Why can't constant values be used to define an array's initial size?


There are times when constant values can be used and there are times when they can't. A C program can use what C considers to be constant expressions, but not everything C++ would accept.

When defining the size of an array, you need to use a constant expression. A constant expression will always have the same value, no matter what happens at runtime, and it's easy for the compiler to figure out what that value is. It might be a simple numeric literal:

char a[ 512 ];

Or it might be a "manifest constant" defined by the preprocessor:

#define MAX     512
/* ... */
char    a[ MAX ];

Or it might be a sizeof:

char a[ sizeof( struct cacheObject ) ];

Or it might be an expression built up of constant expressions:

char buf[ sizeof( struct cacheObject ) * MAX ];

Enumerations are allowed too.

An initialized const int variable is not a constant expression in C:

int max = 512; /* not a constant expression in C */

char buffer[ max ]; /* not valid C */

Using const ints as array sizes is perfectly legal in C++; it's even recommended. That puts a burden on C++ compilers (to keep track of the values of const int variables) that C compilers don't need to worry about. On the other hand, it frees C++ programs from using the C preprocessor quite so much.


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