Why is the use of tuples in C++ not more common?

Why does nobody seem to use tuples in C++, either the Boost Tuple Library or the standard library for TR1? I have read a lot of C++ code, and very rarely do I see the use of tuples, but I often see lots of places where tuples would solve many problems (usually returning multiple values from functions).

Tuples allow you to do all kinds of cool things like this:

tie(a,b) = make_tuple(b,a); //swap a and b

That is certainly better than this:

temp=a;
a=b;
b=temp;

Of course you could always do this:

swap(a,b);

But what if you want to rotate three values? You can do this with tuples:

tie(a,b,c) = make_tuple(b,c,a);

Tuples also make it much easier to return multiple variable from a function, which is probably a much more common case than swapping values. Using references to return values is certainly not very elegant.

Are there any big drawbacks to tuples that I'm not thinking of? If not, why are they rarely used? Are they slower? Or is it just that people are not used to them? Is it a good idea to use tuples?

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Because it's not yet standard. Anything non-standard has a much higher hurdle. Pieces of Boost have become popular because programmers were clamoring for them. (hash_map leaps to mind). But while tuple is handy, it's not such an overwhelming and clear win that people bother with it.

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I have a feeling that many use Boost.Any and Boost.Variant (with some engineering) instead of Boost.Tuple.

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The C++ tuple syntax can be quite a bit more verbose than most people would like.

Consider:

typedef boost::tuple<MyClass1,MyClass2,MyClass3> MyTuple;

So if you want to make extensive use of tuples you either get tuple typedefs everywhere or you get annoyingly long type names everywhere. I like tuples. I use them when necessary. But it's usually limited to a couple of situations, like an N-element index or when using multimaps to tie the range iterator pairs. And it's usually in a very limited scope.

It's all very ugly and hacky looking when compared to something like Haskell or Python. When C++0x gets here and we get the 'auto' keyword tuples will begin to look a lot more attractive.

The usefulness of tuples is inversely proportional to the number of keystrokes required to declare, pack, and unpack them.

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Certainly tuples can be useful, but as mentioned there's a bit of overhead and a hurdle or two you have to jump through before you can even really use them.

If your program consistently finds places where you need to return multiple values or swap several values, it might be worth it to go the tuple route, but otherwise sometimes it's just easier to do things the classic way.

Generally speaking, not everyone already has Boost installed, and I certainly wouldn't go through the hassle of downloading it and configuring my include directories to work with it just for its tuple facilities. I think you'll find that people already using Boost are more likely to find tuple uses in their programs than non-Boost users, and migrants from other languages (Python comes to mind) are more likely to simply be upset about the lack of tuples in C++ than to explore methods of adding tuple support.

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Not everyone can use boost, and TR1 isn't widely available yet.

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A cynical answer is that many people program in C++, but do not understand and/or use the higher level functionality. Sometimes it is because they are not allowed, but many simply do not try (or even understand).

As a non-boost example: how many folks use functionality found in <algorithm>?

In other words, many C++ programmers are simply C programmers using C++ compilers, and perhaps std::vector and std::list. That is one reason why the use of boost::tuple is not more common.

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But what if you want to rotate three values?

swap(a,b);
swap(b,c);  // I knew those permutation theory lectures would come in handy.

OK, so with 4 etc values, eventually the n-tuple becomes less code than n-1 swaps. And with default swap this does 6 assignments instead of the 4 you'd have if you implemented a three-cycle template yourself, although I'd hope the compiler would solve that for simple types.

You can come up with scenarios where swaps are unwieldy or inappropriate, for example:

tie(a,b,c) = make_tuple(b*c,a*c,a*b);

is a bit awkward to unpack.

Point is, though, there are known ways of dealing with the most common situations that tuples are good for, and hence no great urgency to take up tuples. If nothing else, I'm not confident that:

tie(a,b,c) = make_tuple(b,c,a);

doesn't do 6 copies, making it utterly unsuitable for some types (collections being the most obvious). Feel free to persuade me that tuples are a good idea for "large" types, by saying this ain't so :-)

For returning multiple values, tuples are perfect if the values are of incompatible types, but some folks don't like them if it's possible for the caller to get them in the wrong order. Some folks don't like multiple return values at all, and don't want to encourage their use by making them easier. Some folks just prefer named structures for in and out parameters, and probably couldn't be persuaded with a baseball bat to use tuples. No accounting for taste.

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For me, it's habit, hands down: Tuples don't solve any new problems for me, just a few I can already handle just fine. Swapping values still feels easier the old fashioned way -- and, more importantly, I don't really think about how to swap "better." It's good enough as-is.

Personally, I don't think tuples are a great solution to returning multiple values -- sounds like a job for structs.

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When using C++ on embedded systems, pulling in Boost libraries gets complex. They couple to each other, so library size grows. You return data structures or use parameter passing instead of tuples. When returning tuples in Python the data structure is in the order and type of the returned values its just not explicit.

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As many people pointed out, tuples are just not that useful as other features.

  1. The swapping and rotating gimmicks are just gimmicks. They are utterly confusing to those who have not seen them before, and since it is pretty much everyone, these gimmicks are just poor software engineering practice.

  2. Returning multiple values using tuples is much less self-documenting then the alternatives -- returning named types or using named references. Without this self-documenting, it is easy to confuse the order of the returned values, if they are mutually convertible, and not be any wiser.

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You rarely see them because well-designed code usually doesn't need them- there are not to many cases in the wild where using an anonymous struct is superior to using a named one. Since all a tuple really represents is an anonymous struct, most coders in most situations just go with the real thing.

Say we have a function "f" where a tuple return might make sense. As a general rule, such functions are usually complicated enough that they can fail.

If "f" CAN fail, you need a status return- after all, you don't want callers to have to inspect every parameter to detect failure. "f" probably fits into the pattern:

struct ReturnInts ( int y,z; }
bool f(int x, ReturnInts& vals);


int x = 0;
ReturnInts vals;
if(!f(x, vals)) {
..report error..
..error handling/return...
}

That isn't pretty, but look at how ugly the alternative is. Note that I still need a status value, but the code is no more readable and not shorter. It is probably slower too, since I incur the cost of 1 copy with the tuple.

std::tuple<int, int, bool> f(int x);
int x = 0;
std::tuple<int, int, bool> result = f(x); // or "auto result = f(x)"
if(!result.get<2>()) {
... report error, error handling ...
}

Another, significant downside is hidden in here- with "ReturnInts" I can add alter "f"'s return by modifying "ReturnInts" WITHOUT ALTERING "f"'s INTERFACE. The tuple solution does not offer that critical feature, which makes it the inferior answer for any library code.

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As a data-store std::tuple has the worst characteristics of both a struct and an array; all access is nth position based but one cannot iterate through a tuple using a for loop.

So if the elements in the tuple are conceptually an array, I will use an array and if the elements are not conceptually an array, a struct (which has named elements) is more maintainable. ( a.lastname is more explanatory than std::get<1>(a)).

This leaves the transformation mentioned by the OP as the only viable usecase for tuples.


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