TIP: Be Careful with Dummy Reference Arguments

How many times have you used functions that return results through reference parameters, such as this:

void func(int & ra1, int & ra2, int & ra3) ;

You would probably call this function this way:

func(a1, a2, a3) ;

Sometimes, if you do not want the outputs of ra1 and ra2 arguments, but only interested in ra3, you might be tempted to make a call to the function as follows:

int dummy, res ;
func(dummy, dummy, res) ;

This way, you might end up saving the space that is required for that one extra int argument by using dummy in place of arguments a1 and a2. The function may even work properly. But, what's important to note is that if the output argument ra3 is dependent on the values already computed in ra1 and ra2 by func, you might be looking at an entirely different scenario. Take the following example implementation of func:

# include <iostream.h>

void
func(int & ra1, int & ra2, int & ra3)
{
   ra1 = 1 ;
   ra2 = 2 ;
   ra3 = ra1 + ra2 ;
}

void
main()
{
   int dummy, res ;
   func(dummy, dummy, res) ;

   cout << res ;
}

No points for guessing the output. It's not 3 (as expected), but 4. This is obviously due to ra1 and ra2 being allotted the same address, that of dummy. Disaster!! You would have successfully traded accuracy/correctness for a simple saving of 4 bytes in virtual space.

Therefore, in cases where you are not sure whether the values of one result parameter affects others, safely use different variables to hold output parameters, as in:

int  dummy1, dummy2, res ;
func(dummy1, dummy2, res) ;


About the Author

Ashwin Kumar

A mind that's ready to keep learning.

Comments

  • Is it not the other way round?

    Posted by prantlf on 02/24/2009 04:30pm

    A method implementation should not make assumptions how the parameters must be entered. Otherwise the implementation will be difficult to maintain and a cause of many difficult to find troubles. In this case, local variables should be used and the output parameters filled at the end. Don't let yourself discouraged and keep learning :-) Cheers, Ferda Prantl

    Reply
Leave a Comment
  • Your email address will not be published. All fields are required.

Top White Papers and Webcasts

  • Live Event Date: May 18, 2015 @ 1:00 p.m. ET / 10:00 a.m. PT While the idea of using facial and or gesture recognitions to create a modern, intuitive game seems attractive, some developers may want to leverage Unity 3D as a way to accelerate their development. There are many different ways in which Intel and Unity Technologies have been working together to helps speed the develop of games with the Intel® RealSense™ SDK (Software Developer Kit), so come hear from a panel of experts on what we've done …

  • Are you truly leading your team or simply managing them? Organizations need leaders and your team needs someone to follow. With some ongoing development, you could become that leader. Learn the top leadership qualities that inspire others to want to follow you and the direction of your company.

Most Popular Programming Stories

More for Developers

RSS Feeds

Thanks for your registration, follow us on our social networks to keep up-to-date