When your in a situation where you
need to return two things in a single
method, what is the best approach?
It depends on WHY you are returning two things.
Basically, as everyone here seems to agree, #2 and #4 are the two best answers...
I understand the philosophy that a
method should do one thing only, but
say you have a method that runs a
database select and you need to pull
two columns. I'm assuming you only
want to traverse through the database
result set once, but you want to
return two columns worth of data.
If the two pieces of data from the database are related, such as a customer's First Name and Last Name, I would indeed still consider this to be doing "one thing."
On the other hand, suppose you have come up with a strange SELECT statement that returns your company's gross sales total for a given date, and also reads the name of the customer that placed the first sale for today's date. Here you're doing two unrelated things!
If it's really true that performance of this strange SELECT statement is much better than doing two SELECT statements for the two different pieces of data, and both pieces of data really are needed on a frequent basis (so that the entire application would be slower if you didn't do it that way), then using this strange SELECT might be a good idea - but you better be prepared to demonstrate why your way really makes a difference in perceived response time.
The options I have come up with:
1 Use global variables to hold returns. I personally try and avoid
globals where I can.
There are some situations where creating a global is the right thing to do. But "returning two things from a function" is not one of those situations. Doing it for this purpose is just a Bad Idea.
2 Pass in two empty variables as parameters then assign the variables
inside the method, which now is a
void.
Yes, that's usually the best idea. This is exactly why "by reference" (or "output", depending on which language you're using) parameters exist.
I don't like the idea of methods that have a side effects.
Good theory, but you can take it too far. What would be the point of calling SaveCustomer() if that method didn't have a side-effect of saving the customer's data?
By Reference parameters are understood to be parameters that contain returned data.
3 Return a collection that contains two variables. This can lead to confusing code.
True. It wouldn't make sense, for instance, to return an array where element 0 was the first name and element 1 was the last name. This would be a Bad Idea.
4 Build a container class to hold the double return. This is more self-documenting then a collection containing other collections, but it seems like it might be confusing to create a class just for the purpose of a return.
Yes and no. As you say, I wouldn't want to create an object called FirstAndLastNames just to be used by one method. But if there was already an object which had basically this information, then it would make perfect sense to use it here.