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 wrote: Thanks Joaquín On Wed, Mar 18, 2015 at 3:45 PM, Joaquin M Lopez Munoz Nick Stokes 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_iteratorstd::string(std::cout," ")); Joaquín M López Muñoz
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