Is CRTP used much? and why it is/isn't?

Question

I'm thinking about curiously recurring template pattern.

I find it quite useful, and it's pretty well adjustable for many tasks. But I have to admit, I don't see much of CRTP code in the wild.

Is it hard to grasp? Are virtual functions easier overall? Shouldn't it be used for 90% of cases where virtual keyword appears?

Answer 1

The simple fact is that many, if not most, C++ users can hardly grasp that vector is better than malloc and free, so the number who can not only appreciate but implement advanced template metaprogramming is not very high. This implies that even if it were good for 90% of situations, you'd see it in far less, simply because people don't know about it or how to use it.

However, virtual functions do have advantages, namely, virtual functions are much easier to use than the CRTP. C++ templates are notorious for being bitchy about virtually anything and everything, and the inclusion model sucks balls.

However, they [virtual functions] are distinctly slower and less powerful. For example, I right now am writing code using the CRTP, where you inherit operator overloads for use with expression templates. There's no way virtual functions could possibly make that work.

Answer 2

WTL uses the CRTP. Chromium browser is a well known example of software that uses WTL, so I'd say it's used "in the wild".

About the 90%? That's definitely an overstatement. Have you ever used a container of polymorphic objects? That's a very common usage of polymorphism and you can't do that with CRTP because Base<DerivedA> is not the same as Base<DerivedB>.

Answer 3

Here's reasons why it's not used more widely:

1. It's a hack
2. As a hack, it's not guaranteed you don't run into trouble when you use it. The normal OO stuff has been used by large number of projects, and found to work properly/problems are well known, the CRTP is found in only small number of projects, and no data is available
3. there is no good reason to use it - most reasons sited are just not valid
4. it can make some things more complicated