Use multiple inheritance to represent the union of implementation attributes

Reason

Some forms of mixins have state and often operations on that state. If the operations are virtual the use of inheritance is necessary, if not using inheritance can avoid boilerplate and forwarding.

Example

class iostream : public istream, public ostream {   // very simplified
    // ...
};

istream provides the interface to input operations (and some data); ostream provides the interface to output operations (and some data). iostream provides the union of the istream and ostream interfaces and the synchronization needed to allow both on a single stream.

Note

This is a relatively rare use because implementation can often be organized into a single-rooted hierarchy.

Example

Sometimes, an "implementation attribute" is more like a "mixin" that determines the behavior of an implementation and injects members to enable the implementation of the policies it requires. For example, see std::enable_shared_from_this or various bases from boost.intrusive (e.g. list_base_hook or intrusive_ref_counter).

Enforcement

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