Re: [Boost-users] [fusion] transform & iterators

Subject: Re: [Boost-users] [fusion] transform & iterators
From: Christopher Schmidt (mr.chr.schmidt_at_[hidden])
Date: 2010-03-12 06:40:25

David A. Greene schrieb:
> On Thursday 11 March 2010 11:39:10 Steven Watanabe wrote:
> 
>>> Ugh.  That makes things difficult then.  There's no simple way to define
>>> a function that just takes a pair of iterators and does something with
>>> them.  Surely others have tried to do things like this before.  What's
>>> the typical solution?
>>
>> Use a recursive function template or use one of fusion's algorithms.
> 
> But the non-compiling example in the first testcase shows that a fusion 
> algorithm doesn't provide a general solution.  The sequence could be the 
> result of an application of transform() or some other algorithm that causes 
> problems.  Unless there's some other technique I've missed.

Change the signature of square's invocation operator to

result_type operator()(int i) const

and the sample should compile fine.

> In any case, I wasn't completely clear about what my goal is.  Ultimately
> I'd like to initialize a class with a pair of iterators:
> 
[snip]
> 
> Unfortunately, it appears this isn't possible and foo will need at least two 
> constructors:
> 
> struct foo {
>   vector<int> data;
> 
>   template<typename InputIterator>
>   foo(InputIterator begin, InputIterator end) : data(begin, end) {}
> 
>   template<typename Sequence>
>   foo(Sequence d) {
>     // Some fusion algorithm
>   }
> };

That's correct.

> But that doesn't seem to be a general solution either due to the
> transform() problem described above.

Joel referred to fusion::transform taking the sequence cref qualified
only. This should not be a limitation for your particular use-case though.

-Christopher