What exactly does “static” mean when declaring “global” variables in C++?

Question

This is an expansion of the scope of a previous question of mine.

What exactly is "static", how is it used, and what is the purpose of using "static" when dealing with C++?

Thanks.

Answer 1

The keyword static has different meanings in C++, depending on the context.

When declaring a free function or a global variable it means that the function is not to be available outside of this single translation unit:

// test.cpp
static int a = 1;
static void foo() {}

If the result of compiling that translation unit is linked with a different translation unit containing symbols a and foo it will not break the One Definition Rule, as in this particular translation unit a and foo are private symbols. This use has been obsoleted by unnamed namespaces.

// test2.cpp
namespace {
   static int a = 1;
   static void foo() {}
}

When declaring a local variable within a function it means that the lifetime of the variable will extend from the first call to the function to the end of the program, and not only for the duration of the call:

int foo() {
   static int counter = 0;
   return ++counter;
}
int main() {
  for ( int i = 0; i < 10; ++i ) { 
     std::cout << foo() << std::endl;
  }
}

In the previous code, counter is initialized once when foo is called for the first time, but the variable will outlive the function and keep the value across different function calls. The previous code will print "1 2 3 4... 10". If the variable was not declared static then the output would be "1 1 1... 1".

Within a class scope, static means that the member is a member of the class, and not of a particular instance. This use is equivalent to the use in your other question: usage of that particular member is not bound to any specific object.

struct test {
   int x;
   static int y;
};
int test::y;       // need to define it in one translation unit
int main() {
   // test::x = 5; // !error cannot access a non-static member variable
                   // without an instance
   test::y = 5;    // ok
   test t, other;
   t.x = 10;       // ok
   t.y = 15;       // ok, the standard allows calling a static member through
                   // an instance, but this is the same as test::y
}

In this case, the member x is a non-static member attribute, and as such there is a different x for each instance of the class. In the sample program t.x and other.x refer to different integers. On the other hand y is static and thus there is a single instance of test::y in the program. Even if the standard allows to call t.y and other.y both uses refer to the same variable. The same goes with member methods. If they are static they are class-level methods and can be called without an instance, while if they are non-static they are applied to a concrete instance and the a.b or a->b syntax must be used.

This use of static is similar to the use of the same keyword in Java, while the other two are not present in that language. There is one use of the keyword in Java that is not present in C++, and that is the use of static class initializers (a block of code at class level surrounded by static { ... }). In Java that block of code will be executed when the class is loaded and only once. Initialization of static member variables in C++ must be done in the initializer of the variable definition.

Answer 2

It means that the variable is local to a translation unit (simply put, to a single source file), and cannot be accessed from outside it. This use of static is in fact deprecated in the current C++ Standard - instead you are supposed to use anonymous namespaces:

static int x = 0;    

should be:

namespace {
    int x = 0;    
}

Answer 3

This stuff seems to be fairly well covered here.

But to paraphrase, there are 2 uses in C

1. Prevent the use of a global variable outside the scope of the file that defines it.

2. Allow local variables within a function to persist accross invocations of the function, as in

   int getNextId() { static int id = 0; return id++; }

C++ inherits both of these, and adds two uses of its own.

1. static member variables: Variables that are "shared" accross all instances of a class, and can also be accesses without reference to an instance of the class. Shared seems like the wrong word, but in essence I beleive that the result is that any reference to a static member variable references the same memory location.
2. static methods: Methods that can be called without reference to a specific instance of the class that defines it.

Answer 4

Static basically means that a variable is tied to the lifetime of the program and not of any given function or class instance. When should you use it? Don't. What is the purpose? Debugging data, mostly.

Generally, in C++, if you find yourself using static data, you've done it wrong. There are times when it's appropriate, but they're very rare.

Answer 5

When static is used in a class in C++, it means more or less the same thing that it does in Java. For variables it means that one one instance of the variable exists for all classes and for functions, it means that the function does not implicitly access the this pointer at all.

In C and C++ when static is used for a global variable or function, then it means that the variable may only be referenced in the current C or C++ file. In other words, the compiler must not generate any relocation symbols for the variable or function.

When static is used next to a variable in a local function, it means that the variable does not go out of scope but will retain its value from function-call to function-call. The variable become effectively a global variable that can only be accessed from the given function.

Answer 6

Static class members are data and functions that are associated with the class itself, rather than with the objects of the class.

In the following example, class Fred has a static data member x_ and an instance data member y_. There is only one copy of Fred::x_ regardless of how many Fred objects are created (including no Fred objects), but there is one y_ per Fred object. Thus x_ is said to be associated with the class and y_ is said to be associated with an individual object of the class. Similarly class Fred has a static member function f() and an instance member function g().

class Fred {
    public:
        static void f() throw();                           <-- 1
        void g() throw();                                  <-- 2
    protected:
        static int x_;                                     <-- 3
        int y_;                                            <-- 4
};

(1) Member function associated with the class

(2) Member function associated with an individual object of the class

(3) Data member associated with the class

(4) Data member associated with an individual object of the class

Usage:

When you want to keep tract of the number of instances of a class created you use static variable. For example in a 'Car' class each Car instance may have a unique serial number(_y in this case) and the company may want to keep track of the number of cars produced(_x in this case).