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 not required if you are using sizeof with a variable type (e.g. sizeof(int)).