I've been trying to define a defaulted move constructor in a class with a boost::optional
member variable.
#include <boost/optional.hpp>
#include <utility>
#include <vector>
struct bar {std::vector<int> vec;};
struct foo {
foo() = default;
foo(foo&&) = default;
boost::optional<bar> 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.
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.
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.