CODEWITHSUNDEEP
c++cstatic
Mustansar Saeed
Mustansar Saeed

Reputation: 2790

Initialization of static variables

Can anyone please tell me when the static variables/functions are allocated memory and in which memory segment? i.e. static global variable, static member variable, static local variable etc all are initialized once before the beginning of program and all retains its values?

Furthermore, If Class MyClass has static variable count, when I declare MyClass obj in main, then MyClass object is created and count is given memory, If I declared MyClass obj2, what happens in terms of memory? Is there any count in obj2 which refers to the count of obj1? Or there is only separate memory allocation. Hope so that I am able to clearly ask the question.

Thanks in advance.

Upvotes: 3

Views: 1999

Answers (3)

ajay
ajay

Reputation: 9680

The keyword static has different meaning depending on the context it is used in. static variables are allocated on the heap and their lifetime extends across the entire run of the program.

  1. A static global variable is same as a global variable except that the static keyword limits the scope of the variable to the file containing it only. This is called file linkage or internal linkage.

  2. A static member variable is created when the class containing it is defined. This means it can be accessed using the class's name. This variable is shared across all instances of the class.

  3. A static local variable has the same lifetime as that of a static global variable except that its scope is the immediate block containing it.

  4. When a function is qualified with the static keyword, this means that the scope of the function is the file containing it only. The function cannot be called by a function in another file. This is, again, called file scope or internal linkage.

Upvotes: 0

haccks
haccks

Reputation: 106122

When we declare a member of a class as static it means no matter how many objects of the class are created, there is only one copy of the static member. It exists even though no objects of the static data member's class exist.

A static member is shared by all objects of the class.

If I declared MyClass obj2, what happens in terms of memory? Is there any count in obj2 which refers to the count of obj1?

Yes. There is only one count for all objects. This test program would explain this a bit clear; 

class Something
{
    public:
    static int s_nValue;
};

int Something::s_nValue = 1;

int main()
{
    Something cFirst;
    cFirst.s_nValue = 2;

    Something cSecond;
    std::cout << cSecond.s_nValue;

    return 0;
}

output:

2

Because s_nValue is a static member variable, s_nValue is shared between all objects of the class. Consequently, cFirst.s_nValue is the same as cSecond.s_nValue.

Furthermore, If Class MyClass has tatic variable count, when I declare MyClass obj in main, then MyClass object is created and count is given memory

No. In fact, count exists even if there are no objects of the class have been instantiated!

Upvotes: 4

barak manos
barak manos

Reputation: 30156

The keyword static can probably be seen as somewhat "overloaded".

The following usage-options are all viable:

  • Static local variables
  • Static global variables
  • Static member variables
  • Static global functions
  • Static member functions

Variables:

In terms of runtime, all types of static variables are essentially the same. They all reside in the data-section of the program, and their addresses remain constant throughout the execution of the program. So the only difference between them is during compilation, in the scope of declaration:

  • Static local variable: recognized by the compiler only in the scope of the function
  • Static global variable: recognized by the compiler only in the scope of the file
  • Static member variable: recognized by the compiler only in the scope of the class

Functions:

In terms of runtime, all types of functions (static and non-static) are essentially the same. They all reside in the code-section of the program, and their addresses remain constant throughout the execution of the program. So the only difference between them is during compilation, in the scope of declaration:

  • Static global function: recognized by the compiler only in the scope of the file
  • Static member function: recognized by the compiler only in the scope of the class

Upvotes: 0

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