How to declare a global const initialized by a function in the header?

I have an older source code, like this, in a header used from many places in my project:

const int myVar = myFunc();

What I want:

• Being a global, const variable, I would like if it would be linked only once in the binary.
• Thus, also myFunc() should be called only once in the global variable initialization phase.

Now the problem is that I get this warning from the .cc I compile:

In file included from mySource.cc:7:0:
myHeader.h:59:11: warning: ‘myVar’ defined but not used [-Wunused-variable]
 const int myVar = myFunc();
       ^

Note, mySource.cc really doesn't use myVar, thus the warning is okay, but other sources yes.

I think, the best would be if I would declare myVar only in the header, some like so:

myHeader.h:

 int myVar;

mySource.cc:

 int myVar = myFunc();

But in this case, I can't declare it as const. This variable should be a const. Yes, I know it will be on a writable memory page, only the c++ will see it as a constant, but this is exactly what I want.

Thus, I also want to avoid this warning. Furthermore, I think myFunc() would be called many times, what I don't want.

How can I do this?