C++ sizeof Keyword

The sizeof keyword in C++ is used to determine the size, in bytes, of a data type or an object. It is a compile-time operator and is particularly useful for understanding memory usage, ensuring compatibility between platforms, and managing low-level memory operations.

When sizeof is applied to an object, it returns the size of the object in memory, including any padding bytes added for alignment. Similarly, when used with a type, it provides the size of that type in bytes.

Syntax

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sizeof(expression);
sizeof(type);
expression
An object or variable whose size is to be determined.
type
The data type for which the size is required.

Examples

Example 1: Using sizeof with Basic Data Types

In this example, we will use sizeof to get the sizes of basic data types.

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#include <iostream>
using namespace std;

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

Output:

Size of int: 4 bytes
Size of float: 4 bytes
Size of double: 8 bytes
Size of char: 1 byte

Explanation:

  1. The sizeof operator is applied to data types directly to get their sizes in bytes.
  2. Output may vary depending on the compiler and platform.

Example 2: Using sizeof with Variables

In this example, we will show how sizeof is used to determine the size of variables and objects.

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#include <iostream>
using namespace std;

int main() {
    int num = 42;
    double pi = 3.14;
    char letter = 'A';

    cout << "Size of num: " << sizeof(num) << " bytes" << endl;
    cout << "Size of pi: " << sizeof(pi) << " bytes" << endl;
    cout << "Size of letter: " << sizeof(letter) << " bytes" << endl;

    return 0;
}

Output:

Size of num: 4 bytes
Size of pi: 8 bytes
Size of letter: 1 byte

Explanation:

  1. The size of variables is determined based on their data types.
  2. The sizeof operator evaluates the memory footprint of the specific variable or object.

Example 3: Using sizeof with Arrays

In this example, we will learn how sizeof works with arrays.

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#include <iostream>
using namespace std;

int main() {
    int numbers[5] = {1, 2, 3, 4, 5};

    cout << "Size of array: " << sizeof(numbers) << " bytes" << endl;
    cout << "Size of one element: " << sizeof(numbers[0]) << " bytes" << endl;
    cout << "Number of elements: " << sizeof(numbers) / sizeof(numbers[0]) << endl;

    return 0;
}

Output:

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

Explanation:

  1. The total size of the array is calculated as the number of elements multiplied by the size of each element.
  2. The size of one element is determined using sizeof(numbers[0]).
  3. The number of elements is calculated by dividing the total array size by the size of one element.

Key Points to Remember about sizeof Keyword

  1. sizeof is a compile-time operator that determines the size of a type or an object.
  2. It includes padding bytes used for alignment.
  3. The result of sizeof is always returned as an unsigned integer.
  4. For arrays, sizeof returns the total size, not the number of elements.
  5. Output depends on the system architecture and compiler settings.