为什么C++ destuctor会影响返回值优化的行为

我简化了我的代码如下.

#include 
class NoncopyableItem {
 public:
  NoncopyableItem() { }
  NoncopyableItem(NoncopyableItem &&nt) { };
};
class Iterator {
  friend class Factory;
 public:
  ~Iterator() { }  // weird
 private:
  Iterator() { }
  std::vector buffer_;
};
class Factory {
 public:
  Iterator NewIterator() {
    return Iterator();
  }
};
int main() {
  Factory fa;
  auto it = fa.NewIterator();
  return 0;
}

我想在函数中利用RVO(返回值优化)NewIterator,但是我收到以下错误:

In file included from /usr/lib/gcc/x86_64-pc-cygwin/4.9.3/include/c++/vector:62:0,
                 from /cygdrive/c/Users/DELL/ClionProjects/destructor_test/main.cpp:1:
/usr/lib/gcc/x86_64-pc-cygwin/4.9.3/include/c++/bits/stl_construct.h: In instantiation of 'void std::_Construct(_T1*, _Args&& ...) [with _T1 = NoncopyableItem; _Args = {const NoncopyableItem&}]':
/usr/lib/gcc/x86_64-pc-cygwin/4.9.3/include/c++/bits/stl_uninitialized.h:75:53:   required from 'static _ForwardIterator std::__uninitialized_copy<_TrivialValueTypes>::__uninit_copy(_InputIterator, _InputIterator, _ForwardIterator) [with _InputIterator = __gnu_cxx::__normal_iterator >; _ForwardIterator = NoncopyableItem*; bool _TrivialValueTypes = false]'
/usr/lib/gcc/x86_64-pc-cygwin/4.9.3/include/c++/bits/stl_uninitialized.h:126:41:   required from '_ForwardIterator std::uninitialized_copy(_InputIterator, _InputIterator, _ForwardIterator) [with _InputIterator = __gnu_cxx::__normal_iterator >; _ForwardIterator = NoncopyableItem*]'
/usr/lib/gcc/x86_64-pc-cygwin/4.9.3/include/c++/bits/stl_uninitialized.h:279:63:   required from '_ForwardIterator std::__uninitialized_copy_a(_InputIterator, _InputIterator, _ForwardIterator, std::allocator<_Tp>&) [with _InputIterator = __gnu_cxx::__normal_iterator >; _ForwardIterator = NoncopyableItem*; _Tp = NoncopyableItem]'
/usr/lib/gcc/x86_64-pc-cygwin/4.9.3/include/c++/bits/stl_vector.h:324:32:   required from 'std::vector<_Tp, _Alloc>::vector(const std::vector<_Tp, _Alloc>&) [with _Tp = NoncopyableItem; _Alloc = std::allocator]'
/cygdrive/c/Users/DELL/ClionProjects/destructor_test/main.cpp:7:7:   required from here
/usr/lib/gcc/x86_64-pc-cygwin/4.9.3/include/c++/bits/stl_construct.h:75:7: error: use of deleted function 'constexpr NoncopyableItem::NoncopyableItem(const NoncopyableItem&)'
     { ::new(static_cast(__p)) _T1(std::forward<_Args>(__args)...); }
       ^
/cygdrive/c/Users/DELL/ClionProjects/destructor_test/main.cpp:2:7: note: 'constexpr NoncopyableItem::NoncopyableItem(const NoncopyableItem&)' is implicitly declared as deleted because 'NoncopyableItem' declares a move constructor or move assignment operator
 class NoncopyableItem {
       ^
CMakeFiles/destructor_test.dir/build.make:62: recipe for target 'CMakeFiles/destructor_test.dir/main.cpp.o' failed

根据cppreference.com,NewIterator()应符合RVO的要求.但是,似乎编译器试图调用默认的复制构造函数Iterator,然后因为Iterator.buffer_不可复制而失败.

好吧,令我惊讶的是,如果我删除IteratorL#13中的析构函数,代码工作正常.

为什么析构函数会影响编译器的RVO行为？

首先,在这种情况下忘记RVO.这是一种合法的优化,但即使它确实发生了,代码必须是合法的,没有它.

所以考虑到这一点,我们来看看

auto it = fa.NewIterator();

该行尝试Iterator从临时构造一个新的Iterator.为此,我们需要以下两个中的任何一个^†:

Iterator(const Iterator&); //or
Iterator(Iterator&&);

现在在您发布的代码中,尝试使用隐式声明Iterator(const Iterator&);将导致您显示的编译器错误,因为非静态成员的复制构造函数buffer_无法编译.

未生成第二个候选项,因为Iterator具有用户定义的析构函数.

如果删除用户定义的析构函数,编译器将生成移动构造函数Iterator(Iterator&&);并在我们从临时构造时使用它.或者它可能不会和RVO相反,但它可以使用它,这是重要的部分.

^†或者其他一些用户声明的构造函数当然使该行合法.但上面是两个常见的编译器生成的,你显然要问的.