views:

1091

answers:

22

When your in a situation where you need to return two things in a single method, what is the best approach?

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.

The options I have come up with:

  1. Use global variables to hold returns. I personally try and avoid globals where I can.
  2. Pass in two empty variables as parameters then assign the variables inside the method, which now is a void. I don't like the idea of methods that have a side effects.
  3. Return a collection that contains two variables. This can lead to confusing code.
  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.
A: 

Use std::vector, QList, or some managed library container to hold however many X you want to return:

QList<X> getMultipleItems()
{
  QList<X> returnValue;
  for (int i = 0; i < countOfItems; ++i)
  {
    returnValue.push_back(<your data here>);
  }
  return returnValue;
}
Bill
I'd think a vector would be better, just for cache reasons.
GMan
@GMan - I'm used to QList, which is mostly a vector under the covers, thanks. (Although if the OP is really only returning a couple of objects it's pretty moot. :-P )
Bill
A: 

Use var/out parameters or pass variables by reference, not by value. In Delphi:

function ReturnTwoValues(out Param1: Integer):Integer;
begin
  Param1 := 10;
  Result := 20;
end;

If you use var instead of out, you can pre-initialize the parameter.

With databases, you could have an out parameter per column and the result of the function would be a boolean indicating if the record is retrieved correctly or not. (Although I would use a single record class to hold the column values.)

Workshop Alex
This is just a specialized version of #2 on his list (or is #2 a specialized version of out params :)
DVK
That's a pretty confusing approach from the readability standpoint. Most people expect parameters to be function inputs, not outputs.
Anna Lear
@Anna, you're right but then again, most people expect functions to only return one result, not two. For the few exceptions, using an out parameter is still the most optimal solution. (Code completion will notify developers about the parameter being an out parameter.)
Workshop Alex
@DVK, basically, it also depends on the language you're using. In Delphi (code example) solution #2 happens to be the most common solution. Pascal couldn't return structures as function results originally so structures are often returned as parameters with some additional result value indicating success or error.
Workshop Alex
@Workshop Alex: True enough, but either refactoring the approach or using a DTO/collection still seems more straightforward to me.
Anna Lear
@Anna, what you also need to consider is the history of certain programming languages and the programmers that have been using those languages for over 20 years already. (I started with Pascal in 1986!) Even though programming languages can introduce new technique, most of those old dudes will just be happy using those older techniques. Option 2 is better than #1 and #3 and #4 used to be too complex a decade or two ago, even though back then OO would still be an option. #3 and #4 are reasonable new techniques compared to #1 and #2 for quite a few languages.
Workshop Alex
+13  A: 

I would create data transfer objects. If it is a group of information (first and last name) I would make a Name class and return that. #4 is the way to go. It seems like more work up front (which it is), but makes it up in clarity later.

If it is a list of records (rows in a database) I would return a Collection of some sort.

I would never use globals unless the app is trivial.

Nemi
(+1) When you do this, you actually change the method to just return a single value again. :-)
Workshop Alex
+1  A: 

If I was returning two of the exact same thing, a collection might be appropriate, but in general I would usually build a specialized class to hold exactly what I needed.

And if if you are returning two things today from those two columns, tomorrow you might want a third. Maintaining a custom object is going to be a lot easier than any of the other options.

Adam Batkin
But adding one more class does add more code, more source files and thus more complexity. It's one more object that needs to be created and freed.
Workshop Alex
Why the downvote? This is the same as the top-voted answer, except in that case it was given a better name (DTO). And adding classes when they are needed does not increase complexity, that's what they are there for! In this case, the clarity that a specialized class would provide far outweighs the perceived complexity of adding an additional class.
Adam Batkin
+1  A: 
  1. Some languages make doing #3 native and easy. Example: Perl. "return ($a, $b);". Ditto Lisp.

  2. Barring that, check if your language has a collection suited to the task, ala pair/tuple in C++

  3. Barring that, create a pair/tuple class and/or collection and re-use it, especially if your language supports templating.

DVK
A: 

As much as it pains me to do it, I find the most readable way to return multiple values in PHP (which is what I work with, mostly) is using a (multi-dimensional) array, like this:

function doStuff($someThing)
{
    // do stuff
    $status  = 1;
    $message = 'it worked, good job';

    return array('status' => $status, 'message' => $message);
}

Not pretty, but it works and it's not terribly difficult to figure out what's going on.

