C++ - Why do I have to include .cpp file along with/ instead of .h file to acces the value of a global variable in the following case? - class

Question

I am trying to properly declare and define global variables in separate files and include them in a third file which deals with class declaration.
The three files are:
1) global.h
#ifndef GLOBAL_H_INCLUDED
#define GLOBAL_H_INCLUDED
extern const int marker_num;
extern const int dim;
using namespace std;
#endif // GLOBAL_H_INCLUDED
2) global.cpp
#include <iostream>
#include <cstdio>
#include <cmath>
#include "global.h"
#include "WorldState.h"
#include "Robot.h"
#include "Sensor.h"
#include "Marker.h"
constexpr const int marker_num = 10;
constexpr const int dim = (2 * marker_num) + 3;
3) WorldState.h
#ifndef WORLDSTATE_H
#define WORLDSTATE_H
#include "global.h"
#include "global.cpp"
class WorldState{
public:
WorldState(float a[], float b[dim][dim]);
get_wstate();
protected:
private:
float w_state[];
float covar_matrix[dim][dim];
};
#endif // WORLDSTATE_H
I am using the global variable dim to declare and define a multidimensional array. I have declared dim inside global.h and defined it inside global.cpp. Now, I have a class called WorldState and inside its header, I am using dim. If I comment out #include "global.cpp", it throws the following error:
C:\Users\syamp\Documents\codeblocks\slam\WorldState.h|10|error: array bound is not an integer constant before ']' token
My understanding is that including the .h file includes the corresponding .cpp as well, and that all declarations should be inside .h and all definitions should be inside .cpp. However, it doesn't seem to work in this case.
1) If I decide to include global.cpp file inside WorldState.h, isn't it bad programming practice? I am trying to write a good code not just a code that works.
2) An alternative is to define values of variable(s) dim (and marker_num) inside global.h. Is that good programming practice?
3) I believe there is something that I am missing. Kindly suggest the best method to resolve this issue. I am using codeblocks and C++11. Thanks in advance.

Answer 1

I am using the global variable dim to declare and define a multidimensional array.
When declaring a fixed-length array at compile-time, the value(s) of its dimension(s) must be known to the compiler, but your separation prevents the value of dim from being known to the compiler at all, so dim cannot be used to specify fixed array dimensions. Any code that uses dim will just compile into a reference to it, and then the linker will resolve the references after compilation is done. Just because dim is declared as const does not make it suitable as a compile-time constant. To do that, you must define its value in its declaration, eg:
#ifndef GLOBAL_H_INCLUDED
#define GLOBAL_H_INCLUDED
static constexpr const int marker_num = 10;
static constexpr const int dim = (2 * marker_num);
using namespace std;
#endif // GLOBAL_H_INCLUDED
Otherwise, if you keep dim's declaration and definition in separate files, you will have to dynamically allocate the array at run-time instead of statically at compile-time.
I have declared dim inside global.h and defined it inside global.cpp.
That is fine for values you don't need to use until run-time. That will not work for values you need to use at compile-time.
My understanding is that including the .h file includes the corresponding .cpp as well
That is not even remotely true. The project/makefile brings in the .cpp file when invoking the compiler. The .h file has nothing to do with that.
that all declarations should be inside .h and all definitions should be inside .cpp.
Typically yes, but not always.
If I decide to include global.cpp file inside WorldState.h, isn't it bad programming practice?
Yes.
An alternative is to define values of variable(s) dim (and marker_num) inside global.h. Is that good programming practice?
Yes, if you want to use them where compile-time constants are expected.