When refactoring away some #defines
I came across declarations similar to the following in a C++ header file:
static const unsigned int VAL = 42;
const unsigned int ANOTHER_VAL = 37;
The question is, what difference, if any, will the static make? Note that multiple inclusion of the headers isn't possible due to the classic #ifndef HEADER
#define HEADER
#endif
trick (if that matters).
Does the static mean only one copy of VAL
is created, in case the header is included by more than one source file?
The static
and extern
tags on file-scoped variables determine whether they are accessible in other translation units (i.e. other .c
or .cpp
files).
static
gives the variable internal linkage, hiding it from other translation units. However, variables with internal linkage can be defined in multiple translation units.
extern
gives the variable external linkage, making it visible to other translation units. Typically this means that the variable must only be defined in one translation unit.
The default (when you don't specify static
or extern
) is one of those areas in which C and C++ differ.
In C, file-scoped variables are extern
(external linkage) by default. If you're using C, VAL
is static
and ANOTHER_VAL
is extern
.
In C++, file-scoped variables are static
(internal linkage) by default if they are const
, and extern
by default if they are not. If you're using C++, both VAL
and ANOTHER_VAL
are static
.
From a draft of the C specification:
6.2.2 Linkages of identifiers ... -5- If the declaration of an identifier for a function has no storage-class specifier, its linkage is determined exactly as if it were declared with the storage-class specifier extern. If the declaration of an identifier for an object has file scope and no storage-class specifier, its linkage is external.
From a draft of the C++ specification:
7.1.1 - Storage class specifiers [dcl.stc] ... -6- A name declared in a namespace scope without a storage-class-specifier has external linkage unless it has internal linkage because of a previous declaration and provided it is not declared const. Objects declared const and not explicitly declared extern have internal linkage.