FAQ-0556 Why is it better to use the Pre-increment Operator rather than the Post-Increment Increment Operator?


FAQ-0556 Why is it better to use the Pre-increment Operator rather than the Post-Increment Increment Operator?

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Classification:	C++	Category:	Development
Created:	01/19/2001	Modified:	06/12/2001
Number:	FAQ-0556
Platform:	Not Applicable

Question:
Why is it better to use the Pre-Increment Operator (e.g. ++var) rather than the Post-Increment Operator (e.g. var++)?

Answer:
In C++ terms, if a copy constructor has been defined for the class you are incrementing, then using the post-increment operator requires the creation of a temporary object.
Looking at things in more detail, here are the steps performed by each operator:

pre-increment:

*this += 1;
return( *this );

post-increment:

Object temp = *this;
++(*this);
return( temp );

Hence the pre-increment operator just increments and returns, whilst the post-increment operator has to take a copy, increment the object and return the un-incremented copy. So for C++, the post-increment operator needs to construct and destruct a new object in order to return the un-incremented value. It is therefore more efficient to use the pre-increment operator rather than the post-increment operator.

In EPOC terms, if the class being incremented is a T or R class there is a small saving (the bitwise copy of the class into a temporary). As for C classes, you will need to use the pre-increment operator as you're not allowed copy constructors on C classes.

Symbian coding standards recommend using the pre-increment operator wherever possible, so that you get into the habit of it, and are more likely to use it automatically in situations where it actually makes a difference.