Hi, Indeed everything compiles again with boost 1.34! Thanks alot, Andrej --- Joaquín Mª López Muñoz <joaquin@tid.es> wrote:
Hello Andrej,
Andrej van der Zee ha escrito:
Hi,
I'm trying to store these classes in a multi_index:
struct Foo { int a; };
struct Bar :Foo { };
typedef multi_index_container< Bar, indexed_by< ordered_unique< member<Foo, int, &Bar::a> > >
Bar_container;
Notice I have to use "Foo" in the "member<" line.
Yes, this is an odd C++ restriction with pointers to member functions when used as non-type template parameters: although Bar inherits from Foo, &Bar::a (which is an alias for &Foo::a) is not seen (statically) as a valid int (Bar:: *) --usual base-derived relationships don't apply here. That's why you had to use Foo as the base class of member. BTW, the expression
member<Foo, int, &Bar::a>
is entirely equivalent to
member<Foo, int, &Foo::a>
although the latter is probably clearer.
However, even with this, when trying to do a search: [...] I get this:
/usr/include/boost/multi_index/member.hpp: [...] multitest2.cc:55: instantiated from here /usr/include/boost/multi_index/member.hpp:84: error: no match for 'operator*' in '*x'
I've tried a few alternatives such as providing a custom compare function but I always get this error. It seems to have something to do with the fact that the key is in the base class. If I move the key to the derived class (and change the "member<" line), it works fine.
Surely there's a way to do this. Anyone know how ?
Using gcc version 4.1.2 20061115 (prerelease) (Debian 4.1.1-21) and libboost 1.33.1.
Your code will work out of the box with Boost 1.34, where some issues about key extractors being fed derived classes are solved (see bullet 3 of
http://boost.org/libs/multi_index/doc/release_notes.html#boost_1_34
). In case you can't upgrade to Boost 1.34, you can use a custom key extractor to work around the problem:
struct Bar_a_extractor { typedef int result_type;
int operator()(const Bar& b)const { return b.a; } };
using namespace boost::multi_index;
typedef multi_index_container< Bar, indexed_by< ordered_unique< Bar_a_extractor > >
Bar_container;
Hope this helps,
Joaquín M López Muñoz Telefónica, Investigación y Desarrollo
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