取消引用空指针

发布于 2024-09-02 16:33:50 字数 97 浏览 1 评论 0原文

int* p = 0;
int* q = &*p;

这是未定义的行为吗？我浏览了一些相关的问题，但这个具体方面没有出现。

原文

int* p = 0;
int* q = &*p;

Is this undefined behavior or not? I browsed some related questions, but this specific aspect didn't show up.

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各自安好 2024-09-09 16:33:50

这个问题的答案是：这取决于您遵循哪种语言标准:-)。

在 C90 和 C++ 中，这是无效的，因为您对空指针执行间接寻址（通过执行 *p），并且这样做会导致未定义的行为。

然而，在 C99 中，这个是有效的、格式良好且定义良好的。在 C99 中，如果 unary-& 的操作数是通过应用 unary-* 或执行下标 ([])，则既不应用 &，也不应用 * 或 []。例如：

int* p = 0;
int* q = &*p; // In C99, this is equivalent to int* q = p;

同样，

int* p = 0;
int* q = &p[0]; // In C99, this is equivalent to int* q = p + 0;

来自 C99 §6.5.3.2/3：

如果[一元 & 运算符] 的操作数是一元 * 运算符的结果，则该运算符和 & 都不是> 运算符被求值，结果就像两者都被省略一样，除了对运算符的约束仍然适用并且结果不是左值。
同样，如果操作数是 [] 运算符的结果，则隐含的 & 运算符和一元 * 都不是由 [] 进行求值，结果就好像删除了 & 运算符并将 [] 运算符更改为 + 运算符。

（及其脚注，#84）：

因此，&*E 等价于 E（即使 E 是空指针）

The answer to this question is: it depends which language standard you are following :-).

In C90 and C++, this is not valid because you perform indirection on the null pointer (by doing *p), and doing so results in undefined behavior.

However, in C99, this is valid, well-formed, and well-defined. In C99, if the operand of the unary-& was obtained as the result of applying the unary-* or by performing subscripting ([]), then neither the & nor the * or [] is applied. For example:

int* p = 0;
int* q = &*p; // In C99, this is equivalent to int* q = p;

Likewise,

int* p = 0;
int* q = &p[0]; // In C99, this is equivalent to int* q = p + 0;

From C99 §6.5.3.2/3:

If the operand [of the unary & operator] is the result of a unary * operator, neither that operator nor the & operator is evaluated and the result is as if both were omitted, except that the constraints on the operators still apply and the result is not an lvalue.
Similarly, if the operand is the result of a [] operator, neither the & operator nor the unary * that is implied by the [] is evaluated and the result is as if the & operator were removed and the [] operator were changed to a + operator.

(and its footnote, #84):