如何重载下标操作符以返回可选的左值？

Question

我正在通过实现一个octree类来学习一些C++特性。我希望这个类上的下标操作符返回octant corresponding to an index。我应该如何在类上定义下标操作符，以便我可以(i)赋值给结果和(ii)检查结果是否为空？

目标(i)通常是通过使下标操作符返回引用来实现的。但是引用不能引用nothing，如果八元数为空，则这是一个合法的返回值。

目标(ii)可以通过使下标运算符返回一个可选的。但是，修改指向八元数的指针就变得很重要了--在std::optional<T>中，T不能作为引用。

这是一个不完整的例子(除了在真实代码中所有的optional实例都以experimental为前缀，因为我的GCC对C++17只有实验性的支持)。

#include <optional>
#include <iostream>

using namespace std;

class Octree {
    Octree* branch[8];
public:
    Octree();
    ~Octree();
    optional<Octree&> operator[](int index);
};

Octree::Octree() : branch{}
{
}

Octree::~Octree()
{
    for (int i = 0; i < 8; i++) {
        if (branch[i])
            delete branch[i];
    }
}

optional<Octree&> Octree::operator[](int index)
{
    if (branch[index] == NULL)
        return nullopt;
    else
        return &branch[index];
}

int main(int argc, char *argv[])
{
    Octree o;
    if (o[0])
        cout << "Octant o[0] is not empty.\n";
    else
        cout << "Octant o[0] is empty.\n";
    o[0] = new Octree(); // The intent is to modify o
    return 0;
}

