Storage Classes in C++

storage_class var_data_type var_name; 

There are five types of storage classes, they are follows

1. Automatic
2. Register
3. Static
4. External
5. Mutable

1. Auto Storage Class in C++

Automatic storage class is the default storage class for all local variables, which are declared inside a block. Keyword auto is used for declaring automatic variables.

Syntax of auto strorage class in C++

auto datatype var_name1 [= value];

Program of auto storage class in C++

#include <iostream>
using namespace std;
 
void autoStorageClass()
{
 
    cout << "Auto class\n";
 
    auto a = 56;
    auto b = 5.6;
    auto c = "StudyExperts";
    auto d = 'S';
 
    cout << a << " \n";
    cout << b << " \n";
    cout << c << " \n";
    cout << d << " \n";
}
 
int main()
{
    autoStorageClass();
 
    return 0;
}

Output

Auto class
56 
5.6 
StudyExperts
S

2. Register Storage Class

register keyword is used for specifying register variables.

Register variables are similar to auto variables that is having same functionality and exists inside a particular function only. It is supposed to be faster than the variables stored in memory during runtime program

Program of register storage class in C++

#include <iostream>
using namespace std;
 
void registerStorageClass()
{
 
    cout << " Register class\n";
 
 
    register char b = 'G';
 
    
    cout << "Value of the variable 'b'"
         << " declared as register: " << b;
}
int main()
{
 
    registerStorageClass();
    return 0;
}

Output

Register class
Value of the variable 'b' declared as register: G

3. Static Storage Class

static keyword is used to declare a static variable.

The static variable is initialized only one time and exists till the end of a program. It kepp its value between multiple functions call.

The static variable has the default value zero (0) which is provided by compiler.

#include <iostream>  
using namespace std;  
void func() {    
   static int i=0; //static variable    
   int j=0; //local variable    
   i++;    
   j++;    
   cout<<"i=" << i<<" and j=" <<j<<endl;    
}    
int main()  
{  
 func();    
 func();    
 func();    
}

i= 1 and j= 1
i= 2 and j= 1
i= 3 and j= 1

#include <iostream>
using namespace std;
 
int x;
void externStorageClass()
{
 
    cout << " Extern class\n";
 
      extern int x;
 
    
    cout << "Value of the variable 'x'"
         << "declared, as extern: " << x << "\n";
 
    x = 2;
 
      
    cout<< "Modified value of the variable 'x'"
        << " declared as extern: \n"
        << x;
}
 
int main()
{
 
    externStorageClass();
 
    return 0;

Extern class
Value of the variable 'x'declared, as extern: 0
Modified value of the variable 'x' declared as extern: 
2

#include <iostream>
using std::cout;
 
class Test {
public:
    int x;
     mutable int y;
 
    Test()
    {
        x = 4;
        y = 10;
    }
};
 
int main()
{
    const Test t1;
 
   
    t1.y = 20;
    cout << t1.y;
 
    return 0;
}

20