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849. missing type traits to compute root class and derived class of types in a class hierachy

Section: 21.3.8.7 [meta.trans.other] Status: NAD Submitter: Thorsten Ottosen Opened: 2008-06-05 Last modified: 2016-01-28

Priority: Not Prioritized

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Discussion:

The type traits library contains various traits to dealt with polymorphic types, e.g. std::has_virtual_destructor, std::is_polymorphic and std::is_base_of. However, there is no way to compute the unique public base class of a type if such one exists. Such a trait could be very useful if one needs to instantiate a specialization made for the root class whenever a derived class is passed as parameter. For example, imagine that you wanted to specialize std::hash for a class hierarchy---instead of specializing each class, you could specialize the std::hash<root_class> and provide a partial specialization that worked for all derived classes.

This ability---to specify operations in terms of their equivalent in the root class---can be done with e.g. normal functions, but there is, AFAIK, no way to do it for class templates. Being able to access compile-time information about the type-hierachy can be very powerful, and I therefore also suggest traits that computes the directly derived class whenever that is possible.

If the computation can not be done, the traits should fall back on an identity transformation. I expect this gives the best overall usability.

Proposed resolution:

Add the following to the synopsis in 21.3.3 [meta.type.synop] under "other transformations":

template< class T > struct direct_base_class;
template< class T > struct direct_derived_class;
template< class T > struct root_base_class;

Add three new entries to table 51 (21.3.8.7 [meta.trans.other]) with the following content

Template	Condition	Comments
template< class T > struct direct_base_class;	T shall be a complete type.	The member typedef type shall equal the accessible unambiguous direct base class of T. If no such type exists, the member typedef type shall equal T.
template< class T > struct direct_derived_class;	T shall be a complete type.	The member typedef type shall equal the unambiguous type which has T as an accessible unambiguous direct base class. If no such type exists, the member typedef type shall equal T.
template< class T > struct root_base_class;	T shall be a complete type.	The member typedef type shall equal the accessible unambiguous most indirect base class of T. If no such type exists, the member typedef type shall equal T.

Rationale:

2008-9-16 San Francisco: Issue pulled by author prior to being reviewed by the LWG.