The problem is the following: 

class TypeDictionary {

public: 

   // implementation be here -- and the question is about this

}; 

void userOfTypeDictionary() {

    TypeDictionary td;

    

    td.insert( Foo() ); 

    td.insert( Bar() );

    td,insert( double(3.14) ); 

    // and other unknown (to TypeDictionary) 

    // list of types attached

    

    // later on, in a different scope perhaps

    Foo& f = td.get<Foo>() ;

    Bar& f = td.get<Bar>(); 

    double pi = tg.get<double>();

    // ...

}

Now the question so far seems like right up in the alley of Boost.Fusion, perhaps with a boost::fusion::set, but I am unsure how to manipulate a set of types unknown ahead of time as in code above, and with a  -- rather obvious -- extension whereby different instances of TypeDictionary   would be used to contain a different set of types, potentially in different compilation units. 

As for a realistic motivating use case, think of a  plugin manager. At any given time of its life an arbitrary heterogeneous set of  objects are attached to it, and the it is the manager's job to manage the life time of the attached objects and return (a reference) to them when queried in a type-safe manner. 

Any ideas on whether such a thing is possible, and how, would be deeply appreciated. 

Thanks many,

Nick