不出所料，编译器会五颜六色地拒绝可选引用。

In file included from parc.cpp:1:0:
/usr/include/c++/6/experimental/optional: In instantiation of ‘class std::experimental::fundamentals_v1::optional<Octree&>’:
parc.cpp:26:61:   required from here
/usr/include/c++/6/experimental/optional:507:7: error: static assertion failed: Invalid instantiation of optional<T>
       static_assert(__and_<__not_<is_same<remove_cv_t<_Tp>, nullopt_t>>,
       ^~~~~~~~~~~~~
/usr/include/c++/6/experimental/optional:713:7: error: forming pointer to reference type ‘Octree&’
       operator->() const
       ^~~~~~~~
/usr/include/c++/6/experimental/optional:723:7: error: forming pointer to reference type ‘Octree&’
       operator->()
       ^~~~~~~~
parc.cpp: In member function ‘std::experimental::fundamentals_v1::optional<Octree&> Octree::operator[](int)’:
parc.cpp:31:10: error: could not convert ‘&((Octree*)this)->Octree::branch[index]’ from ‘Octree**’ to ‘std::experimental::fundamentals_v1::optional<Octree&>’
   return &branch[index];
          ^~~~~~~~~~~~~~
parc.cpp: In function ‘int main(int, char**)’:
parc.cpp:41:24: error: no match for ‘operator=’ (operand types are ‘std::experimental::fundamentals_v1::optional<Octree&>’ and ‘Octree*’)
      o[0] = new Octree();
                        ^
In file included from parc.cpp:1:0:
/usr/include/c++/6/experimental/optional:595:7: note: candidate: std::experimental::fundamentals_v1::optional<_Tp>& std::experimental::fundamentals_v1::optional<_Tp>::operator=(std::experimental::fundamentals_v1::nullopt_t) [with _Tp = Octree&]
       operator=(nullopt_t) noexcept
       ^~~~~~~~
/usr/include/c++/6/experimental/optional:595:7: note:   no known conversion for argument 1 from ‘Octree*’ to ‘std::experimental::fundamentals_v1::nullopt_t’
/usr/include/c++/6/experimental/optional:609:9: note: candidate: template<class _Up> std::enable_if_t<std::__and_<std::__not_<std::is_same<std::experimental::fundamentals_v1::optional<_Tp>, typename std::decay<_Up>::type> >, std::is_constructible<_Tp, _Up>, std::__not_<std::__and_<std::is_scalar<_Tp>, std::is_same<_Tp, typename std::decay<_Up>::type> > >, std::is_assignable<_Tp&, _Up> >::value, std::experimental::fundamentals_v1::optional<_Tp>&> std::experimental::fundamentals_v1::optional<_Tp>::operator=(_Up&&) [with _Up = _Up; _Tp = Octree&]
         operator=(_Up&& __u)
         ^~~~~~~~
/usr/include/c++/6/experimental/optional:609:9: note:   template argument deduction/substitution failed:
/usr/include/c++/6/experimental/optional:628:9: note: candidate: template<class _Up> std::enable_if_t<std::__and_<std::__not_<std::is_same<_T1, _U1> >, std::is_constructible<_Tp, const _Up&>, std::is_assignable<_Tp&, _Up>, std::__not_<std::__or_<std::is_constructible<_Tp, const std::experimental::fundamentals_v1::optional<_Up>&>, std::is_constructible<_Tp, std::experimental::fundamentals_v1::optional<_Up>&>, std::is_constructible<_Tp, const std::experimental::fundamentals_v1::optional<_Up>&&>, std::is_constructible<_Tp, std::experimental::fundamentals_v1::optional<_Up>&&>, std::is_convertible<const std::experimental::fundamentals_v1::optional<_Up>&, _Tp>, std::is_convertible<std::experimental::fundamentals_v1::optional<_Up>&, _Tp>, std::is_convertible<const std::experimental::fundamentals_v1::optional<_Up>&&, _Tp>, std::is_convertible<std::experimental::fundamentals_v1::optional<_Up>&&, _Tp> > >, std::__not_<std::__or_<std::is_assignable<_Tp&, const std::experimental::fundamentals_v1::optional<_Up>&>, std::is_assignable<_Tp&, std::experimental::fundamentals_v1::optional<_Up>&>, std::is_assignable<_Tp&, const std::experimental::fundamentals_v1::optional<_Up>&&>, std::is_assignable<_Tp&, std::experimental::fundamentals_v1::optional<_Up>&&> > > >::value, std::experimental::fundamentals_v1::optional<_Tp>&> std::experimental::fundamentals_v1::optional<_Tp>::operator=(const std::experimental::fundamentals_v1::optional<_Up>&) [with _Up = _Up; _Tp = Octree&]
         operator=(const optional<_Up>& __u)
         ^~~~~~~~
/usr/include/c++/6/experimental/optional:628:9: note:   template argument deduction/substitution failed:
parc.cpp:41:24: note:   mismatched types ‘const std::experimental::fundamentals_v1::optional<_Tp>’ and ‘Octree*’
      o[0] = new Octree();
                        ^
In file included from parc.cpp:1:0:
/usr/include/c++/6/experimental/optional:653:9: note: candidate: template<class _Up> std::enable_if_t<std::__and_<std::__not_<std::is_same<_T1, _U1> >, std::is_constructible<_Tp, _Up>, std::is_assignable<_Tp&, _Up>, std::__not_<std::__or_<std::is_constructible<_Tp, const std::experimental::fundamentals_v1::optional<_Up>&>, std::is_constructible<_Tp, std::experimental::fundamentals_v1::optional<_Up>&>, std::is_constructible<_Tp, const std::experimental::fundamentals_v1::optional<_Up>&&>, std::is_constructible<_Tp, std::experimental::fundamentals_v1::optional<_Up>&&>, std::is_convertible<const std::experimental::fundamentals_v1::optional<_Up>&, _Tp>, std::is_convertible<std::experimental::fundamentals_v1::optional<_Up>&, _Tp>, std::is_convertible<const std::experimental::fundamentals_v1::optional<_Up>&&, _Tp>, std::is_convertible<std::experimental::fundamentals_v1::optional<_Up>&&, _Tp> > >, std::__not_<std::__or_<std::is_assignable<_Tp&, const std::experimental::fundamentals_v1::optional<_Up>&>, std::is_assignable<_Tp&, std::experimental::fundamentals_v1::optional<_Up>&>, std::is_assignable<_Tp&, const std::experimental::fundamentals_v1::optional<_Up>&&>, std::is_assignable<_Tp&, std::experimental::fundamentals_v1::optional<_Up>&&> > > >::value, std::experimental::fundamentals_v1::optional<_Tp>&> std::experimental::fundamentals_v1::optional<_Tp>::operator=(std::experimental::fundamentals_v1::optional<_Up>&&) [with _Up = _Up; _Tp = Octree&]
         operator=(optional<_Up>&& __u)
         ^~~~~~~~
/usr/include/c++/6/experimental/optional:653:9: note:   template argument deduction/substitution failed:
parc.cpp:41:24: note:   mismatched types ‘std::experimental::fundamentals_v1::optional<_Tp>’ and ‘Octree*’
      o[0] = new Octree();
                        ^
In file included from parc.cpp:1:0:
/usr/include/c++/6/experimental/optional:493:11: note: candidate: std::experimental::fundamentals_v1::optional<Octree&>& std::experimental::fundamentals_v1::optional<Octree&>::operator=(const std::experimental::fundamentals_v1::optional<Octree&>&)
     class optional
           ^~~~~~~~
/usr/include/c++/6/experimental/optional:493:11: note:   no known conversion for argument 1 from ‘Octree*’ to ‘const std::experimental::fundamentals_v1::optional<Octree&>&’
/usr/include/c++/6/experimental/optional:493:11: note: candidate: std::experimental::fundamentals_v1::optional<Octree&>& std::experimental::fundamentals_v1::optional<Octree&>::operator=(std::experimental::fundamentals_v1::optional<Octree&>&&)
/usr/include/c++/6/experimental/optional:493:11: note:   no known conversion for argument 1 from ‘Octree*’ to ‘std::experimental::fundamentals_v1::optional<Octree&>&&’
/usr/include/c++/6/experimental/optional: In instantiation of ‘void std::experimental::fundamentals_v1::_Optional_base<_Tp, false>::_M_construct(_Args&& ...) [with _Args = {Octree}; _Tp = Octree&]’:
/usr/include/c++/6/experimental/optional:384:11:   required from ‘std::experimental::fundamentals_v1::_Optional_base<_Tp, false>::_Optional_base(std::experimental::fundamentals_v1::_Optional_base<_Tp, false>&&) [with _Tp = Octree&]’
/usr/include/c++/6/experimental/optional:493:11:   required from here
/usr/include/c++/6/experimental/optional:439:11: error: new cannot be applied to a reference type
           ::new (std::__addressof(this->_M_payload))
           ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
             _Stored_type(std::forward<_Args>(__args)...);
             ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

