C++ Storage Classes

In C++, storage classes are used to specify the scope, life time, and visibility of variables, functions, and objects. There are four main storage classes in C++: auto, register, extern, static, and mutable.

Auto

The auto storage class is the default storage class in C++ and is used for automatic variables. These variables are declared within a function and are automatically destroyed at the end of the function.

Example:

1#include <iostream>
2using namespace std;
3 
4void func()
5{
6    auto int a = 10;
7    cout << "The value of a is: " << a;
8}
9 
10int main()
11{
12    func();
13    return 0;
14}

Output: The value of a is: 10

Register

The register storage class is used for variables that are stored in the CPU's register. This can improve the performance of a program by reducing the amount of memory access required.

Example:

1#include <iostream>
2using namespace std;
3 
4int main()
5{
6    register int a = 10;
7    cout << "The value of a is: " << a;
8    return 0;
9}

Output: The value of a is: 10

Extern

The extern storage class is used to declare variables or functions that are defined in another source file. This is useful when you need to use a variable or function in multiple files.

Example:

1// file1.cpp
2#include <iostream>
3using namespace std;
4 
5extern int a;
6 
7void func() {
8    cout << "The value of a is: " << a;
9}
10 
11// file2.cpp
12#include <iostream>
13using namespace std;
14 
15int a = 10;
16 
17int main() {
18    func();
19    return 0;
20}

Output: The value of a is: 10

Static

The static storage class is used for variables that have a single instance of the variable for the entire program. This means that the value of the variable persists between function calls and is not destroyed when a function ends.

Example:

1#include <iostream>
2using namespace std;
3 
4void func()
5{
6    static int a = 10;
7    a++;
8    cout << "The value of a is: " << a;
9}
10 
11int main()
12{
13    func();
14    func();
15    func();
16    return 0;
17}

Output:

1The value of a is: 11
2The value of a is: 12
3The value of a is: 13

Mutable

The mutable storage class is used for class member variables. It allows the value of a member variable to be changed even if the member function that changes it is declared as a constant.

Example:

1#include <iostream>
2using namespace std;
3 
4class A
5{
6public:
7    mutable int x;
8    int y;
9 
10    void func() const {
11        x = 10;
12        // y = 10; // error: cannot as
13		const member function 
14	}
15};
16
17int main() { 
18	A a; 
19	a.x = 5; 
20	a.y = 5;
21	a.func(); 
22	cout << "The value of x is: " << a.x << endl; 
23	cout << "The value of y is: " << a.y << endl; 
24	return  0;
25}

Output:

1The value of x is: 10 
2The value of y is: 5