Boost::Tuples vs Structs for return values

Question

I'm trying to get my head around tuples (thanks @litb), and the common suggestion for their use is for functions returning > 1 value.

This is something that I'd normally use a struct for , and I can't understand the advantages to tuples in this case - it seems an error-prone approach for the terminally lazy.

Borrowing an example, I'd use this

struct divide_result {
    int quotient;
    int remainder;
};

Using a tuple, you'd have

typedef boost::tuple<int, int> divide_result;

But without reading the code of the function you're calling (or the comments, if you're dumb enough to trust them) you have no idea which int is quotient and vice-versa. It seems rather like...

struct divide_result {
    int results[2]; // 0 is quotient, 1 is remainder, I think
};

...which wouldn't fill me with confidence.

So, what are the advantages of tuples over structs that compensate for the ambiguity?

Answer 1

I tend to use tuples in conjunction with typedefs to at least partially alleviate the 'nameless tuple' problem. For instance if I had a grid structure then:

//row is element 0 column is element 1
typedef boost::tuple<int,int> grid_index;

Then I use the named type as :

grid_index find(const grid& g, int value);

This is a somewhat contrived example but I think most of the time it hits a happy medium between readability, explicitness, and ease of use.

Or in your example:

//quotient is element 0 remainder is element 1
typedef boost:tuple<int,int> div_result;
div_result div(int dividend,int divisor);

Answer 2

tuples

I think i agree with you that the issue with what position corresponds to what variable can introduce confusion. But i think there are two sides. One is the call-side and the other is the callee-side:

int remainder; 
int quotient;
tie(quotient, remainder) = div(10, 3);

I think it's crystal clear what we got, but it can become confusing if you have to return more values at once. Once the caller's programmer has looked up the documentation of div, he will know what position is what, and can write effective code. As a rule of thumb, i would say not to return more than 4 values at once. For anything beyond, prefer a struct.

output parameters

Output parameters can be used too, of course:

int remainder; 
int quotient;
div(10, 3, &quotient, &remainder);

Now i think that illustrates how tuples are better than output parameters. We have mixed the input of div with its output, while not gaining any advantage. Worse, we leave the reader of that code in doubt on what could be the actual return value of div be. There are wonderful examples when output parameters are useful. In my opinion, you should use them only when you've got no other way, because the return value is already taken and can't be changed to either a tuple or struct. operator>> is a good example on where you use output parameters, because the return value is already reserved for the stream, so you can chain operator>> calls. If you've not to do with operators, and the context is not crystal clear, i recommend you to use pointers, to signal at the call side that the object is actually used as an output parameter, in addition to comments where appropriate.

returning a struct

The third option is to use a struct:

div_result d = div(10, 3);

I think that definitely wins the award for clearness. But note you have still to access the result within that struct, and the result is not "laid bare" on the table, as it was the case for the output parameters and the tuple used with tie.

I think a major point these days is to make everything as generic as possible. So, say you have got a function that can print out tuples. You can just do

cout << div(10, 3);

And have your result displayed. I think that tuples, on the other side, clearly win for their versatile nature. Doing that with div_result, you need to overload operator<<, or need to output each member separately.

Answer 3

Another option is to use a Boost Fusion map (code untested):

struct quotient;
struct remainder;

using boost::fusion::map;
using boost::fusion::pair;

typedef map<
    pair< quotient, int >,
    pair< remainder, int >
> div_result;

You can access the results relatively intuitively:

using boost::fusion::at_key;

res = div(x, y);
int q = at_key<quotient>(res);
int r = at_key<remainder>(res);

There are other advantages too, such as the ability to iterate over the fields of the map, etc etc. See the doco for more information.

Answer 4

I agree with you 100% Roddy.

To return multiple values from a method, you have several options other than tuples, which one is best depends on your case:

1. Creating a new struct. This is good when the multiple values you're returning are related, and it's appropriate to create a new abstraction. For example, I think "divide_result" is a good general abstraction, and passing this entity around makes your code much clearer than just passing a nameless tuple around. You could then create methods that operate on the this new type, convert it to other numeric types, etc.

2. Using "Out" parameters. Pass several parameters by reference, and return multiple values by assigning to the each out parameter. This is appropriate when your method returns several unrelated pieces of information. Creating a new struct in this case would be overkill, and with Out parameters you emphasize this point, plus each item gets the name it deserves.

Tuples are Evil.

Answer 5

One feature of tuples that you don't have with structs is in their initialization. Consider something like the following:

struct A
{
  int a;
  int b;
};

Unless you write a make_tuple equivalent or constructor then to use this structure as an input parameter you first have to create a temporary object:

void foo (A const & a)
{
  // ...
}

void bar ()
{
   A dummy = { 1, 2 };
   foo (dummy);
}

Not too bad, however, take the case where maintenance adds a new member to our struct for whatever reason:

struct A
{
  int a;
  int b;
  int c;
};

The rules of aggregate initialization actually mean that our code will continue to compile without change. We therefore have to search for all usages of this struct and updating them, without any help from the compiler.

Contrast this with a tuple:

typedef boost::tuple<int, int, int> Tuple;
enum {
  A
  , B
  , C
};

void foo (Tuple const & p) {
}

void bar ()
{
  foo (boost::make_tuple (1, 2));  // Compile error
}

The compiler cannot initailize "Tuple" with the result of make_tuple, and so generates the error that allows you to specify the correct values for the third parameter.

Finally, the other advantage of tuples is that they allow you to write code which iterates over each value. This is simply not possible using a struct.

void incrementValues (boost::tuples::null_type) {}

template <typename Tuple_>
void incrementValues (Tuple_ & tuple) {
   // ...
   ++tuple.get_head ();
   incrementValues (tuple.get_tail ());
}

Answer 6

Prevents your code being littered with many struct definitions. It's easier for the person writing the code, and for other using it when you just document what each element in the tuple is, rather than writing your own struct/making people look up the struct definition.