inkedmn
You should edit your post to show how you can use list($var1, $var2) to unpack the array.
ryeguy
what's so unpretty about this solution? I don't think it makes the code unreadable. but it's not a multidimensional array, it's just a simple array with two key => value pairs.
tharkun
What makes this EXTREMELY unpretty is that it forces tight coupling between the caller and doStuff. If you don't believe me try writing the caller that uses the return values without looking at the implementation of DoStuff.
JohnFx
@inkedmn:You have multi-dimensional mixed up with associative. Your example shows what is called an associative array in PHP.@JohnFx:Unless the caller just passes the object along, I don't see how any other implementation wouldn't need you to look at it's implementation (or at least at the docs). How would returning any other type losen coupling?
fireeyedboy
I do this in JavaScript with an object. If you're careful, it can make the code short and clear.
Nosredna
+3  A: 

In the C/C++ world it would actually be quite common to pass two variables by reference (an example, your no. 2).

middus
+15  A: 

This is not entirely language-agnostic: in Lisp, you can actually return any number of values from a function, including (but not limited to) none, one, two, ...

(defun returns-two-values ()
  (values 1 2))

The same thing holds for Scheme and Dylan. In Python, I would actually use a tuple containing 2 values like

def returns_two_values():
   return (1, 2)

As others have pointed out, you can return multiple values using the out parameters in C#. In C++, you would use references.

void 
returns_two_values(int& v1, int& v2)
{
    v1 = 1; v2 = 2;
}

In C, your method would take pointers to locations, where your function should store the result values.

void 
returns_two_values(int* v1, int* v2)
{
    *v1 = 1; *v2 = 2;
}

For Java, I usually use either a dedicated class, or a pretty generic little helper (currently, there are two in my private "commons" library: Pair<F,S> and Triple<F,S,T>, both nothing more than simple immutable containers for 2 resp. 3 values)

Dirk
Yeah, your absolutely right this isn't completely agnostic. Still there are solutions to certain groups of languages. For instance, the C++ solution you present could be implemented in Java, which is the language I am actually using.
James McMahon
Dirk
The Java standard library has org.omg.CORBA.IntHolder somewhat resembles a reference in C++ and was intended for use as an out parameter. But I've never seen it used in idiomatic code.
Pete Kirkham
@Dirk, well for primitives it is impossible to pass by reference, but the accessors for objects use a reference to the original memory space, equivalent to -> in C++.
James McMahon
@nemo: Yes. But if using pre-built container objects for passing return values would be a special case of your `#2` or that `IntHolder` thing mentioned by Pete Kirkham.
Dirk
Using the built in data type of Tuple is what you are after. If you don't have one in your language of choice, it's often possible to make one. This way, it's not specific to that method and can be reasonably used elsewhere.
CaptainCasey
+3  A: 

I think the most preferred approach is to build a container (may it be a class or a struct - if you don't want to create a separate class for this, struct is the way to go) that will hold all the parameters to be returned.

Ashwin
+2  A: 

Personally I try to use languages that allow functions to return something more than a simple integer value.

First, you should distinguish what you want: an arbitrary-length return or fixed-length return.

If you want your method to return an arbitrary number of arguments, you should stick to collection returns. Because the collections--whatever your language is--are specifically tied to fulfill such a task.

But sometimes you just need to return two values. How does returning two values--when you're sure it's always two values--differ from returning one value? No way it differs, I say! And modern languages, including perl, ruby, C++, python, ocaml etc allow function to return tuples, either built-in or as a third-party syntactic sugar (yes, I'm talking about boost::tuple). It looks like that:

tuple<int, int, double> add_multiply_divide(int a, int b) {
  return make_tuple(a+b, a*b, double(a)/double(b));
}

Specifying an "out parameter", in my opinion, is overused due to the limitations of older languages and paradigms learned those days. But there still are many cases when it's usable (if your method needs to modify an object passed as parameter, that object being not the class that contains a method).

The conclusion is that there's no generic answer--each situation has its own solution. But one common thing there is: it's not violation of any paradigm that function returns several items. That's a language limitation later somehow transferred to human mind.

Pavel Shved
Oh, I didn't mention globals. Because they suck. Period.
Pavel Shved
A: 

I generally use tuples. I mainly work in C# and its very easy to design generic tuple constructs. I assume it would be very similar for most languages which have generics. As an aside, 1 is a terrible idea, and 3 only works when you are getting two returns that are the same type unless you work in a language where everything derives from the same basic type (i.e. object). 2 and 4 are also good choices. 2 doesn't introduce any side effects a priori, its just unwieldy.

Steve
+12  A: 

Not my own thoughts (Uncle Bob's):

If there's cohesion between those two variables - I've heard him say, you're missing a class where those two are fields. (He said the same thing about functions with long parameter lists.)

On the other hand, if there is no cohesion, then the function does more than one thing.

azheglov
A: 

For the situation you described, pulling two fields from a single table, the appropriate answer is #4 given that two properties (fields) of the same entity (table) will exhibit strong cohesion.

