant expression.

Puppy · Accepted Answer · 2011-01-13 19:22:11Z

3

The main version works because your compiler has an extension to allow for variable length arrays. Array accesses cannot be constant expressions in C++03, even if the array and the index are both constant expressions, which is the source of the error.

answered Jan 13, 2011 at 19:22

Puppy

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Comments

Edward Strange · Accepted Answer · 2011-01-13 19:19:27Z

1

The size of an array must be a constant expression. I don't believe that constant elements in an array qualify as such.

The version in main() working is probably due to a compiler extension.

answered Jan 13, 2011 at 19:19

Edward Strange

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