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Showing content from https://www.tutorialspoint.com/cplusplus/cpp_sizeof_operator.htm below:

C++ sizeof Operator

The sizeof is a keyword, but it is a compile-time operator that determines the size, in bytes, of a variable or data type.

The sizeof operator can be used to get the size of classes, structures, unions and any other user defined data type.

The syntax of using sizeof is as follows −

sizeof (data type)

Where data type is the desired data type including classes, structures, unions and any other user defined data type.

Try the following example to understand all the sizeof operator available in C++. Copy and paste following C++ program in test.cpp file and compile and run this program.

#include <iostream>
using namespace std;
 
int main() {
   cout << "Size of char : " << sizeof(char) << endl;
   cout << "Size of int : " << sizeof(int) << endl;
   cout << "Size of short int : " << sizeof(short int) << endl;
   cout << "Size of long int : " << sizeof(long int) << endl;
   cout << "Size of float : " << sizeof(float) << endl;
   cout << "Size of double : " << sizeof(double) << endl;
   cout << "Size of wchar_t : " << sizeof(wchar_t) << endl;
   
   return 0;
}

When the above code is compiled and executed, it produces the following result, which can vary from machine to machine −

Size of char : 1
Size of int : 4
Size of short int : 2
Size of long int : 4
Size of float : 4
Size of double : 8
Size of wchar_t : 4

The following examples demonstrate the common usage of the sizeof operator in C++:

In the following example, we are finding the memory size occupied by different variables.

#include <iostream>
using namespace std;

int main() {
    int age = 25;
    float price = 99.99;
    char grade = 'A';

    // Printing size of variables
    cout << "Size of age (int): " << sizeof(age) << " bytes" << endl;
    cout << "Size of price (float): " << sizeof(price) << " bytes" << endl;
    cout << "Size of grade (char): " << sizeof(grade) << " bytes" << endl;

    return 0;
}

When executed, this program outputs:

Size of age (int): 4 bytes
Size of price (float): 4 bytes
Size of grade (char): 1 bytes

In the following example, we are finding the total size of an integer array and the number of elements it contains.

#include <iostream>
using namespace std;

int main() {
    int scores[] = {85, 90, 78, 92, 88};

    // Finding the number of elements in the array
    int totalSize = sizeof(scores);
    int elementSize = sizeof(scores[0]);
    int length = totalSize / elementSize;

    cout << "Total size of array: " << totalSize << " bytes" << endl;
    cout << "Size of one element: " << elementSize << " bytes" << endl;
    cout << "Number of elements: " << length << endl;

    return 0;
}

When executed, this program outputs:

Total size of array: 20 bytes
Size of one element: 4 bytes
Number of elements: 5

In the following example, we are finding the memory size occupied by an object of a class.

#include <iostream>
using namespace std;

class Student {
    int rollNumber;
    double marks;
public:
    Student(int r = 1, double m = 95.5) : rollNumber(r), marks(m) {}
};

int main() {
    // Finding size of class Student
    cout << "Size of Student class: " << sizeof(Student) << " bytes" << endl;
    return 0;
}

When executed, this program outputs:

Size of Student class: 16 bytes

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