[cfe-dev] RFC: Introduce a section to the programmers manual about type hierarchies, polymorphism, and virtual dispatch.

[Chandler Carruth]

The proposed addition to ProgrammersManuel.rst is below the break...

This is essentially trying to explain the emerging design techniques being
used in LLVM these days somewhere more accessible than the comments on a
particular piece of infrastructure. It covers the "concepts-based
polymorphism" that caused some confusion during initial reviews of the new
pass manager as well as the tagged-dispatch mechanism used pervasively in
LLVM and Clang.

Perhaps most notably, I've tried to provide some criteria to help
developers choose between these options when designing new pieces of
infrastructure.

Many thanks to Richard Smith who helped me crystalize this, as well as
Eric, Dave, and Nick who helped me talk it through.

You can find the actual patch and make comments on Phabricator here:
http://reviews.llvm.org/D7191

I'm just going to paste the text below however as I think that'll more
easily facilitate discussion!

========

.. _polymorphism:

Designing Type Hiercharies and Polymorphic Interfaces
-----------------------------------------------------

There are two different design patterns that tend to result in the use of
virtual dispatch for methods in a type hierarchy in C++ programs. The first
is
a genuine type hierarchy where different types in the hierarchy model
a specific subset of the functionality and semantics, and these types nest
strictly within each other. Good examples of this can be seen in the
``Value``
or ``Type`` type hierarchies.

A second is the desire to dispatch dynamically across a collection of
polymorphic interface implementations. This latter use case can be modeled
with
virtual dispatch and inheritance by defining an abstract interface base
class
which all implementations derive from and override. However, this
implementation strategy forces an **"is-a"** relationship to exist that is
not
actually meaningful. There is often not some nested hierarchy of useful
generalizations which code might interact with and move up and down.
Instead,
there is a singular interface which is dispatched across a range of
implementations.

An alternate implementation strategy for the second use case which should be
preferred within LLVM is to that of generic programming. For example, a
template
over some type parameter ``T`` can be instantiated across any particular
implementation that conforms to the interface. A good example here is the
highly generic properties of any type which models a node in a directed
graph.
LLVM models these primarily through templates and generic programming. Such
templates include the ``LoopInfoBase`` and ``DominatorTreeBase``. When this
type of polymorphism truly needs **dynamic** dispatch, you can generalize it
using a technique called *concept-based polymorphism* which emulates the
interfaces and behaviors of templates while using a very limited form of
virtual dispatch for type erasure inside its implementation. You can find
examples of this technique in the ``PassManager.h`` system, and there is a
more
detailed introduction to it by Sean Parent in several of his talks and
papers:

#. `Sean Parent's Papers and Presentations
   <
http://github.com/sean-parent/sean-parent.github.com/wiki/Papers-and-Presentations
>`_
   - A Github project full of links to slides, video, and sometimes code.
#. `Value Semantics and Concepts-based Polymorphism
   <http://www.youtube.com/watch?v=_BpMYeUFXv8>`_ - The C++Now! 2012 talk
   describing this technique.
#. `Inheritance Is The Base Class of Evil
   <
http://channel9.msdn.com/Events/GoingNative/2013/Inheritance-Is-The-Base-Class-of-Evil
>`_
   - The GoingNative 2013 talk describing this technique.

When deciding between creating a type hierarchy and using virtual dispatch
and
using templates or concepts-based polymorphism, consider whether there is
some
refinement of an abstract base class which is actually a useful type on an
interface boundary. If anything more refined than the root abstract
interface
is meaningless to talk about as a partial extension of the semantic model,
then
your use case fits better with polymorphism and you should avoid using
virtual
dispatch. However, there may be some exigent circumstances that require one
technique or the other to be used.

If you do need to introduce a type hierarchy, we often prefer to used
explicitly closed type hierarchies with manual tagged dispatch rather than
the
open inheritance model and virtual dispatch that is more common in C++ code.
This is because LLVM rarely encourages library consumers to extend its core
types, and leverages the closed and tag-dispatched nature of its
hierarchies to
generate significantly more efficient code. We have also found that a large
amount of our usage of type hierarchies fits better with tag-based pattern
matching rather than dynamic dispatch across a common interface.
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