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 #include #include struct bar {std::vector vec;}; struct foo { foo() = default; foo(foo&&) = default; boost::optional hello; }; int main() { foo a; foo b(std::move(a)); } ``` My compiler supports both move semantics and defaulted move constructors, but I cannot get this to work. ``` % clang++ foo.cc -std=c++11 -stdlib=libc++ foo.cc:15:7: error: call to deleted constructor of 'foo' foo b(std::move(a)); ^ ~~~~~~~~~~~~ foo.cc:9:3: note: function has been explicitly marked deleted here foo(foo&&) = default; ^ 1 error generated. ``` Is there a way to move a `boost::optional` without modifying Boost's source code? Or should I wait until Boost supports moving? I had the same problem with `boost::any` in the past.

Accepted Answer

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.

Is it possible to move a boost::optional?

Solution

Problem