Declare and define static variable in C++ header?

Question

Many other questions deal with how to allocate a variable by declaring it in a header file and defining it (allocating) in a .cpp file.

What I want to do is not use any .cpp files for my class, and to define all functions as inline (in the header file). The problem that I run into is how to define static member variables so that even when the .h file is included in multiple compilation units I don't get the "first defined here" linker error.

I'm open to preprocessor hacks, etc. if it gets the job done. I just want to avoid any .cpp files.

If it matters I'm using GCC.

You simply declare your variable as static. A new instance of the variable will be created in each translation unit you include the header file in, but at link time, there will be no conflicts since the objects have static linkage... — user529758
"defining it (allocating)": defining and allocating are two different things. — Pete Becker
@cmaster - allocating is part of defining, but they are not the same thing. — Pete Becker
@PeteBecker it's true that the words have a different meaning, however, for variables they are essentially synonyms, so there's nothing wrong with "defining it (allocating)". But this discussion leads to nothing, so I suggest we both just delete our comments. — cmaster - reinstate monica

cmaster - reinstate monica cmaster - reinstate monica · Accepted Answer · 2013-08-03T11:34:49

You can abuse the singleton pattern if you really must avoid any .cpp files:

class Foo {
    public:
        static Bar& getMyStatic() {
            static Bar bar;
            return bar;
        };
};

This works because now the variable is a static variable inside a function, and static has a different meaning within a function context than within a class context. And for functions, the linker does recognize multiple identical definitions and throws away the copies.

But, of course, I would strongly advise against avoiding .cpp files: It means that you get into a situation where you have to build the entire program, or at least large parts of it, in one big piece. Every change you do will necessitate a complete rebuilt which slows down your change-compile-test cycle significantly. For very small projects that might not be a problem, but it is for medium to large ones.

Declare and define static variable in C++ header?

3 Answers