In this exercise, you prevent a function from changing the value of a named constant pas...

In this exercise, you prevent a function from changing the value of a named constant passed to it.

a. If necessary, create a new project named Advanced22 Project. Copy the instructions from the Lab10-2.cpp file into a source file named Advanced22.cpp. (Alternatively, you can enter the instructions from Figure 10-31 into the Advanced22.cpp file.) Change the filename in the first comment to Advanced22.cpp.

b. The program passes the value of the UNIT _CHG named constant to the calcBill function, which stores the value it receives (.09) in the chgPerUni t variable. Because the function stores the value in a variable, the value can be changed by the function. First, you will verify that the cal cBill function can change the value stored in the chg PerUn i t variable. Insert a blank line above the totChg = numUni is * chg PerUn i t; statement in the calcBill function. In the blank line, type chgPerUni t = . 25; . Save and then run the program. Enter 3000 and 2000 as the current and previous readings. The program displays 1000 as the number of units used, which is correct. How-ever, rather than displaying $90.00 as the total charge, the pro-gram displays $250.00.

c. To prevent the calcBill function from changing the charge per unit value, you must use the const keyword to indicate that the value being passed is a constant. Make the appropriate modification to the cal cBill function's prototype and its header. Save and then run the program. The compiler displays an error message indicating that you cannot assign a value to the chgP rUn i t variable, which is defined using the const keyword.

d. Delete the chg PerUni t = .25 ; statement from the calcBill function. Save and then run the program. Enter 3000 and 2000 as the current and previous readings. The program correctly displays 1000 and $90.00 as the units used and total charge, respectively.