当“移动的”对象在联合中具有“非平凡”成员时,为什么强制复制构造函数?

我注意到了 move 可以应用于 不 拥有一个与“非平凡”(不确切知道,但如基本类型很好)成员的联合。例如,编译以下代码(C++14,Clang):

#include <vector>
#include <string>

class Foo {
public:
  union {
    int i;
    bool b;
  };

  Foo() {};
  ~Foo() {};

  // Move constructor to default.
  Foo(Foo &&) = default;

  // Copy constructor deleted.
  Foo(const Foo &) = delete;
};

int main() {
  std::vector<Foo> v;
  v.push_back(Foo());
}

请注意,复制构造函数已被删除。自从 std::vector 的 push_back 可以接受右值引用,在这种情况下,它将使用右值引用,而不接受右值引用 copy 将发生。但是,一旦将“非平凡”类型添加到联合中,就会强制复制构造函数,因此它将不会编译:

#include <vector>
#include <string>

class Foo {
public:
  union {
    int i;
    bool b;
    std::string s; // <-- Added element causing compile error.
  };

  Foo() {};
  ~Foo() {};

  // Move constructor to default.
  Foo(Foo &&) = default;

  // Copy constructor deleted.
  Foo(const Foo &) = delete;
};

int main() {
  std::vector<Foo> v;
  v.push_back(Foo());
}

编译器错误消息的相关部分:

In file included from experiment/miniso.cpp:1:
In file included from /usr/bin/../lib/gcc/x86_64-linux-gnu/7.2.0/../../../../include/c++/7.2.0/vector:61:
In file included from /usr/bin/../lib/gcc/x86_64-linux-gnu/7.2.0/../../../../include/c++/7.2.0/bits/allocator.h:46:
In file included from /usr/bin/../lib/gcc/x86_64-linux-gnu/7.2.0/../../../../include/x86_64-linux-gnu/c++/7.2.0/bits/c++allocator.h:33:
/usr/bin/../lib/gcc/x86_64-linux-gnu/7.2.0/../../../../include/c++/7.2.0/ext/new_allocator.h:136:23: error: call to deleted constructor of 'Foo'
        { ::new((void *)__p) _Up(std::forward<_Args>(__args)...); }
                             ^   ~~~~~~~~~~~~~~~~~~~~~~~~~~~
/usr/bin/../lib/gcc/x86_64-linux-gnu/7.2.0/../../../../include/c++/7.2.0/bits/alloc_traits.h:475:8: note: in instantiation of function template
      specialization '__gnu_cxx::new_allocator<Foo>::construct<Foo, Foo>' requested here
        { __a.construct(__p, std::forward<_Args>(__args)...); }
              ^
/usr/bin/../lib/gcc/x86_64-linux-gnu/7.2.0/../../../../include/c++/7.2.0/bits/vector.tcc:100:21: note: in instantiation of function template
      specialization 'std::allocator_traits<std::allocator<Foo> >::construct<Foo, Foo>' requested here
            _Alloc_traits::construct(this->_M_impl, this->_M_impl._M_finish,
                           ^
/usr/bin/../lib/gcc/x86_64-linux-gnu/7.2.0/../../../../include/c++/7.2.0/bits/stl_vector.h:954:9: note: in instantiation of function template
      specialization 'std::vector<Foo, std::allocator<Foo> >::emplace_back<Foo>' requested here
      { emplace_back(std::move(__x)); }
        ^
experiment/miniso.cpp:24:5: note: in instantiation of member function 'std::vector<Foo, std::allocator<Foo> >::push_back' requested here
  v.push_back(Foo());
    ^
experiment/miniso.cpp:19:3: note: 'Foo' has been explicitly marked deleted here
  Foo(const Foo &) = delete;
  ^

目标编译器是C++14。