Is it possible to move a boost::optional?

Question

I\'ve been trying to define a defaulted move constructor in a class with a boost::optional member variable.

#include 
         


        

Answer 1

Looking at boost::optional source code, it doesn't define move constructors or move assignments. However, it does define copy constructor and assignment, which prevent the compiler from auto-generating move constructor and assignment for it.

See Move constructor — Q&A:

the compiler will implicitly generate move constructor as member-wise moves, unless you have explicitly defined a copy constructor or copy/move assignment or a destructor

As Luc Danton suggests there needs to be a cure for that. Let's try using swap to move boost::optional in the hope that creating an empty boost::optional is cheap and then we just swap the default-constructed boost::optional with the one from the r-value foo, e.g.:

foo(foo&& b) {
    hello.swap(b.hello);
}

And same for the move assignment.

Answer 2

It's not clear how high on the agenda C++11 feature support for boost::optional is.

I found a nice fellow on the Internet who gave me some of his own code under a boost license...which is still undergoing some tweaks. It's at a point where it works for me. Maybe it will help anyone looking at this question:

https://github.com/hostilefork/CopyMoveConstrainedOptional

There is also a larger group effort to define a std::optional which has been going on...but their reference implementation wasn't able to handle some of the cases I had:

http://kojot.sggw.waw.pl/~akrzemi1/optional/tr2.optional.proposal.html

Answer 3

Boost.Optional supports move construction only since Boost version 1.56. See the release notes.