How to use non-const global variable that defined in a namespace

Question

I have an header file that i put some global const variables. Now I need another project wide modifiable variable. But it gives linker error as expected. I am trying to solve the issue without using inline keyword. My codes:

constants.h:

#ifndef CONSTANTS_H
#define CONSTANTS_H

namespace constants {
    bool myVar;
}

#endif // CONSTANTS_H

I am using this variable in my classes with constants::myVar. And I got a linker error. Any solution without using inline (for backward compiler compability)?

Answer 1

I completely agree with the comments above about looking at alternatives for this design. Also as you seems to be aware, c++17 inline variables offer a better solution.

Having said that, if you must keep the current design - you can do the following:

1. Change the header file to declare myVar as extern:

   namespace constants {
   //--vvvvvv------------
       extern bool myVar;
   }
   

2. Add constants.cpp file (if it doesn't exists yet), with a definition for myVar (preferably with some initialization):

   namespace constants {
       bool myVar;
       // or better: something like:
       bool myVar = false;
   }