An Example of Pass-By-Value

Figure 8.6 and Fig. 8.7 present two versions of a function that cubes an integer. Figure 8.6 passes variable number by value (line 14) to function cubeByValue (lines 19–22), which cubes its argument and passes the new value back to main using a return statement (line 21). The new value is assigned to number (line 14) in main. The calling function has the opportunity to examine the function call’s result before modifying variable number’s value. For example, we could have stored the result of cubeByValue in another variable, examined its value and assigned the result to number only after determining that the returned value was reasonable.

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 1   // Fig. 8.6: fig08_06.cpp
 2   // Pass-by-value used to cube a variable's value.
 3   #include <iostream>
 4   using namespace std;
 5
 6   int cubeByValue( int ); // prototype
 7
 8   int main()
 9   {
10      int number = 5;
11
12      cout << "The original value of number is " << number;
13
14      number = cubeByValue( number ); // pass number by value to cubeByValue
15      cout << " The new value of number is " << number << endl;
16   } // end main
17
18   // calculate and return cube of integer argument               
19   int cubeByValue( int n )                                       
20   {                                                              
21      return n * n * n; // cube local variable n and return result
22   } // end function cubeByValue                                  

The original value of number is 5
The new value of number is 125

Fig. 8.6. Pass-by-value used to cube a variable’s value.

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 1   // Fig. 8.7: fig08_07.cpp
 2   // Pass-by-reference with a pointer argument used to cube a
 3   // variable's value.
 4   #include <iostream>
 5   using namespace std;
 6
 7   void cubeByReference( int * ); // prototype
 8
 9   int main()
10   {
11      int number = 5;
12
13      cout << "The original value of number is " << number;
14
15      cubeByReference( &number ); // pass number address to cubeByReference
16
17      cout << " The new value of number is " << number << endl;
18   } // end main
19
20   // calculate cube of *nPtr; modifies variable number in main
21   void cubeByReference( int *nPtr )                           
22   {                                                           
23      *nPtr = *nPtr * *nPtr * *nPtr; // cube *nPtr             
24   } // end function cubeByReference                           

The original value of number is 5
The new value of number is 125

Fig. 8.7. Pass-by-reference with a pointer argument used to cube a variable’s value.

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