我确信有一种重载赋值的方法，这样我就可以返回一个可选的，然后像上面的main一样赋值给它。感谢您的指点！;-)

Answer 1

目标(I)可以通过返回重载=操作符的帮助器类来实现。目标(Ii)可以通过返回重载bool操作符的帮助器类来实现。

考虑一下当您的operator[]返回一个如下所示的类时会发生什么：

class Octree {

    // Other declarations...

public:

    // Other declarations...

    struct value_at {

        Octree *ptr;

        operator bool() const { return ptr != nullptr; }

        Octree &operator=(const Octree &v)
        {
            return *ptr=v;
        }
    };

    value_at operator[](int index);
};

构造value_at将是您的作业；但是很明显，带有空ptr的返回对象表示一个不存在的值，否则它指向要返回的值。

现在，可以在布尔上下文中使用[]运算符returns，该上下文的计算结果表示是否返回了一个值，并且将某些内容赋给返回值最终将赋值给[]应该返回的值。

作为调试辅助，=操作符重载还可以检查ptr是否为null，并抛出异常。

helper类还可以声明operator Octree() const重载，以便返回的对象看起来更加透明。

如上所述:您也可以返回一个std::optional<std::reference_wrapper<Octree>>，它实际上与您的问题中描述的对象更接近。然而，在实践中，使用它可能需要一些繁琐的语法(赋值给这样的std::optional可能不一定会产生您想要的效果)。像这样一个简单的助手类通常会导致更自然、更透明的用法。