如何将下标运算符重载为 return 一个可以是左值的可选值？
How can I overload the subscript operator to return an optional which can be an lvalue?
我正在通过实现 octree class. I want the subscript operator on this class to return the octant corresponding to an index 来学习一些 C++ 功能。我应该如何在 class 上定义下标运算符，以便我既可以 (i) 分配给结果，又可以 (ii) 检查结果是否为空？
目标 (i) 是通过使下标运算符 return 成为引用而惯用地实现的。但是引用不能引用任何内容，如果八分圆为空，这是合法的 return 值。
目标 (ii) 可以通过使下标运算符 return 成为可选的来实现。但是随后修改指向八分圆的指针变得非常重要 - std::optional<T>，T 不能作为引用。
这是错误的示例（除了 optional 的所有实例在真实代码中都以 experimental 为前缀，因为我的 GCC 仅对 C++17 提供实验性支持）。
#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;
}
正如预期的那样，编译器拒绝了可选引用。
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)...);
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
我确定有一种重载赋值的方法，这样我就可以 return 一个可选的，然后像我在上面的 main 中那样赋值给它。感谢您的指点！ ;-)
目标 (i) 可以通过 return 重载 = 运算符的助手 class 来实现。目标 (ii) 可以通过 return 重载 bool 运算符的助手 class 来实现。
考虑一下当您的 operator[] return 是一个如下所示的 class 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);
};
构建 value_at 将是您的家庭作业；但很明显，具有空 ptr 的 returned 对象表示不存在的值，否则它指向的值是 returned.
现在，您的 [] 运算符 returns 可以在布尔上下文中使用，它评估一个值是否被 returned 的指示，并分配一些东西returned 值最终分配给 [] 应该 returned.
的值
= 运算符重载还可以检查 ptr 是否为 null，并抛出异常，作为调试辅助。
助手 class 也可以声明一个 operator Octree() const 重载，这样 returned 对象看起来更加透明。
综上所述：您还可以 return std::optional<std::reference_wrapper<Octree>> 实际上更接近您问题中描述的对象。然而，在实践中使用它可能需要一些繁琐的语法（分配给这样的 std::optional 可能不一定具有您正在寻找的效果）。像这样的简单助手 class 通常会导致更自然、更透明的使用。
我正在通过实现 octree class. I want the subscript operator on this class to return the octant corresponding to an index 来学习一些 C++ 功能。我应该如何在 class 上定义下标运算符，以便我既可以 (i) 分配给结果，又可以 (ii) 检查结果是否为空？
目标 (i) 是通过使下标运算符 return 成为引用而惯用地实现的。但是引用不能引用任何内容，如果八分圆为空，这是合法的 return 值。
目标 (ii) 可以通过使下标运算符 return 成为可选的来实现。但是随后修改指向八分圆的指针变得非常重要 - std::optional<T>，T 不能作为引用。
这是错误的示例（除了 optional 的所有实例在真实代码中都以 experimental 为前缀，因为我的 GCC 仅对 C++17 提供实验性支持）。
#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;
}
正如预期的那样，编译器拒绝了可选引用。
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)...);
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
我确定有一种重载赋值的方法，这样我就可以 return 一个可选的，然后像我在上面的 main 中那样赋值给它。感谢您的指点！ ;-)
目标 (i) 可以通过 return 重载 = 运算符的助手 class 来实现。目标 (ii) 可以通过 return 重载 bool 运算符的助手 class 来实现。
考虑一下当您的 operator[] return 是一个如下所示的 class 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);
};
构建 value_at 将是您的家庭作业；但很明显，具有空 ptr 的 returned 对象表示不存在的值，否则它指向的值是 returned.
现在，您的 [] 运算符 returns 可以在布尔上下文中使用，它评估一个值是否被 returned 的指示，并分配一些东西returned 值最终分配给 [] 应该 returned.
= 运算符重载还可以检查 ptr 是否为 null，并抛出异常，作为调试辅助。
助手 class 也可以声明一个 operator Octree() const 重载，这样 returned 对象看起来更加透明。
综上所述：您还可以 return std::optional<std::reference_wrapper<Octree>> 实际上更接近您问题中描述的对象。然而，在实践中使用它可能需要一些繁琐的语法（分配给这样的 std::optional 可能不一定具有您正在寻找的效果）。像这样的简单助手 class 通常会导致更自然、更透明的使用。