I'm talking about something like this

    std::relation<Foo, Bar> foobars;
    ...
    auto& bars = foobars.forward(foo);
    ...
    auto& foos = foobars.backward(bar);

Edit: Boost apparently has it, see e.g. https://stackoverflow.com/questions/1128144/c-maps-for-many-...

Boost.Bimap is a special case (and built on top) of Boost.Multiindex that allows indexing a logical table with an arbitrary subset of fields.