Question

I have a global unique value, which I wish to use it across within a exe and different DLLs.

For any project which wish to use it, they may just include defs.h

I was wondering, should I declare it as

// defs.h
const UINT UNIQUE_MESSAGE = 
    RegisterWindowMessage(_T("UNIQUE_MESSAGE-{E5476FDB-3E7E-4113-8132-1D87709BC46C}"));

---

OR

// defs.h
const static UINT UNIQUE_MESSAGE = 
    RegisterWindowMessage(_T("UNIQUE_MESSAGE-{E5476FDB-3E7E-4113-8132-1D87709BC46C}"));

---

Currently, I do not find any different from code consumer point of view. Is there any catch behind? Should I use static or not?

Answer 1

In C++, namespace level const are implicitly static (i.e it has internal linkage), unless you write extern const. So both of your syntax is basically same.

§7.1.1/6 says,

Objects declared const and not explicitly declared extern have internal linkage.

So I would suggest this: (with minimal consideration, like excluding static initialization order fiasco and many other issues with static and global variables)

//in defs.h:
extern const UINT UNIQUE_MESSAGE;

//defs.cpp
const UINT UNIQUE_MESSAGE =  RegisterWindowMessage(_T("whatever"));