C++ sizeof Operator

Last Updated : 16 Jan, 2026

The sizeof operator is a unary, compile-time operator in C++ used to determine the memory size (in bytes) of variables, data types, constants, as well as user-defined types such as structures, unions, and classes.

For example:

C++ 
#include <iostream>
using namespace std;

int main() {
    cout << sizeof(int);
    return 0;
}

Output
4

Explanation: The above code outputs the size of the integer data type for the compiler on which the program is executed. Here, it is 4 bytes.

This article covers the syntax, usage, and common examples of sizeof operator in C++:

Syntax of sizeof in C++

sizeof (expression)

  • Parameters: expression: The variable, data type, or expression whose size (in bytes) is to be determined.
  • Return Type: Returns a value of type size_t, representing the size in bytes.

Examples of sizeof in C++

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

1. Size of Different Primitive Data Types

C++ 
#include <bits/stdc++.h>
using namespace std;

int main() {
    
    cout << "Size of char: " << sizeof(char) << endl;
    cout << "Size of int: " << sizeof(int) << endl;
    cout << "Size of float: " << sizeof(float) << endl;
    cout << "Size of double: " << sizeof(double) << endl;
    cout << "Size of long: " << sizeof(long);
    return 0;
}

Output
Size of char: 1
Size of int: 4
Size of float: 4
Size of double: 8
Size of long: 8

2. Size of Different Variables

C++ 
#include <bits/stdc++.h>
using namespace std;

int main() {
    int a;
    float b;
    char g;

    // Printing size of all three variables
    cout << "Size of a:  " << sizeof(a) << endl;
    cout << "Size of b:  " << sizeof(b) << endl;
    cout << "Size of g:  " << sizeof(g);

    return 0;
}

Output
Size of a:  4
Size of b:  4
Size of g:  1

3. Size of an Array Using sizeof

C++ 
#include <bits/stdc++.h>
using namespace std;

int main()  {
	int arr[] = {1, 2, 3, 5, 6};
  
  	// Finding the length of array
  	int n = sizeof(arr) / sizeof(arr[0]);
  	cout << n;
  	return 0;
}

Output
5

4. Size of Class

C++ 
#include <bits/stdc++.h>
using namespace std;

class A {
    int x;
    A(int val = 10) : x(val){}
};

int main() {
  	// Finding size of class A (size of the objects
  	// of class A)
    cout << "Size of Class a: " << sizeof(A);
    return 0;
}

Output
Size of Class a: 4
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