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 a_description_of_the_C_and C++_data types 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