c++ extern constant int for array size

Question

I have the following three files in my code (with most of the code removed. This is just to isolate the issue).

global.h:

//global.h
#ifndef GLOBAL_H
#define GLOBAL_H
extern const int ARRAYSIZEX;
extern const int ARRAYSIZEY;
extern const int ARRAYSIZEZ;
#endif //GLOBAL_H

global.cpp:

//global.cpp
#include "global.h"
const int ARRAYSIZEX = 5;
const int ARRAYSIZEY = 2;
const int ARRAYSIZEZ = 4;

main:

//main
#include "global.h"
using namespace std;

someType mySomeTypeArray[ARRAYSIZEX][ARRAYSIZEY][ARRAYSIZEZ];

int main(int argc, char **argv)
{
//...
}

Compiling gives me three errors at the declaration of mySomeTypeArray.

error: array bound is not an integer constant before ']' token

I want to keep my global variable and array size definitions in global.h/cpp for this application, just for organization, so that all my configuration parameters are in one place. What's the proper way to achieve what I'm trying to do?

Thanks

Answer 1

Your declaration is failing because array sizes need to be evaluated at compile-time and your encapsulation scheme is actually hiding the values from the compiler. This is true because compilers work on individual translation units. While compiling main.cpp your compiler sees only extern const int ARRAYSIZEX thanks to the include statement, but not the value which is visible in a separate translation unit so it can't figure out the memory layout.

While const variables can used as array sizes in some contexts, the language provides the more appropriate constexpr qualifier which comes with a set of restrictions that enforce its compile-time evaluation and suitability for array sizes. I recommend always using it when appropriate because it will point you to the error in situations such as this. In this case, you would get a compiler error because an extern constexpr declaration is ill-formed which hints at the proper solution: to hold the values for compile-time constants directly inside the header file.

global.h

constexpr int ARRAYSIZEX = ...;
constexpr int ARRAYSIZEY = ...;
constexpr int ARRAYSIZEZ = ...;

main.cpp

#include "global.h"
someType mySomeTypeArray[ARRAYSIZEX][ARRAYSIZEY][ARRAYSIZEZ];