Storage Classes in C++

In this tutorial we will study and understand the concept and implementation of Storage Classes in C++

Storage Classes in C++
Storage Classes in C++

Storage Classes in C++ define the scope (visibility) and life-time of variables and/or functions within a C++ Program. Lifetime means the duration till which the variable remains active and visibility defines in which module of the program the variable is accessible. There are 5 types of Storage Classes in C++ :

  • Automatic
  • External
  • Static
  • Register
  • Mutable
Storage ClassKeywordLifetimeVisibilityInitial Value
AutomaticautoFunction BlockLocalGarbage
ExternalexternWhole ProgramGlobalZero
StaticstaticWhole ProgramLocalZero
RegisterregisterFunction BlockLocalGarbage
MutablemutableClassLocalGarbage
Automatic Storage Class

Automatic storage class assigns a variable to its default storage type. auto keyword is used to declare automatic variables. However, if a variable is declared without any keyword inside a function, it is automatic by default. This variable is visible only within the function it is declared and its lifetime is same as the lifetime of the function as well. Once the execution of function is finished, the variable is destroyed. Example of Automatic Storage Class:

External Storage Class

External storage class assigns variable a reference to a global variable declared outside the given program. extern keyword is used to declare external variables. They are visible throughout the program and its lifetime is same as the lifetime of the program where it is declared. This visible to all the functions present in the program. Example of External Storage Class:

A variable test is declared as external in main.cpp. It is a global variable and it is assigned to 100 in sub.cpp. It can be accessed in both files. The function multiply() multiplies the value of test with the parameter passed to it while invoking it. The program performs the multiplication and changes the global variable test to 500. Note: Run the main.cpp program
Output:
100
500

Static Storage Class

The static storage class instructs the compiler to keep a local variable in existence during the life-time of the program instead of creating and destroying it each time it comes into and goes out of scope. Therefore, making local variables static allows them to maintain their values between function calls.
The static modifier may also be applied to global variables. When this is done, it causes that variable’s scope to be restricted to the file in which it is declared.
In C++, when static is used on a class data member, it causes only one copy of that member to be shared by all objects of its class. Example of Static Storage Class:

Output
i is 6 and count is 9
i is 7 and count is 8
i is 8 and count is 7
i is 9 and count is 6
i is 10 and count is 5
i is 11 and count is 4
i is 12 and count is 3
i is 13 and count is 2
i is 14 and count is 1
i is 15 and count is 0
Register Storage Class

Register storage assigns a variable’s storage in the CPU registers rather than primary memory. It has its lifetime and visibility same as automatic variable. The purpose of creating register variable is to increase access speed and makes program run faster. If there is no space available in register, these variables are stored in main memory and act similar to variables of automatic storage class. So only those variables which requires fast access should be made register.For example:

Mutable Storage Class

In C++, a class object can be kept constant using keyword const. This doesn’t allow the data members of the class object to be modified during program execution. But, there are cases when some data members of this constant object must be changed. Example of Mutable Storage Class:

A class test is defined in the program. It consists of a mutable data member a. A constant object of class test is created and the value of data members are initialized using user-defined constructor. Since, b is a normal data member, its value can’t be changed after initialization. However a being mutable, its value can be changed which is done by invoking square_a() method. display() method is used to display the value the data members.

Output
Total Objects: 1
Initial value
a = 2
b = 3
Final value
a = 4
b = 3

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