How to ensure comparison operator is "complete"?
-
Let's say I have a class with n member variables. I want to provide a comparison operator for the whole class.
How can I ensure I do not forget about the operator== when adding members later on, or otherwise changing the class? Same goes for the copy constructor, BTW.
Usually, it's no big deal. But there's always a time when you change code in a hurry, and later on wonder why some other perfectly fine code doesn't do as expected. I'd like to have some kind of safety net.
Ideas?
-
-Rely on the automatically generated ones?-
Edit:
If your classes derive from QObject, you could use Qt's property system to do your comparisons. If you expose all relevant properties via Q_PROPERTY, your implementation of the comparison operator could iterate over these properties and always work, even for subclasses. However, that has two downsides:- It only move the responsibility of keeping the operators in sync to making sure all properties are Q_PROPERYs
- It would run slower than using direct comparisons.
I can't think of another way though.
Edit 2:
There are no default generated ones, thanks for correcting me.
-
I often have non-trivial comparisons, like fuzzyCompare for qreal values.
-
How should any algorithm know whether a new attribute in your class is relevant for a comparison?
Even if the answer in your case would be "any one is relevant", that would be out of the scope of a compiler. One would have to analyze the source code and then to analyze the operator== method.
The method Andre did outline, works too. But it cannot handle the cases where your forgot to make the new attribute a property. So you're not gaining that much from that.
For our projects, I just would add a HUGE comment at the place where the attributes are added in the header:
@
/************************- CAUTION!
- if you add an attribute here
- ADD IT TO THE comparison methods too!
************************/
@
:-)
-
I was afraid this would be the answer...
well, one could hope.
-
Yes, hope dies last :-)
-
Perhaps you can find a static code analysis tool that can warn for these cases?
-
Yes, I also thought this would be in the realm of static code analysis.
If anyone knows about a static code analysis tool that can provide that, please speak up.
-
Maybe this is a stupid question but do you need the operator==() for comparison?
(edit:I meant to say, do you need == within the comparison?)
It could be that it suffices for your application to compare the allocated memory
blocks using sizeof(Class1/2) and memcmp()? OK, this is C not C++...
-
As the topic starter stated, he needs to do non-trival comparisons, like fuzzy comparisons of reals. That is never going to work with doing a memcmp. -Otherwise, relying on the default comparison operator would have been just as effective.-
Edit: there is no such thing as a default comparison operator in C++, thanks for correcting me.
-
[quote author="Andre" date="1333107436"]Rely on the automatically generated ones?[/quote]
Took me a while to notice...since when do automatically generated comparison operators exist?
-
[quote author="Asperamanca" date="1334563036"][quote author="Andre" date="1333107436"]Rely on the automatically generated ones?[/quote]
Took me a while to notice...since when do automatically generated comparison operators exist?
[/quote]
Sorry, you are right: they don't. I thought there were, but I was wrong on this count. I stand corrected.
StackOverflow has an interesting "discusson":http://stackoverflow.com/questions/217911/why-dont-c-compilers-define-operator-and-operator on the why (not) of automatic operator==() definition.
-
I think it's the same one I stumbled upon. ;-)
