Re: [Boost-users] [multi_index] iterating over keys and "extracting" key extractors

Subject: Re: [Boost-users] [multi_index] iterating over keys and "extracting" key extractors
From: Nick Stokes (randomaccessiterator_at_[hidden])
Date: 2015-03-19 14:20:17

... forgot to add, if I use a custom functor which does include the & in
result_type, like:

struct FooName {  typedef const string& result_type;    result_type
operator()(const Foo& f) const { return f.name;} };

in the same ranged for as above:

for( string& s : foos.get<1>() | boost::adaptors::transformed( FooName() )
) ..

then there is no warning from the compiler.

On Thu, Mar 19, 2015 at 2:17 PM, Nick Stokes <randomaccessiterator_at_[hidden]
> wrote:

> There is a problem...   I noticed, when used as follows with boost::range,
>
>
> for( string& s : foos.get<1>() | boost::adaptors::transformed(
> m.get<1>().key_extractor() ) )
>    ...
>
> Visual Studio 2012 generates an ugly warning " warning C4172: returning
> address of local variable or temporary", and compiles.  It also compiles on
> clang and gcc (sorry for not being too specific with versions, modern
> ones).   But this is suspicious enough so I poked around a little bit.
> Sure enough, the key_from_value (the return type of key_extractor()) is a
> functor which is like
>
> template< .. class Value  .. >
> struct key_from_value {
>      typedef  Value result_type;   // not Value&
>
>      Value& operator()( Foo&.. ) const {  .. }
>
> };
>
> So isn't result_type being a non-reference type a problem here, given
> operator() is returning &?
>
> Regards,
> Nick
>
> On Thu, Mar 19, 2015 at 9:09 AM, Nick Stokes <
> randomaccessiterator_at_[hidden]> wrote:
>
>> Thanks Joaquín
>>
>> On Wed, Mar 18, 2015 at 3:45 PM, Joaquin M Lopez Munoz <joaquin_at_[hidden]>
>> wrote:
>>
>>> Nick Stokes <randomaccessiterator <at> gmail.com> writes:
>>>
>>> >
>>> > Q1:  Is there a builtin in way in MI to iterate over keys rather
>>> > than elements? I.e. the iterator's value type will be that of the
>>> > key, not the element.
>>>
>>> No, there's none. Some iterator adaption is needed as you mention
>>> below.
>>>
>>> > Q2:  Is there a way to obtain the type of key extractors given a
>>> > multi index container?  (it is related to Q1, as this sort of a
>>> > thing could be passed to boost::transform iterator to obtain an
>>> > alternative answer for Q1)
>>>
>>>   auto beg=boost::make_transform_iterator(
>>>              foos.get<1>().begin(),foos.get<1>().key_extractor()),
>>>        end=boost::make_transform_iterator(
>>>              foos.get<1>().end(),foos.get<1>().key_extractor());
>>>
>>>   std::copy(
>>>     beg,end,std::ostream_iterator<std::string>(std::cout," "));
>>>
>>> Joaquín M López Muñoz
>>> Telefónica
>>> _______________________________________________
>>> Boost-users mailing list
>>> Boost-users_at_[hidden]
>>> http://lists.boost.org/mailman/listinfo.cgi/boost-users
>>
>>
>>
>