Re: [boost] [Range] Range adaptor approach for temporary range lifetime issue

Subject: Re: [boost] [Range] Range adaptor approach for temporary range lifetime issue
From: Dave Abrahams (dave_at_[hidden])
Date: 2012-06-22 14:37:12

on Sun Jun 17 2012, Michel Morin <mimomorin-AT-gmail.com> wrote:

> Dave Abrahams wrote:
>>> Side note:
>>>
>>> * There is also a fully automatic approach (i.e. when an rvalue range
>>> is adapted,
>>> `moved_range` is automatically used without piping it to `moved` adaptor).
>>> But this approach incurs unnecessary overhead when passing them to functions,
>>> because function arguments do not have the lifetime issue and we don't need
>>> to use `moved_range`. So I prefer the "manual" approach.
>>
>> Could you illustrate this with an example? Â I'd like to understand the
>> trade-off you're making.
>
> Here is an example of the manual approach:
>
>     template <typename Range> void f(Range&&);
>
>     // No lifetime problem.
>     f(std::string("Hello world!") | reversed);
>
>     // `moved` is not necessary.
>     f(std::string("Hello world!") | moved | reversed);
>
> In the automatic approach, `std::string("Hello world!") | reversed` returns
> `moved_range<std::string, boost::fusion::vector<reverse_forwarder> >`.

As opposed to what?  You haven't shown me what it returns in the manual approach.

> 
> So there is unavoidable overhead (i.e. moving a temporary container
> into range adaptors) in `f(std::string("Hello world!") | reversed);`.

Also, I don't know enough about these types to see the overhead.  More
detail would be appreciated.

-- 
Dave Abrahams
BoostPro Computing
http://www.boostpro.com