是指向对象生命周期之外的已分配内存的指针“无效指针"吗?或“对象的指针"? [英] Are pointers to allocated memory outside object's lifetime "invalid pointer[s]" or "pointer[s] to an object"?

问题描述

C ++ 17(草稿N4659) [basic.compound] /3 说:

C++17 (draft N4659) [basic.compound]/3 says:

指针类型的每个值都是以下之一:

Every value of pointer type is one of the following:

• 指向对象或函数的指针(据说该指针指向该对象或函数)，或者

• a pointer to an object or function (the pointer is said to point to the object or function), or

指向对象([expr.add])末尾的指针，或者

a pointer past the end of an object ([expr.add]), or

该类型的空指针值([conv.ptr])，或

the null pointer value ([conv.ptr]) for that type, or

无效的指针值.

在以下对象程序的生命周期之外，这些类别中的哪一个属于指向已分配内存的指针，特别是// (1)到// (3)的a和// (4)的b的值?

To which of these categories belong pointers to allocated memory outside the lifetime of objects, specifically the values of a at // (1) through // (3) and b at // (4) in the following program?

#include<new>
#include<algorithm>

struct S {
    ~S() { /* Non-trivial destructor */ }
};

struct T {
    ~T() { /* Non-trivial destructor */ }
};

int main() {
    void* a = operator new(std::max(sizeof(S), sizeof(T)));
    // (1)
    a = new(a) S;
    static_cast<S*>(a)->~S();
    // (2)
    a = new(a) T;
    static_cast<T*>(a)->~T();
    // (3)
    operator delete(a);   

    void* b = operator new(42);
    // (4)
    operator delete(b);
}

---

据我了解，指针值在释放时变为无效，而不是在对象的生命周期结束时失效，但如果指针值为"指向对象的指针"，则为他们指向哪个对象?

---

In my understanding a pointer value becomes invalid when deallocated, not when the life time of an object ends, but if the pointer values are "pointer[s] to an object", to which object do they point?

推荐答案

在以下对象程序的生命周期之外，这些类别中的哪一个属于指向已分配内存的指针，特别是在// (1)到// (3)的a和在// (4)的b的值?

To which of these categories belong pointers to allocated memory outside the lifetime of objects, specifically the values of a at // (1) through // (3) and b at // (4) in the following program?

当前尚未指定从分配函数(// (1)的a和// (4)的b)返回的

指针值，几乎无法根据[basic.compound]/3中的分类法对其进行分类. ，请参见 https://groups.google. com/a/isocpp.org/d/msg/std-discussion/4NQawIytVzM/eMKo2AJ9BwAJ

Pointer values returned from allocation functions (a at // (1) and b at // (4)) are not currently specified and it is barely possible to classify them according to the taxonomy in [basic.compound]/3, see https://groups.google.com/a/isocpp.org/d/msg/std-discussion/4NQawIytVzM/eMKo2AJ9BwAJ

据我了解，指针值在释放时变为无效，而不是在对象的生命周期结束时变为无效，但是如果指针值是指向对象的指针"，则它们指向哪个对象?

In my understanding a pointer value becomes invalid when deallocated, not when the life time of an object ends, but if the pointer values are "pointer[s] to an object", to which object do they point?

他们指向对象时该对象还活着.

To the object they pointed to when the object was alive.

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