Your concern that "it might be confusing to create a class just for the purpose of a return" is probably not that realistic. If your application is non-trivial you are likely going to need to re-use that class/object elsewhere anyway.

JohnFx
+1  A: 

If your function has return value(s), it's presumably returning it/them for assignment to either a variable or an implied variable (to perform operations on, for instance.) Anything you can usefully express as a variable (or a testable value) should be fair game, and should dictate what you return.

Your example mentions a row or a set of rows from a SQL query. Then you reasonably should be ready to deal with those as objects or arrays, which suggests an appropriate answer to your question.

le dorfier
+3  A: 

I think it all depends on the scenario.

Thinking from a C# mentality:

1: I would avoid globals as a solution to this problem, as it is accepted as bad practice.

4: If the two return values are uniquely tied together in some way or form that it could exist as its own object, then you can return a single object that holds the two values. If this object is only being designed and used for this method's return type, then it likely isn't the best solution.

3: A collection is a great option if the returned values are the same type and can be thought of as a collection. However, if the specific example needs 2 items, and each item is it's 'own' thing -> maybe one represents the beginning of something, and the other represents the end, and the returned items are not being used interchangably, then this may not be the best option.

2: I like this option the best, if 4, and 3 do not make sense for your scenario. As stated in 3, if you wanted to get two objects that represent the beginning and end items of something. Then I would use parameters by reference (or out parameters, again, depending on how it's all being used). This way your parameters can explicitly define their purpose: MethodCall(ref object StartObject, ref object EndObject)

Scott
A: 

My choice is #4. Define a reference parameter in your function. That pointer references to a Value Object.

In PHP:

class TwoValuesVO { public $expectedOne; public $expectedTwo; }

/* parameter $_vo references to a TwoValuesVO instance */ function twoValues( & $_vo ) { $vo->expectedOne = 1; $vo->expectedTwo = 2; }

In Java:

class TwoValuesVO { public int expectedOne; public int expectedTwo; }

class TwoValuesTest { void twoValues( TwoValuesVO vo ) { vo.expectedOne = 1; vo.expectedTwo = 2; } }

[]'s,

And Past

apast
+2  A: 

Python (like Lisp) also allows you to return any number of values from a function, including (but not limited to) none, one, two

def quadcube (x):
     return x**2, x**3

a, b = quadcube(3)
Dragos Toader
A: 

You should also consider whether the design of your method is primarily returning a single value, and you are getting another value for reference along with it, or if you really have a single returnable thing like first name - last name.

For instance, you might have an inventory module that queries the number of widgets you have in inventory. The return value you want to give is the actual number of widgets.. However, you may also want to record how often someone is querying inventory and return the number of queries so far. In that case it can be tempting to return both values together. However, remember that you have class vars availabe for storing data, so you can store an internal query count, and not return it every time, then use a second method call to retrieve the related value. Only group the two values together if they are truly related. If they are not, use separate methods to retrieve them separately.

Zak
+1  A: 

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.

Allan Woloshin
A: 

Haskell also allows multiple return values using built in tuples:

sumAndDifference        :: Int -> Int -> (Int, Int)
sumAndDifference x y    = (x + y, x - y)

> let (s, d) = sumAndDifference 3 5 in s * d
-16

Being a pure language, options 1 and 2 are not allowed.

Even using a state monad, the return value contains (at least conceptually) a bag of all relevant state, including any changes the function just made. It's just a fancy convention for passing that state through a sequence of operations.

Chris Smith
A: 

I will usually opt for approach #4 as I prefer the clarity of knowing what the function produces or calculate is it's return value (rather than byref parameters). Also, it lends to a rather "functional" style in program flow.

The disadvantage of option #4 with generic tuple classes is it isn't much better than returning a collection (the only gain is type safety).

public IList CalculateStuffCollection(int arg1, int arg2)
public Tuple<int, int> CalculateStuffType(int arg1, int arg2)

var resultCollection = CalculateStuffCollection(1,2);
var resultTuple = CalculateStuffTuple(1,2);

resultCollection[0]    // Was it index 0 or 1 I wanted? 
resultTuple.A          // Was it A or B I wanted?

I would like a language that allowed me to return an immutable tuple of named variables (similar to a dictionary, but immutable, typesafe and statically checked). But, sadly, such an option isn't available to me in the world of VB.NET, it may be elsewhere.

I dislike option #2 because it breaks that "functional" style and forces you back into a procedural world (when often I don't want to do that just to call a simple method like TryParse).

A: 

I have sometimes used continuation-passing style to work around this, passing a function value as an argument, and returning that function call passing the multiple values.

Objects in place of function values in languages without first-class functions.

Lang Martin