C++ Basics - Functions for All Subtasks

Slide 5- 5
Copyright © 2007 Pearson Education, Inc. Publishing as Pearson Addison-Wesley
void-Functions

In top-down design, a subtask might produce

No value (just input or output for example)

One value

More than one value

We have seen how to implement functions that
return one value

A void-function implements a subtask that
returns no value or more than one value
Slide 5- 6
Copyright © 2007 Pearson Education, Inc. Publishing as Pearson Addison-Wesley
Display 5.1
void-Function Definition

Two main differences between void-function
definitions and the definitions of functions
that return one value

Keyword void replaces the type of the value returned

void means that no value is returned by the function

The return statement does not include and expression

Example:
void show_results(double f_degrees, double c_degrees)
{
using namespace std;
cout << f_degrees
<< “ degrees Fahrenheit is euivalent to “ << endl
<< c_degrees << “ degrees Celsius.” << endl;
return;
}
Slide 5- 7
Copyright © 2007 Pearson Education, Inc. Publishing as Pearson Addison-Wesley
Using a void-Function

void-function calls are executable statements

They do not need to be part of another statement

They end with a semi-colon

Example:
show_results(32.5, 0.3);
NOT: cout << show_results(32.5, 0.3);
Slide 5- 8
Copyright © 2007 Pearson Education, Inc. Publishing as Pearson Addison-Wesley
void-Function Calls

Mechanism is nearly the same as the function
calls we have seen

Argument values are substituted for the formal
parameters

It is fairly common to have no parameters in
void-functions

In this case there will be no arguments in the function call

Statements in function body are executed

Optional return statement ends the function

Return statement does not include a value to return

Return statement is implicit if it is not included
Slide 5- 9
Copyright © 2007 Pearson Education, Inc. Publishing as Pearson Addison-Wesley
Display 5.2 (1)
Display 5.2 (2)
Example:
Converting Temperatures

The functions just developed can be used in a
program to convert Fahrenheit temperatures to
Celcius using the formula
C = (5/9) (F – 32)

Do you see the integer division problem?
Slide 5- 10
Copyright © 2007 Pearson Education, Inc. Publishing as Pearson Addison-Wesley
Display 5.3
void-Functions
Why Use a Return?

Is a return-statement ever needed in a
void-function since no value is returned?

Yes!

What if a branch of an if-else statement requires
that the function ends to avoid producing more
output, or creating a mathematical error?

void-function in Display 5.3, avoids division by zero
with a return statement
Slide 5- 11
Copyright © 2007 Pearson Education, Inc. Publishing as Pearson Addison-Wesley
The Main Function

The main function in a program is used like a
void function…do you have to end the program
with a return-statement?

Because the main function is defined to return a
value of type int, the return is needed

C++ standard says the return 0 can be omitted, but
many compilers still require it
Slide 5- 12
Copyright © 2007 Pearson Education, Inc. Publishing as Pearson Addison-Wesley
Section 5.1 Conclusion

Can you

Describe the differences between void-
functions and functions that return one value?

Tell what happens if you forget the return-
statementin a void-function?

Distinguish between functions that are used as
expressions and those used as statements?
Copyright © 2007 Pearson Education, Inc. Publishing as Pearson Addison-Wesley
5.2
Call-By-Reference Parameters
Slide 5- 14
Copyright © 2007 Pearson Education, Inc. Publishing as Pearson Addison-Wesley
Call-by-Reference Parameters

Call-by-value is not adequate when we need
a sub-task to obtain input values

Call-by-value means that the formal parameters
receive the values of the arguments

To obtain input values, we need to change the
variables that are arguments to the function

Recall that we have changed the values of
formal parameters in a function body, but we have not
changed the arguments found in the function call

Call-by-reference parameters allow us to change
the variable used in the function call

Arguments for call-by-reference parameters must be
variables, not numbers

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