c++ - 内存重用不当导致的未定义行为

Question

标准引用了下面的例子(3.8/7 N3797):

struct C 
{
    int i;
    void f();
    const C& operator=( const C& );
};

const C& C::operator=( const C& other) 
{
    if ( this != &other ) 
    {
        this->~C(); // lifetime of *this ends
        new (this) C(other); // new object of type C created
        f(); // well-defined
    }
return *this;
}

C c1;
C c2;
c1 = c2; // well-defined
c1.f(); // well-defined; c1 refers to a new object of type C

如果我们按如下方式实现operator=是否有UB:

const C& C::operator=( const C& other) 
{
    if ( this != &other ) 
    {                        // Note that there is no more explcicitly destructor call,
                             // since at the time of memory reusing the lifetime of 
                             // this is still going on
        new (this) C(other); // new object of type C created
        f(); // well-defined
    }
return *this;
}

相关引用是:

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

没有规则:“在对象占用的存储位置创建新对象”。 同时，我们有一个适用于 const 对象的规则。很明显:

第 3.8/9 节:

Creating a new object at the storage location that a const object with static, thread, or automatic storage duration occupies or, at the storage location that such a const object used to occupy before its lifetime ended results in undefined behavior.

Answer 1

相关规则是这样的:

3.8/4 A program may end the lifetime of any object by reusing the storage which the object occupies or by explicitly calling the destructor for an object of a class type with a non-trivial destructor. For an object of a class type with a non-trivial destructor, the program is not required to call the destructor explicitly before the storage which the object occupies is reused or released; however, if there is no explicit call to the destructor or if a delete-expression (5.3.5) is not used to release the storage, the destructor shall not be implicitly called and any program that depends on the side effects produced by the destructor has undefined behavior.

你的例子，正如所写的那样，是合法的，因为 C 有一个普通的析构函数。