放置 new 并赋值 class with const member

Question

为什么会出现这种未定义的行为？

struct s
{
    const int id; // <-- const member

    s(int id):
        id(id)
    {}

    s& operator =(const s& m) {
        return *new(this) s(m); // <-- undefined behavior?
    }
};

（引用标准会更好）。

这个问题来自this answer。

Answer 1

显示的代码片段没有任何内在的 UB。但是，几乎可以肯定，在任何正常使用情况下，UB 都会立即出现。

来自 [basic.life]/8（强调我的）

If, after the lifetime of an object has ended and before the storage which the object occupied is reused or released, a new object is created at the storage location which the original object occupied, a pointer that pointed to the original object, a reference that referred to the original object, or the name of the original object will automatically refer to the new object and, once the lifetime of the new object has started, can be used to manipulate the new object, if:

the storage for the new object exactly overlays the storage location which the original object occupied, and

the new object is of the same type as the original object (ignoring the top-level cv-qualifiers), and

the type of the original object is not const-qualified, and, if a class type, does not contain any non-static data member whose type is const-qualified or a reference type, and

the original object was a most derived object of type T and the new object is a most derived object of type T (that is, they are not base class subobjects).

由于s中有一个const成员，调用operator=后使用原始变量将是UB。

s var{42};
var = s{420};         // OK
do_something(var.id); // UB! Reuses s through original name
do_something(std::launder(&var)->id);  // OK, this is what launder is used for

放置 new 并赋值 class with const member

Placement new and assignment of class with const member

c++

constants

placement-new

assignment-operator

language-lawyer