c++ - 当数组对象的元素存储被重用时，它的生命周期是否结束？

Question

在随后的引述中，我指的是 ISO 标准草案 N4713。

§ 6.6.3, 第 1 段说:

...The lifetime of an object o of type T ends when:
...
— the storage which the object occupies is released, or is reused by an object that is not nested within o (6.6.2).

请回答代码注释中的问题:

#include <new>

int main() {
    int x[2] = {0, 1};
    char* p = new (x + 1) char{0}; // Has x ended its' lifetime?
    int z = x[0];                  // Is this UB?
}

如果我使用 unsigned char作为数组元素类型，数组对象 x将为 *p 提供存储空间，根据第 6.6.2 节第 3 段:

If a complete object is created (8.5.2.4) in storage associated with another object e of type “array of N unsigned char” or of type “array of N std::byte” (21.2.1), that array provides storage for the created object if:
— the lifetime of e has begun and not ended, and
— the storage for the new object fits entirely within e, and
— there is no smaller array object that satisfies these constraints.

请验证我在代码注释中的陈述:

#include <new>

int main() {
    unsigned char x[2] = {0, 1};
    char* p = new (x + 1) char{0}; // Only x[1] have ended its' lifetime.
    int z = x[0];                  // This is OK.
}

虽然我不明白前面引用的最后一条规则，请举个例子？

there is no smaller array object that satisfies these constraints.

Answer 1

根据以下对标准的阅读，数组生命周期不会因放置 new 表达式而结束:

[基本生活]/1.5:

The lifetime of an object o of type T ends when:

• [...]

• the storage which the object occupies is released, or is reused by an object that is not nested within o ([intro.object]).

根据这个规则，如果在数组中创建的新对象没有嵌套在数组中，则数组生命周期结束。否则它不会结束。

[介绍对象]/2

If an object is created in storage associated with a member subobject or array element e (which may or may not be within its lifetime), the created object is a subobject of e's containing object if:

• the lifetime of e's containing object has begun and not ended, and

• the storage for the new object exactly overlays the storage location associated with e, and

• the new object is of the same type as e (ignoring cv-qualification).

最后两个项目符号没有填满，所以很明显，新的 char 对象不是数组的子对象。

然后 [intro.object]/3:

If a complete object is created ([expr.new]) in storage associated with another object e of type “array of N unsigned char” or of type “array of N std​::​byte” ([cstddef.syn]), that array provides storage for the created object if:

• the lifetime of e has begun and not ended, and

• the storage for the new object fits entirely within e, and

• there is no smaller array object that satisfies these constraints.

所以 unsigned char 数组为新的 char 对象提供了存储空间。

[介绍对象]/4:

An object a is nested within another object b if:

• a is a subobject of b, or

• b provides storage for a, or

• there exists an object c where a is nested within c, and c is nested within b.

正如我们上面提到的，第一个项目符号没有填满。第二个满载而归。所以根据[basic.life]/1.5，数组的生命周期并没有结束，因为新的char对象嵌套在数组中:

int main() {
    unsigned char x[2] = {0, 1};
    char* p = new (x + 1) char{0}; // x provides storage for the char object
    int z = x[0];                  // OK The first element is within its lifetime.
}

如果不是这种情况，可能大多数低级代码将不符合标准!