在C ++中是否定义了& * NULL? [英] Is &*NULL well-defined in C++?

问题描述

以下是什么定义明确的C ++标准版本(如果有)?

In what version(s) of the C++ standards (if any) is the following well-defined?

void foo(void) {
    char *nullPtr = NULL;
    &*nullPtr;
}

请注意，我在这里专门询问有关&*nullPtr的问题.我知道*nullPtr只是未定义-但这是一个单独的问题，因此当前链接的重复"不是重复的.

Note that I am specifically asking about &*nullPtr here. I am aware that simply *nullPtr is undefined - but this is a separate question and hence the currently-linked "duplicate" is not a duplicate.

请注意，我没有将结果分配给任何内容-第二行是一个简单的语句.

Note that I am not assigning the result to anything - the second line is a simple statement.

这个应该是一个带有明显答案的问题，但是(似乎在此类问题上经常发生)我听到很多人说答案显然没有定义"为明显"定义".

This should be a question with an obvious answer, but (as seemingly happens way too often on such questions) I have heard just as many people say the answer is "obviously undefined" as "obviously defined".

相对来说，以下内容如何? foo是否应该读取c?

On a rather related note, what about the following? Should foo produce a read of c?

extern volatile char c;

void bar(void) {
    volatile char *nonnullptr = &c;
    &*nonnullptr;
}

(相同问题的C版本:在以下位置定义的& * NULL是否为NULL C?)

(C version of the same question: Is &*NULL well-defined in C?)

推荐答案

这是四个问题合而为一.

This is four questions in one.

&*nullPtr在C语言中定义良好，表示一元&运算符:

&*nullPtr is well-defined in C since C99, which says of the unary & operator:

如果操作数是一元*运算符的结果，则两者都不是 运算符或&运算符都被求值，结果就像 被省略了，[...]

If the operand is the result of a unary * operator, neither that operator nor the & operator is evaluated and the result is as if both were omitted, [...]

请参见WG14 N721 和 DR076 .

&*nullPtr在C ++的所有修订版中都未正式定义(通过省略:一元&被指定为产生指向指定对象"的指针，一元*被指定为产生指向该对象的左值"表达式指向的对象；"空指针值指向没有对象)，尽管核心问题的方向232 是为了使它定义明确.

&*nullPtr is formally undefined in all revisions of C++ (by omission: unary & is specified to produce a pointer to "the designated object", and unary * is specified to produce "an lvalue referring to the object [...] to which the expression points"; a null pointer value points to no object), although the direction of core issue 232 is to make this well-defined.

&*nonnullptr不会产生*nonnullptr的易失性读取.一元&需要一个左值操作数； *nonnullptr不执行左值转换(对于C)或左值到右值转换(对于C ++).

&*nonnullptr produces no volatile read of *nonnullptr. Unary & expects an lvalue operand; no lvalue conversion (for C) or lvalue-to-rvalue conversion (for C++) is performed for *nonnullptr.

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