When to use static keyword before global variables

Question

Can someone explain when you're supposed to use the static keyword before global variables or constants defined in header files?

For example, lets say I have a header file with the line:

const float kGameSpriteWidth = 12.0f;

Should this have static in front of const or not? What are some best practices for using static?

Answer 1

static before a global variable means that this variable is not accessible from outside the compilation module where it is defined.

E.g. imagine that you want to access a variable in another module:

foo.c

int var; // a global variable that can be accessed from another module
// static int var; means that var is local to the module only.
...

bar.c

extern int var; // use the variable in foo.c
...

Now if you declare var to be static you can't access it from anywhere but the module where foo.c is compiled into.

Note, that a module is the current source file, plus all included files. i.e. you have to compile those files separately, then link them together.

Answer 2

static renders variable local to the file which is generally a good thing, see for example this Wikipedia entry.

Answer 3

Rule of thumb for header files:

• declare the variable as extern int foo; and put a corresponding intialization in a single source file to get a modifiable value shared across translation units
• use static const int foo = 42; to get a constant which can be inlined

Answer 4

The static keyword is used in C to restrict the visibility of a function or variable to its translation unit. Translation unit is the ultimate input to a C compiler from which an object file is generated.

Check this: Linkage | Translation unit

Answer 5

Yes, use static

Always use static in .c files unless you need to reference the object from a different .c module.

Never use static in .h files, because you will create a different object every time it is included.

Answer 6

The correct mechanism for C++ in anonymous namespaces. If you want something that is local to your file, you should use an anonymous namespace rather than the static modifier.