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
#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?
static means that there will be one copy of
VAL created for each source file it is included in. But it also means that multiple inclusions will not result in multiple definitions of
VAL that will collide at link time. In C, without the
static you would need to ensure that only one source file defined
VAL while the other source files declared it
extern. Usually one would do this by defining it (possibly with an initializer) in a source file and put the
extern declaration in a header file.
static variables at global level are only visible in their own source file whether they got there via an include or were in the main file.
Editor's note: In C++,
const objects with neither the
extern keywords in their declaration are implicitly
extern tags on file-scoped variables determine whether they are accessible in other translation units (i.e. other
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
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,
In C++, file-scoped variables are
static (internal linkage) by default if they are
extern by default if they are not. If you're using C++, both
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.