xstd::optional

Alternative to boost::optional for C++11, supporting move-only types and optimizations by detecting type-traits.

Notes from Brian (of http://hostilefork.com) who put this code on GitHub:

In terms of compatibility with C++11, boost::optional lacked support for move-only types (among some other issues). In 2012 there did not exist a viable reference implementation for a std::optional from the group discussing it, so Daniel Krügler was willing to share his own implementation called xstd::optional under a boost license. This was not intended to compete with the standards effort, but be a stopgap measure and to experiment with some particular implementation choices.

As of 2014 there is a reference implementation which is being vetted and known as std::experimental::optional. It is available in another GitHub repository as a single header file:

https://github.com/akrzemi1/Optional/

Due to issues brought up during review, that specification is not slated to make it into C++14, but should be adopted in a later release.

Generally speaking the std::experimental::optional managed to handle all the cases I was working with. In order to bring the two into greater alignment, I updated xstd::optional to use nullopt instead of none to indicate an empty optional... and also added the make_optional helper function.

I discussed a situation that xstd::experimental::optional was not able to handle with Andrzej Krzemieński, where xstd::optional was able to work. That specifically related to the type trait of copy constructibility not being inherited by optional from the type it is parameterized with:

akrzemi1/Optional#13

Andrzej felt that using a technique like Daniel's to provide this ability would complicate the code. std::vector and other classes are subject to the same failure to answer the trait correctly, so it didn't seem like a good tradeoff to do it when the scheduled C++ Concepts Lite compiler feature would solve it generally. There may be other differences, but that's the one I noticed.

As a general heads-up, one should observe that moving the contents out of an optional for a move-only type will not register that optional as being a nullopt. It will consider itself to be containing a value unless you explicitly assign it nullopt. There was an extract() function in xstd::optional which addressed this, but that is not currently planned for inclusion in std::experimental::optional:

akrzemi1/Optional#12

Many thanks to Daniel and Andrzej for their code and time (and to everyone else working through the details...)

hostilefork/CopyMoveConstrainedOptional

xstd::optional