Coming back to C++ after many years; trying to catch up to C++11 & 14. I've read about rvalues and move semantics. I thought I understood the concept. Apparently not. I've looked at dozens of examples. But I simply can't get my code to compile. I must be missing something obvious in the examples. I always get the error about the copy ctor being deleted because of unique_ptr<int>
having a user-declared move ctor. There's obviously something I'm missing about the concept, but I can't figure out what it is. Here's the code, stripped down to its essence:
#include <memory>
#include <utility>
#include <vector>
int main(int, char*[]) {
auto oneInt{std::make_unique<int>(0)};
auto someInts{std::vector<std::unique_ptr<int>>{std::move(oneInt)}};
return 0;
}
What am I doing wrong?
Edit: Here's the error from this particular code. Note that I've tried every variation on the code that I can think of, with varying results, but the basic problem is always the same: copy ctor deleted because unique_ptr<int>
has a user-declared move ctor.
Edit: I've updated the code to #include <memory>
, and pasted the new error. I can only wish the problem were something silly like that.
/Applications/Xcode.app/Contents/Developer/Toolchains/XcodeDefault.xctoolchain/usr/bin/../include/c++/v1/memory:1752:31: error: call to implicitly-deleted copy constructor of
'std::__1::unique_ptr<int, std::__1::default_delete<int> >'
::new((void*)__p) _Up(_VSTD::forward<_Args>(__args)...);
^ ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
/Applications/Xcode.app/Contents/Developer/Toolchains/XcodeDefault.xctoolchain/usr/bin/../include/c++/v1/memory:1668:18: note: in instantiation of function template
specialization 'std::__1::allocator<std::__1::unique_ptr<int, std::__1::default_delete<int> > >::construct<std::__1::unique_ptr<int, std::__1::default_delete<int> >,
const std::__1::unique_ptr<int, std::__1::default_delete<int> > &>' requested here
{__a.construct(__p, _VSTD::forward<_Args>(__args)...);}
^
/Applications/Xcode.app/Contents/Developer/Toolchains/XcodeDefault.xctoolchain/usr/bin/../include/c++/v1/memory:1514:14: note: in instantiation of function template
specialization 'std::__1::allocator_traits<std::__1::allocator<std::__1::unique_ptr<int, std::__1::default_delete<int> > > >::__construct<std::__1::unique_ptr<int,
std::__1::default_delete<int> >, const std::__1::unique_ptr<int, std::__1::default_delete<int> > &>' requested here
{__construct(__has_construct<allocator_type, _Tp*, _Args...>(),
^
/Applications/Xcode.app/Contents/Developer/Toolchains/XcodeDefault.xctoolchain/usr/bin/../include/c++/v1/memory:1598:17: note: in instantiation of function template
specialization 'std::__1::allocator_traits<std::__1::allocator<std::__1::unique_ptr<int, std::__1::default_delete<int> > > >::construct<std::__1::unique_ptr<int,
std::__1::default_delete<int> >, const std::__1::unique_ptr<int, std::__1::default_delete<int> > &>' requested here
construct(__a, _VSTD::__to_raw_pointer(__begin2), *__begin1);
^
/Applications/Xcode.app/Contents/Developer/Toolchains/XcodeDefault.xctoolchain/usr/bin/../include/c++/v1/vector:1024:21: note: in instantiation of function template
specialization 'std::__1::allocator_traits<std::__1::allocator<std::__1::unique_ptr<int, std::__1::default_delete<int> > > >::__construct_range_forward<const
std::__1::unique_ptr<int, std::__1::default_delete<int> > *, std::__1::unique_ptr<int, std::__1::default_delete<int> > *>' requested here
__alloc_traits::__construct_range_forward(__a, __first, __last, this->__end_);
^
/Applications/Xcode.app/Contents/Developer/Toolchains/XcodeDefault.xctoolchain/usr/bin/../include/c++/v1/vector:1285:9: note: in instantiation of function template
specialization 'std::__1::vector<std::__1::unique_ptr<int, std::__1::default_delete<int> >, std::__1::allocator<std::__1::unique_ptr<int, std::__1::default_delete<int> >
> >::__construct_at_end<const std::__1::unique_ptr<int, std::__1::default_delete<int> > *>' requested here
__construct_at_end(__il.begin(), __il.end(), __il.size());
^
virtual.cpp:7:21: note: in instantiation of member function 'std::__1::vector<std::__1::unique_ptr<int, std::__1::default_delete<int> >,
std::__1::allocator<std::__1::unique_ptr<int, std::__1::default_delete<int> > > >::vector' requested here
auto someInts{std::vector<std::unique_ptr<int>>{std::move(oneInt)}};
^
/Applications/Xcode.app/Contents/Developer/Toolchains/XcodeDefault.xctoolchain/usr/bin/../include/c++/v1/memory:2621:31: note: copy constructor is implicitly deleted because
'unique_ptr<int, std::__1::default_delete<int> >' has a user-declared move constructor
_LIBCPP_INLINE_VISIBILITY unique_ptr(unique_ptr&& __u) _NOEXCEPT
There are a lot of problems with your code. First, there is an excessive use of {}
initialization that confuses things. Using auto
is fine, but auto x{...};
declarations are fraught with peril, as the meaning of auto x{single_value}
has shifted over time. It's best to use auto x = single_value;
syntax where reasonable.
Second, you cannot insert a unique_ptr
into a container through a {}
initializer list. At all. Items that go through std::initializer_list
must be copyable, and unique_ptr
is not.
What you want is this:
auto oneInt = std::make_unique<int>(0);
std::vector<std::unique_ptr<int>> someInts;
someInts.push_back(std::move(oneInt));
The short answer is that a std::initializer_list
contains values; and they cannot be moved out of, only copied from. To be clear, if you use the initializer_list form of construction for a vector
, the items in the list are copied into the vector.
See this thread for discussion of your exact problem and some suggested workarounds.
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