8.7. sizeof Operator

The compile time unary operator sizeof determines the size in bytes of a built-in array or of any other data type, variable or constant during program compilation. When applied to a built-in array’s name, as in Fig. 8.13 (line 13), the sizeof operator returns the total number of bytes in the array as a value of type size_t. The computer we used to compile this program stores variables of type double in 8 bytes of memory, and numbers is declared to have 20 elements (line 11), so it uses 160 bytes in memory. When applied to a pointer parameter (line 22) in a function that receives a built-in array as an argument, the sizeof operator returns the size of the pointer in bytes (4 on the system we used)—not the built-in array’s size.

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 1   // Fig. 8.13: fig08_13.cpp
 2   // Sizeof operator when applied to a built-in array's name
 3   // returns the number of bytes in the built-in array.
 4   #include <iostream>
 5   using namespace std;
 6
 7   size_t getSize( double * ); // prototype
 8
 9   int main()
10   {
11      double numbers[ 20 ]; // 20 doubles; occupies 160 bytes on our system
12
13      cout << "The number of bytes in the array is " << sizeof( numbers );
14
15      cout << " The number of bytes returned by getSize is "
16         << getSize( numbers ) << endl;
17   } // end main
18
19   // return size of ptr        
20   size_t getSize( double *ptr )
21   {                            
22      return sizeof( ptr );     
23   } // end function getSize    

The number of bytes in the array is 160
The number of bytes returned by getSize is 4

Fig. 8.13. sizeof operator when applied to a built-in array’s name returns the number of bytes in the built-in array.

Image Common Programming Error 8.3

Using the sizeof operator in a function to find the size in bytes of a built-in array parameter results in the size in bytes of a pointer, not the size in bytes of the built-in array.

The number of elements in a built-in array can be determined using the results of two sizeof operations. For example, to determine the number of elements in the built-in array numbers, use the following expression (which is evaluated at compile time):

sizeof numbers / sizeof( numbers[ 0 ] )

The expression divides the number of bytes in numbers (160, assuming 8 byte doubles) by the number of bytes in the built-in array’s zeroth element (8)—resulting in the number of elements in numbers (20).

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