Storage classes in C++

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The following storage classes can be used in a C++ program:

• Automatic
• External
• Static
• Register
• Mutable

Table:

1. 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.

Syntax:

datatype var1[= value];
or
auto datatype var1[= value];

Example:

auto int x;
float y =5.67; // both these have the same storage classes

2. External Storage Class:

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

Syntax:

extern datatype var1;

Example1:C++ program to create and use external storage.

First File : main.cpp

#include <iostream>
int count ;
extern void write_extern();
 
main() {
   count = 5;
   write_extern();
}

Second File support.cpp

#include <iostream>

extern int count;

void write_extern(void) {
   std::cout << "Count is " << count << std::endl;
}

3. Static Storage Class:

• Static storage class ensures a variable has the visibility mode of a local variable but lifetime of an external variable.
• static keyword is used.
• It can be used only within the function where it is declared but destroys only after the program execution has finished.
• Therefore, making local variables static allows them to maintain their values between function calls.
• When a function is called, the variable defined as static inside the function retains its previous value and operates on it.

Syntax:

static datatype var1[= value];

Example:

static int x = 35;
static float addition;

4. Register Storage Class:

• The register storage class is used to define local variables that should be stored in a register instead of RAM.
• It has its visibility and lifetime same as automatic variable.
• register keyword is used.
• The purpose of creating register varaiable is to increase access speed and make program run faster.
• If there is no space in the register, these variables are stored in main memory and act similar to variables of automatic storage class.
• The register should only be used for variables that require quick access such as counters.

Syntax:

register datatype var1[= value];

Example:

register char z;
register int rollno;

5. Mutable Storage Class:

• The 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.
• mutable specifier applies only to class objects.
• mutable keyword is used.
• It allows a member of an object to override const member function.
• That is, a mutable member can be modified by a const member function.

Syntax:

mutable datatype var1;

Example:

mutable char a;