[multi_index] iterating over keys and "extracting" key extractors
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. 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) An example is in order: struct Foo { int id; sting name; }; using namespace boost::multi_index; typedef multi_index_container< Foo, indexed_by< ordered_unique< member< Foo, int, &Foo::id > >, ordered_unique< member< Foo, string, &Foo::name > >
MFoo;
MFoo foos; // .. populate // Now, we want to iterate over sorted name's only, for e.g: auto beg = answer_to_Q1(foos).begin(); auto end = answer_to_Q1(foos).end(); std::copy( beg, end, ostream_iterator<string>(cout, " ") ); // Continuing the example for Q2: auto aFunctorThatExtractsFooName = answer_to_Q2<M>::type(); auto beg = make_transform_iterator( M.get<1>.cbegin(), aFunctorThatExtractsFooName ); auto end = make_transform_iterator( M.get<1>.cend(), aFunctorThatExtractsFooName ); std::copy( beg, end, ostream_iterator<string>(cout, " ") ); Thanks! Nick
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 Telefónica
Thanks Joaquín
On Wed, Mar 18, 2015 at 3:45 PM, Joaquin M Lopez Munoz
Nick Stokes
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_iteratorstd::string(std::cout," "));
Joaquín M López Muñoz Telefónica _______________________________________________ Boost-users mailing list Boost-users@lists.boost.org http://lists.boost.org/mailman/listinfo.cgi/boost-users
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
Telefónica
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Boost-users mailing list
Boost-users@lists.boost.org
http://lists.boost.org/mailman/listinfo.cgi/boost-users
... 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 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@gmail.com> 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
Telefónica
_______________________________________________
Boost-users mailing list
Boost-users@lists.boost.org
http://lists.boost.org/mailman/listinfo.cgi/boost-users
Sorry for being vague about versions. This was with Boost 1.49. Both
Visual Studio 2012 and 2013 generates the warning. Clang 3.5.0 and g++
4.7.2 do not. They all compile fine at the end.
Also I just tried with Boost 1.56. There is no warning in either
compiler! Strangely, the result_type typedef in key extractor is still
non-& in this version. Perhaps Boost.Range changed...
Any explanation of what may have changed and why not having a & in
result_type works would be greatly appreciated.
Thanks many,
On Thu, Mar 19, 2015 at 2:20 PM, Nick Stokes wrote: ... 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@gmail.com> 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@gmail.com> 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
Telefónica
_______________________________________________
Boost-users mailing list
Boost-users@lists.boost.org
http://lists.boost.org/mailman/listinfo.cgi/boost-users
Nick Stokes
Sorry for being vague about versions. This was with Boost 1.49. Both Visual Studio 2012 and 2013 generates the warning. Clang 3.5.0 and g++ 4.7.2 do not. They all compile fine at the end.
Also I just tried with Boost 1.56. There is no warning in either compiler! Strangely, the result_type typedef in key extractor is still non-& in this version. Perhaps Boost.Range changed...
Please don't top-post:
http://www.boost.org/community/policy.html#quoting
Thank you!
So, to sum up the top-posted chain of posts of yours,
boost::multi_index::member
participants (2)
-
Joaquin M Lopez Munoz -
Nick Stokes