c++ - 这是一个 nullptr 吗？

Question

阅读 this问答，我认为 p 不应该是 nullptr，即使 x 是 0。我理解的对吗？

int main()
{
    int x = 0;

    std::cin >> x; // Enter `0`

    void *p = (void *)x; // or: void *p = reinterpret_cast<void*>(x);


    if (!p)
        std::cout << "p is nullptr" << std::endl;
}

在标准输入中输入 0 后，消息 p is nullptr 将显示在我的 GCC 中。根据link ，它不应计算为 nullptr，但结果并不如我所料。

代码是未定义的行为吗？或未指定的结果？为什么它求值为 nullptr？

Answer 1

来自 C++11 5.2.10/5 Reinterpret cast [expr.reinterpret.cast](强调已添加):

A value of integral type or enumeration type can be explicitly converted to a pointer. A pointer converted to an integer of suffcient size (if any such exists on the implementation) and back to the same pointer type will have its original value; mappings between pointers and integers are otherwise implementation-defined.

来自 5.2.10/4 的相关位:

A value of type std::nullptr_t can be converted to an integral type; the conversion has the same meaning and validity as a conversion of (void*)0 to the integral type. [ Note: A reinterpret_cast cannot be used to convert a value of any type to the type std::nullptr_t. —end note ]