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>

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

   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.


  • Seconded

    Posted by Pharap on 12/29/2014 12:27am

    I agree with prantif that in a case like this, the function should be using its own locals anyway. However, that's not to say such an eventuality where this sort of thing would be a problem would not occur, this example is just far to simplistic to show why it could be a problem. It shows how problems could occur, but not with a meaningful enough example.

  • 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

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