An Introduction to Programming with C++

Fifth Edition

Chapter 10Void Functions

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Objectives

• Create a function that does not return a value

• Invoke a function that does not return a value

• Pass information, by reference, to a function

• Use void functions in a .NET C++ program

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Concept Lesson

• More About Functions

• Creating Void Functions

• Passing Variables to a Function

• Passing Variables By Value and By Reference

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More About Functions

• Use functions to avoid duplicating code– They also allow large and complex programs to be

broken into small and manageable tasks

• main() is typically responsible for invoking each of the other functions when needed– Program-defined functions streamline main()

• Functions can be:– Value-returning– Void

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Creating Void Functions

• Void functions do not return a value after completing their assigned tasks– E.g., to display information (such as a title and

column headings) at the top of each page in a report• Avoids repeating code several times in program

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Creating Void Functions (continued)

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Passing Variables to a Function

• A variable has both a value and a unique address (memory location)

• You can pass either the variable’s value or its address to the receiving function– Pass by value– Pass by reference

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Passing Variables By Value

• In a pass by value, the computer passes the contents of the variable to the receiving function– Receiving function is not given access to the variable

in memory• It cannot change value stored inside variable

• By default, variables are passed by value in C++

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Passing Variables By Value (continued)

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Passing Variables By Value (continued)

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Passing Variables By Value (continued)

Passing Variables By Value (continued)

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Passing Variables By Value (continued)

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Passing Variables By Reference

• Passing a variable’s address is referred to as passing by reference– Receiving function has access to passed variable– Use when you want receiving function to change

contents of variable

• To pass a variable by reference in C++– Include an & before the name of the corresponding

formal parameter in receiving function’s header• Called the address-of operator

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Passing Variables By Reference (continued)

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Passing Variables By Reference (continued)

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Passing Variables By Reference (continued)

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Passing Variables By Reference (continued)

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Passing Variables By Reference (continued)

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Passing Variables By Value andBy Reference

• You can mix the way variables are passed when a function is called, passing some by reference and others by value

• Program in Figure 10-15 allows user to enter an employee’s current salary and raise rate– It then calculates and displays the employee’s raise

and new salary amounts

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Passing Variables By Value andBy Reference (continued)

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Passing Variables By Value andBy Reference (continued)

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Passing Variables By Value andBy Reference (continued)

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Passing Variables By Value and By Reference (continued)

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Summary

• Functions can be:– Value-returning– Void

• Function header begins with void

• Pass by value: value stored inside variable is passed to receiving function

• Pass by reference: variable’s address in memory is passed to receiving function– Receiving function can change variable’s contents– Use & before corresponding formal parameter

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Application Lesson: Using Void Functions in a C++ Program

• Lab 10.1: Stop and Analyze

• Lab 10.2– Create program to calculate customer’s water bill

• Lab 10.3– Modified program will use two void functions (rather

than one void function) to calculate the number of gallons used and the water charge

• Lab 10.4: Desk-Check Lab

• Lab 10.5: Debugging Lab