为什么调用复制构造函数来构造空unique_ptr向量?

#include <vector>
#include <memory>
#include <iostream>
class A {
std::vector<std::unique_ptr<int>> vec;
public:
virtual ~A() = 0;
};
A::~A() {}
class B : public A {
public:
B() {}
};
int main () {
B b = B();
return 0;
}

以上是一个最小的可重现示例。它不会编译，并给出以下错误消息：

me:~ $ g++ main2.cc
In file included from /usr/include/c++/7/vector:62:0,
from main2.cc:1:
/usr/include/c++/7/bits/stl_construct.h: In instantiation of ‘void std::_Construct(_T1*, _Args&& ...) [with _T1 = std::unique_ptr<int>; _Args = {const std::unique_ptr<int, std::default_delete<int> >&}]’:
/usr/include/c++/7/bits/stl_uninitialized.h:83:18:   required from ‘static _ForwardIterator std::__uninitialized_copy<_TrivialValueTypes>::__uninit_copy(_InputIterator, _InputIterator, _ForwardIterator) [with _InputIterator = __gnu_cxx::__normal_iterator<const std::unique_ptr<int>*, std::vector<std::unique_ptr<int> > >; _ForwardIterator = std::unique_ptr<int>*; bool _TrivialValueTypes = false]’
/usr/include/c++/7/bits/stl_uninitialized.h:134:15:   required from ‘_ForwardIterator std::uninitialized_copy(_InputIterator, _InputIterator, _ForwardIterator) [with _InputIterator = __gnu_cxx::__normal_iterator<const std::unique_ptr<int>*, std::vector<std::unique_ptr<int> > >; _ForwardIterator = std::unique_ptr<int>*]’
/usr/include/c++/7/bits/stl_uninitialized.h:289:37:   required from ‘_ForwardIterator std::__uninitialized_copy_a(_InputIterator, _InputIterator, _ForwardIterator, std::allocator<_Tp>&) [with _InputIterator = __gnu_cxx::__normal_iterator<const std::unique_ptr<int>*, std::vector<std::unique_ptr<int> > >; _ForwardIterator = std::unique_ptr<int>*; _Tp = std::unique_ptr<int>]’
/usr/include/c++/7/bits/stl_vector.h:331:31:   required from ‘std::vector<_Tp, _Alloc>::vector(const std::vector<_Tp, _Alloc>&) [with _Tp = std::unique_ptr<int>; _Alloc = std::allocator<std::unique_ptr<int> >]’
main2.cc:5:7:   required from here
/usr/include/c++/7/bits/stl_construct.h:75:7: error: use of deleted function ‘std::unique_ptr<_Tp, _Dp>::unique_ptr(const std::unique_ptr<_Tp, _Dp>&) [with _Tp = int; _Dp = std::default_delete<int>]’
{ ::new(static_cast<void*>(__p)) _T1(std::forward<_Args>(__args)...); }
^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
In file included from /usr/include/c++/7/memory:80:0,
from main2.cc:2:
/usr/include/c++/7/bits/unique_ptr.h:388:7: note: declared here
unique_ptr(const unique_ptr&) = delete;
^~~~~~~~~~

这一切看起来就像是unique_ptr的复制构造函数被调用到某个地方。

但我根本不知道为什么要叫它。当一个向量被初始化时，它的大小是0，对吧？根据这一假设，不应构建任何unique_ptr。

我的编译错误的来源是什么？