Study of C++

Multiple Inheritance - the fantastic and the horrible

• Multiple inheritance is great: it simultaneously gives you
  • Interfaces for free: an interface is an abstract class will null methods.
  • A form of Mixin classes (fancier example: Mixin classes in C++)
    - (mixins are an idea originating in CLOS, the Common Lisp Object System)
  -- we will discuss both of these briefly.
• The major technical hiccup is The Diamond Problem. Blech! We will briefly review the C++ solutions to this problem, including virtual inheritance.

Dynamic Typing in C++: RTTI

• This is a simple concept, objects carry their originating class names around at runtime.
• (Recall that declared types and runtime types may differ in languages with subtyping: the static type there means "this type or a subclass / implementing class")
• In Java this always happens due to its need to ensure safety of downcasts (raise an exception if they are bad).
• The downside is there is extra runtime overhead on adding this information to objects.
• In C++ the runtime type information is only added by the compiler if you need it (you are using RTTI on that object).

Templates

• The C++ answer to parametric polyorphism / generics
• Unlike other parametric types, templates are really just a fancier macro mechanism for C++
• Templates are all instantiated by a preprocessor at compile-time with the actual types, then that expanded code is what is compiled
• There is no need for fancy bounded subtyping like in Java - just instantiate and compile; the final code may have a type error due to a subtyping error
• The latter point makes it hard to debug type errors in templates -- you need to explicitly think about staged compilation (see below) to understand what the compiler is doing!
• But, Java generics get really complicated as well due to all of the sub-sub-sub cases and various needed extensions.

Template Metaprogramming

• Partial evaluation: evaluate some of the program code at compile time, e.g. apply functions if (enough) arguments are known at compile-time. This is a whole subfield of research.
• MetaML is a cleaner and more general notion than C++ template metaprogramming - code is first-class, can be combined almost like strings, but in a well-typed way.
  1. Run the first stage, which can do arbitrary computations; the result returned is the second-stage program.
  2. run the second stage, returning third stage program.
  3. etc - run until the final program results.
  4. ... then run that to do the actual application task.
  Observe
  • In terms of this model, C++ template metaprogramming is a 2-stage process.
  • Also the default is reversed: unlabelled C++ code is analogous to the <bracketed> code in MetaML since its running is delayed.

Template Specialization

C++ Overloading

• Overloading is great: such compact syntax with such great power!
• Overloading is horrible: I have no idea which operators were overloaded where so I don't know what the heck this program is doing!

• LOTS! - but, now you have to worry about all of the contentious overlap between fancy feature 1 and fancy feature 2.
• LITTLE! - but, now you have to write more code for various patterns, over and over and over...

Haskell Type Classes

The above webpage has good examples.