What does const static mean in C and C++?

What does const static mean in C and C++?

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 programs 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 doesnt change.

It has uses in both C and C++.

As you guessed, the static part limits its scope to that compilation unit. It also provides for static initialization. const just tells the compiler to not let anybody modify it. This variable is either put in the data or bss segment depending on the architecture, and might be in memory marked read-only.

All that is how C treats these variables (or how C++ treats namespace variables). In C++, a member marked static is shared by all instances of a given class. Whether its private or not doesnt affect the fact that one variable is shared by multiple instances. Having const on there will warn you if any code would try to modify that.

If it was strictly private, then each instance of the class would get its own version (optimizer notwithstanding).

What does const static mean in C and C++?

That line of code can actually appear in several different contexts and alghough it behaves approximately the same, there are small differences.

Namespace Scope

// foo.h
static const int i = 0;

i will be visible in every translation unit that includes the header. However, unless you actually use the address of the object (for example. &i), Im pretty sure that the compiler will treat i simply as a type safe 0. Where two more more translation units take the &i then the address will be different for each translation unit.

// foo.cc
static const int i = 0;

i has internal linkage, and so cannot be referred to from outside of this translation unit. However, again unless you use its address it will most likely be treated as a type-safe 0.

One thing worth pointing out, is that the following declaration:

const int i1 = 0;

is exactly the same as static const int i = 0. A variable in a namespace declared with const and not explicitly declared with extern is implicitly static. If you think about this, it was the intention of the C++ committee to allow const variables to be declared in header files without always needing the static keyword to avoid breaking the ODR.

Class Scope

class A {
public:
  static const int i = 0;
};

In the above example, the standard explicitly specifies that i does not need to be defined if its address is not required. In other words if you only use i as a type-safe 0 then the compiler will not define it. One difference between the class and namespace versions is that the address of i (if used in two ore more translation units) will be the same for the class member. Where the address is used, you must have a definition for it:

// a.h
class A {
public:
  static const int i = 0;
};

// a.cc
#include a.h
const int A::i;            // Definition so that we can take the address

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