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

#ifndef QWERT_H
#define QWERT_H

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


#include <iostream>开发者_JAVA技巧;
#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.

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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?

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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.

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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.