[boost] Re: FOREACH / C++ "for" proposal

From: Thorsten Ottosen (nesotto_at_[hidden])
Date: 2005-05-10 13:21:17

"Dave Harris" <brangdon_at_[hidden]> wrote in message 
news:memo.760661_at_cix.compulink.co.uk...
| In-Reply-To: <427FB7C9.9030708_at_[hidden]>
| sstrasser_at_[hidden] (Stefan Strasser) wrote (abridged):
| > > Although it doesn't seem to say whether "vec" is hoisted, and the
| > > given translation suggests that it isn't.
| >
| > maybe "hoisting" was not the right word, I only know it from a post by
| > eric.
|
| "Hoisting" is fine for the purposes of this thread.
|
|
| > what I meant was copy-the-value-of-end()-at-the-start-of-the-loop.
| > and the proposal does that, see 3.
|
| It says end(vec) is hoisted, but doesn't say whether vec itself is
| hoisted. In "for (int i: vec )" is vec evaluated once or twice? I believe
| BOOST_FOR evaluates it exactly once. The translation given in the proposal
| is:
|
|    using std::begin; // enable ADL
|    using std::end;   // ditto
|    for( auto __begin = begin(vec),
|          __end   = end(vec);
|         __begin != __end; ++__begin )
|    {
|        int i = *__begin;
|        std::cout << i;
|    }
|
| which apparently evaluates vec twice.
| A variable __end is introduced
| explicitly for end(vec), but there is no analogous variable __vec.
|
| I guess we ought to find out for sure which it will be, and then make
| BOOST_FOR do the same.

thanks for bringing this up.

the intend is, of course, that the expression is only evaluated once.

so

auto&& __vec = vec;

would be needed.

-Thorsten