David Abrahams wrote:
"Jost, Andrew" <Andrew_Jost@mentor.com> writes:
From: boost-bounces@lists.boost.org
[mailto:boost-bounces@lists.boost.org] On Behalf Of Eelis van der Weegen
Jost, Andrew wrote:
I am curious if there is support for what I'm calling a "dual_state" template class.
From your description it sounds a lot like Boost.Optional. What are
the main differences?
Eelis
I'll admit I did not even pause at Boost.optional when I scanned the library listing for previous work, a failure in my ability to connect the description, "Discriminated-union wrapper for optional values," with the concept I had in mind.
Oh, you're right! That is a terrible one-line description, because
a. It uses technical terms that many people probably don't know "discriminated union"
b. optional doesn't really act like a union (in any way that matches my intuition)! I understand the theoretical connection, of course, but nobody is thinking that way when they read brief descriptions.
Well, even despite I do understand what "discriminated union" is, it's more an implementation detail than a brief descripton of practical applications for the library. I found Boost.Optional very useful for myself. http://www.boost.org/libs/optional/doc/optional.html#inplace describes a usage scenario which has almost nothing to do with optionality. With boost::optional in-place construction facilities I can easily store non-Copyable or non-DefaultConstructible objects in STL containers and avoid extra construction, copy and dynamic memory allocation operations in many scenarios, even in those not related to containers. For example, I can return a fresh NonCopyable and NonDefaultConstructible object from a function without often expensive heap operations. I even had my own placeholder<> implementation. But I felt myself too weak to perform boost-style "all possible overloads", and lack of the safety prevented me from using that class widely. I wonder how many more pearls are still hidden deeply in Boost documentation. I'm going to contribute to the documentation (at least in Wiki due to my poor English) when I finish with Boost.Build. Andrey