C++0x tuples have no iterators, right?

Question

Looking into the standard N3291 I do not find any reference for tuple to support begin() and end(). But when I look at my notes from years back I seem to have jotted down that I need to look into that later. And here we are.

I can not find any trace of tuple<...>.begin() or tuple<...>.end() in the current C++0x standard, is this correct? It is not possible to pass a tuple with its iterators to an algorithm, nor can one for-loop over it, right?

tuple<int,string,double> val;
for(auto a : val) cerr << val;     // very wrong!

which is of course nonsense, because what should auto be?

I need the confirmation that my notes contain an error, and that there is no way to get those iterators for tuple elements. Or maybe there was an abandoned path in the standards discussion?

Note: I am aware of that one can use TMP or Variadic Templates to implement a do-for-all-elements-of-a-tuple, but my question is really about iterators.

Why not for-loop with index from 0 to `::tuple_size` and access elements via `std::get` ? — osgx, Jul 31 '11 at 12:16
@osgx: The template parameter has to be a compile-time constant! — Kerrek SB, Jul 31 '11 at 12:17
Check this question and its answers http://stackoverflow.com/q/1198260/196561 — osgx, Jul 31 '11 at 12:19

score 7 · Accepted Answer · answered Jul 31 '11 at 20:16

Boost.Fusion has iterable tuples -- boost::fusion::vector<> -- along with many iteration, query, and transformation algorithms for tuples.

It also has code to adapt boost::tuple<> for use with those iterators and algorithms; you could take that code and modify it to work with std::tuple<> instead.

score 3 · Answer 2 · answered Jul 31 '11 at 12:20

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Iterators for tuples would be as useful as iterators for class members. Tuples just aren't meant to have that kind of--iterable--content.

answered Jul 31 '11 at 12:20

mike3996

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That is a good comparison. Although, I am sure, many reflection fans (read: Java fans) find iterators for class members quite useful... ;-) – towi Jul 31 '11 at 12:24
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@towi, oops :) It does have its uses there. – mike3996 Jul 31 '11 at 12:43

score 1 · Answer 3 · answered Jul 31 '11 at 12:17

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No, there are no iterators for tuples. Iterators are a run-time concept, while tuples are a compile-time construct. As you rightly remarked, there isn't even a way to make sense of a generic tuple iterator.

If you need a type-erasing runtime container, you could use a vector of boost::anys.

answered Jul 31 '11 at 12:17

Kerrek SB

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Nono, I just wanted to make sense of my notes. – towi Jul 31 '11 at 12:22

C++0x tuples have no iterators, right?

3 Answers3