Question

I have a C++ code that’s a physics simulation tool.

I would like to store some physical constants, conversion factor between different sets of units, and also some more application specific constants (such as definition like enum Planes {X=0, Y=1}) and I would like to be able to access them from everywhere in my code.

What would be the best way to do that ?

I think one way would be to create a namespace namespace constants (which can then be a nested namespace in my main namespace) with nested namespaces (like ‘constants’, ‘units’, etc.).

Would it be the way you would do that ?

If I use that method, do I have to make it a header file and include it everywhere ?

If I understand correctly the variables in the namespace at global scope have static linkage, but no external linkage. Then if I want to use them without including a file, I also have to declare them extern ?

As you can see I am a bit confused about that…

Answer 1

Namespace constants are the way to go in most cases.

If I use that method, do I have to make it a header file and include it everywhere ?

Yes, or not everywhere but only where it’s really USED.

If I understand correctly the variables in the namespace at global scope have static linkage, but no external linkage. Then if I want to use them without including a file, I also have to declare them extern ?

Yes, you have to do it that way :

// header 

namespace modulename
{
  // maybe add another namespace to specify that you have constants, but taste-dependant

   namespace domain // like maths or physics
   {
        extern const Number THIS_NUMBER; // have to be defined in the cpp
        extern const int THAT_NUMBER = 256; // if it's int-based type, you can define it -here - BUT DON'T IF IT CAN BE CHANGED : all files including this one would have to be recompiled at each value change!!
   }
}

// .cpp, where you have the definitions

namespace modulename
{
   namespace domain // like maths or physics
   {
        const Number THIS_NUMBER = Number( 256.42f ); // definition - static is implicit
   }
}