The Loki library implements some very widely used concepts (smart pointer, visitor, factory, etc.). The associated book "Modern C++ Design" is often mentioned, but the library itself is not widely used. Why is that?
Most developers seem to prefer Boost. In particular, why do people often decide to use Boost's smart pointers rather than Loki's?
You want to use a library that the next programmer is going to know and that is going to be well supported in the future - so you pick a major lib. Because it's a major lib lots of people use it, so it becomes the default choice.
Speaking as someone who's used quite a bit of the Boost library, and also looked at Loki more than once, the biggest problem was the sparsity of documentation. Also, Loki uses some of the hairiest bits of C++ templates. Exciting stuff, but also rather daunting.
Loki is a research/proof-of-concept sort of thing. Alexandrescu pushes new ideas, other people adopt those for real world. Also boost::shared_ptr is almost literally in TR1.
I used Loki once for a little tool (basically an interpreter) and actually liked it. My coworkers were less enthusiastic about the library, so its use remained constrained to this small sub-project.
I actually prefer Loki's way of doing things and I have contributed to Loki myself a Decorator pattern which now sits in the tracker because the project as far as I know is no longer maintained.
I use boost shared_pointer just because it will be the standard very soon, I may dislike the fact that I can't customize it to act exactly the way I want it to act but I have to live with it.
Usage of the standard library is important as it keeps the code maintainable by other programmers. If it's open source and you want to experiment go ahead and use Loki. No one is stopping you.
Actually Windows Vista which uses some of Loki's features in it.
I am guessing they are not using the redundant implementations of smart pointers and visitors.