The Four Polymorphisms in C++

http://www.catonmat.net/blog/cpp-polymorphism

by pkrumins • 16 years ago

53 21 16 years ago

vog• 16 years ago

That's a great summary!

I'm always disappointed if people only know one kind of polymorphism and apply that rigorously to all problems. Overviews like this might help people to make a conscious decision on which language construct would be a good fit for their actual design problem.

Chris_Newton• 16 years ago

That's a fair point, but unfortunately in C++ the various types of polymorphism often do not play nicely together.

Look at any on-line forums discussing C++. If you're a bit more cynical, look at a list of tricky C++ interview questions instead. :-)

You'll find discussions about overloading vs. overriding for class member functions. You'll find discussions of which function gets called when you have both overloaded functions and function templates to consider. You'll find discussions of which sequences of implicit conversions are allowed, and via what mechanisms.

A very high proportion of the nasty edge cases for which C++ has become infamous are caused by trying to combine different forms of polymorphism. I'm all for knowing your tools and evaluating all the options, but I can see why in practice a lot of teams do stick almost entirely to (for example) using classes/OOP and avoiding templates.

DougBTX• 16 years ago

I get the impression that the answer is no, but is there much use of templates in C++ to make generic classes like in Java/C#?

obiterdictum• 16 years ago

It may be a good summary, but I completely disagree with author's (implied) assertion that all these should be lumped under one term. All these concepts are different in their mechanics, which is the reason we have different terminology to describe them. Just call overloading overloading. C++ does not need more ambiguity, it needs less.

scott_s• 16 years ago

I sympathize with your point, but I used to think of runtime and parametric polymorphism as two completely separate things. It wasn't until I heard them both described as a type of polymorphism that I realized the relationship between them.

pkrumins4• 16 years ago

I never implied that they should lumped under one term. On the contrary. I wrote out all the meanings the term can have to disambiguate it.

seliopou2• 16 years ago

Coercion is not a form of polymorphism. In all the other forms, dispatch depends on the type of a value. When there's coercion going on, be it explicit or implicit, the same code is being executed regardless of the value being coerced. The only code that one might say "changes" is any code that's executed to perform the cast. But that change in casting is basically ad-hoc polymorphism.

pkrumins4• 16 years ago

This paper [1] says coercion is a kind of ad-hoc polymorphism.

[1] "On Understanding Types, Data Abstraction, and Polymorphism" http://lucacardelli.name/Papers/OnUnderstanding.A4.pdf

shasta2• 16 years ago

Where do macros fit into this taxonomy?

jpr• 16 years ago

I would say "outside".

shasta2• 16 years ago

You want to offer a working definition of "polymorphism", then?

Macros certainly allow you to write code that's applied to differently typed arguments.

ioquatix• 16 years ago

Since we are talking about polymorphism, here are several blog posts I wrote on this topic:

http://www.oriontransfer.co.nz/blog/2009-05/what-is-abstract...

http://www.oriontransfer.co.nz/blog/2009-05/run-time-polymor...

http://www.oriontransfer.co.nz/blog/2009-05/compile-time-pol...

If you find it interesting that is great. I enjoyed this blog post.

bmm6o• 16 years ago

I know it's just a blog post, but the section on "Parametric Polymorphism" says that the same code executes for any type. But in the example given, "a > b" will evaluate to entirely different operations depending on the type, and in fact only works because of the other types of polymorphism. Since this is such a common situation, I might not make it a first class citizen in the taxonomy.

pkrumins4• 16 years ago

In that example I only wanted to illustrate parametric polymorphism on type T (and ignore the overloading on operator > that happens).

Figs• 16 years ago

This was also posted in Reddit, and there are some other comments over there, apparently:

http://www.reddit.com/r/programming/comments/cgckf/the_four_...

The Four Polymorphisms in C++

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