c++ - 为什么当 T 包含 `std::optional<T>::operator=` 数据成员时删除 `const`？

Question

以下代码会导致编译器错误:

#include <optional>

class A {
};
class B {
private:
    const A a;
};

int main()
{
    B b;
    std::optional<B> bo1;
    bo1 = b;
}

例如，在 gcc 上，错误显示为:

main.cpp: In function 'int main()':
main.cpp:12:7: error: uninitialized const member in 'class B'
12 |     B b;
   |       ^
main.cpp:7:13: note: 'const A B::a' should be initialized
7 |     const A a;
  |             ^
main.cpp:14:11: error: use of deleted function 'std::optional<B>& std::optional<B>::operator=(std::optional<B>&&)'
14 |     bo1 = b;
   |           ^
In file included from main.cpp:1:
/lib/gcc-head/include/c++/12.0.0/optional:663:11: note: 'std::optional<B>& std::optional<B>::operator=(std::optional<B>&&)' is implicitly deleted because the default definition would be ill-formed:
663 |     class optional
    |           ^~~~~~~~
/lib/gcc-head/include/c++/12.0.0/optional:663:11: error: use of deleted function 'std::_Enable_copy_move<true, false, true, false, _Tag>& std::_Enable_copy_move<true, false, true, false, _Tag>::operator=(std::_Enable_copy_move<true, false, true, false, _Tag>&&) [with _Tag = std::optional<B>]'
In file included from /lib/gcc-head/include/c++/12.0.0/optional:43,
             from main.cpp:1:
/lib/gcc-head/include/c++/12.0.0/bits/enable_special_members.h:248:5: note: declared here
248 |     operator=(_Enable_copy_move&&) noexcept                         = delete;
    |     ^~~~~~~~

在 MSVC 上，作为另一个示例，错误显示为:

main.cpp
<source>(14): error C2280: 'std::optional<B> &std::optional<B>::operator =(const std::optional<B> &)': attempting to reference a deleted function
C:/data/msvc/14.29.29917-Pre/include\optional(445): note: compiler has generated 'std::optional<B>::operator =' here
C:/data/msvc/14.29.29917-Pre/include\optional(445): note: 'std::optional<B> &std::optional<B>::operator =(const std::optional<B> &)': function was implicitly deleted because a base class invokes a deleted or inaccessible function 'std::_Deleted_move_assign<std::_Optional_construct_base<_Ty>,_Ty> &std::_Deleted_move_assign<std::_Optional_construct_base<_Ty>,_Ty>::operator =(const std::_Deleted_move_assign<std::_Optional_construct_base<_Ty>,_Ty> &)'
        with
        [
            _Ty=B
        ]
C:/data/msvc/14.29.29917-Pre/include\xsmf_control.h(131): note: 'std::_Deleted_move_assign<std::_Optional_construct_base<_Ty>,_Ty> &std::_Deleted_move_assign<std::_Optional_construct_base<_Ty>,_Ty>::operator =(const std::_Deleted_move_assign<std::_Optional_construct_base<_Ty>,_Ty> &)': function was implicitly deleted because a base class invokes a deleted or inaccessible function 'std::_Deleted_copy_assign<std::_Optional_construct_base<_Ty>,_Ty> &std::_Deleted_copy_assign<std::_Optional_construct_base<_Ty>,_Ty>::operator =(const std::_Deleted_copy_assign<std::_Optional_construct_base<_Ty>,_Ty> &)'
        with
        [
            _Ty=B
        ]
C:/data/msvc/14.29.29917-Pre/include\xsmf_control.h(92): note: 'std::_Deleted_copy_assign<std::_Optional_construct_base<_Ty>,_Ty> &std::_Deleted_copy_assign<std::_Optional_construct_base<_Ty>,_Ty>::operator =(const std::_Deleted_copy_assign<std::_Optional_construct_base<_Ty>,_Ty> &)': function was explicitly deleted
        with
        [
            _Ty=B
        ]
Compiler returned: 2

只要我们去掉const A a;中的const关键字，错误就消失了。 根据referenced standard on std::optional ，在某些情况下， = 运算符的某些重载将被删除，具体取决于 std::is_...able_v 测试。在这种情况下，为什么 const 关键字会影响其中一些测试？

Answer 1

optional使用对象的 = .

具有 const 的类无法分配数据成员。只能构造。

试试这个:

B b0;    
B b1;
b0=b1;

optional不起作用，因为 B没有。

另外，试试 static_assert(!std::is_copy_assignable_v<B>); ，通过了。

现在，std::optional这里有个“后门”。

B b;
std::optional<B> bo1;
bo1.emplace(b);

构造而不是赋值。

理论上，Optional 可以依赖于此；但如果内容存在，则需要将其销毁，然后放置一个新对象。这是有风险的，会打乱语义，而且是一个不好的计划默默地做。