The sizeof operator is a compile-time operator that returns the size of the argument passed to it.

The size is a multiple of the size of a char, which on many personal computers is 1 byte (or 8 bits). The number of bits in a char is stored in the CHAR_BIT constant defined in the <climits> header file.

For example, the following code uses sizeof to display the sizes of a number of variables:

    struct EmployeeRecord {
      int ID;
      int age;
      double salary;
      EmployeeRecord* boss;
    };
 
    ...
 
    cout << "sizeof(int): " << sizeof(int) << endl
         << "sizeof(float): " << sizeof(float) << endl
         << "sizeof(double): " << sizeof(double) << endl
         << "sizeof(char): " << sizeof(char) << endl
         << "sizeof(EmployeeRecord): " << sizeof(EmployeeRecord) << endl;
 
    int i;
    float f;
    double d;
    char c;
    EmployeeRecord er;
 
    cout << "sizeof(i): " << sizeof(i) << endl
         << "sizeof(f): " << sizeof(f) << endl
         << "sizeof(d): " << sizeof(d) << endl
         << "sizeof(c): " << sizeof(c) << endl
         << "sizeof(er): " << sizeof(er) << endl;

On some machines, the above code displays this output:

    sizeof(int): 4
    sizeof(float): 4
    sizeof(double): 8
    sizeof(char): 1
    sizeof(EmployeeRecord): 20
    sizeof(i): 4
    sizeof(f): 4
    sizeof(d): 8
    sizeof(c): 1
    sizeof(er): 20

Note that sizeof can either take a variable type (such as int) or a variable name (such as i in the example above).

It is also important to note that the sizes of various types of variables can change depending on what system you're on. Check out the data types page for more information.

The parentheses around the argument are only required if you are using sizeof with a variable type (e.g. sizeof(int)). Parentheses can be left out if the argument is a variable or array (e.g. sizeof x, sizeof myArray).

Related Topics: C++ Data Types