C++ – What does ‘const static’ mean in C and C++


const static int foo = 42;

I saw this in some code here on StackOverflow and I couldn't figure out what it does. Then I saw some confused answers on other forums. My best guess is that it's used in C to hide the constant foo from other modules. Is this correct? If so, why would anyone use it in a C++ context where you can just make it private?

Best Solution

A lot of people gave the basic answer but nobody pointed out that in C++ const defaults to static at namespace level (and some gave wrong information). See the C++98 standard section 3.5.3.

First some background:

Translation unit: A source file after the pre-processor (recursively) included all its include files.

Static linkage: A symbol is only available within its translation unit.

External linkage: A symbol is available from other translation units.

At namespace level

This includes the global namespace aka global variables.

static const int sci = 0; // sci is explicitly static
const int ci = 1;         // ci is implicitly static
extern const int eci = 2; // eci is explicitly extern
extern int ei = 3;        // ei is explicitly extern
int i = 4;                // i is implicitly extern
static int si = 5;        // si is explicitly static

At function level

static means the value is maintained between function calls.
The semantics of function static variables is similar to global variables in that they reside in the program's data-segment (and not the stack or the heap), see this question for more details about static variables' lifetime.

At class level

static means the value is shared between all instances of the class and const means it doesn't change.