取消引用无效的指针但不使用结果是C ++中的未定义行为吗? [英] Is it undefined behavior in C++ to dereference an invalid pointer but not use the result?

问题描述

请考虑以下C ++代码:

Consider the following piece of C++ code:

int main() {
    int* ptr; // Deliberately uninitialized
    (void) *ptr;     // Dereference, do not use result
}

此代码是否导致未定义的行为?我假定答案是是"，即使*ptr的值实际上没有在任何地方使用.规范中是否有特定部分可以保证这一点?

Does this code result in undefined behavior? I assume that the answer is "yes" even though the value of *ptr is not actually used anywhere. Is there a particular part of the spec that guarantees this?

(对于这是重复的邮件，我深表歉意，但似乎无法在网站上找到其他任何专门询问此问题的内容.)

(I apologize if this is a duplicate, but I can't seem to find anything else on the site that specifically asks this question.)

推荐答案

仅取消引用尚未分配或标记为只读的指针会导致CPUs中的硬件异常内存管理单元.因此，即使您不使用通过解引用包含随机值的指针而不会返回的垃圾值，也无法保证CPU将从此类事件中返回.

Simply dereferencing a pointer that has not been allocated or is marked as read-only can cause a hardware exception in the CPUs memory management unit. So even if you don't use whatever garbage value would be returned by dereferencing a pointer that contains a random value, there is no guarantee the CPU would return from such an event.

但是，根据ISO C++11 Standard，声明为未初始化的指针必须具有奇异值.因此，未初始化的指针的值不是 undefined ，因为它不是垃圾.

However, according to the ISO C++11 Standard a pointer that is declared uninitialized must have a singular value. So the value of an uninitialized pointer is not undefined as in it is not garbage.

然而，标准指出，对于此类奇异值指针的大多数操作均未定义，只是用非奇异值覆盖奇异值 :

However the Standard states that most operations on such a singular value pointer are undefined with the exception of overwriting the singular value with a non singular value:

24.2.1 通常是 [ iterator.requirements.general ]

24.2.1 In general [ iterator.requirements.general ]

5 []示例:在声明未初始化的指针x之后(与int * x;一样)，必须始终假定x具有奇异值指针的值. —结束示例] 对于单数，大多数表达式的结果均未定义；唯一的例外是销毁具有奇异值的迭代器，将非奇异值分配给具有奇异值的迭代器，对于满足DefaultConstructible要求的迭代器，请使用值初始化的迭代器作为复制或移动操作. []注意:默认区别不提供此保证，尽管区别仅适用于琐碎的默认构造函数的类型，例如指针或持有指针的集合. —尾注 ] 在这种情况下，奇异值与任何其他值一样被覆盖.可引用的值总是非奇数.

5 [ Example: After the declaration of an uninitialized pointer x (as with int* x;), x must always be assumed to have a singular value of a pointer. — end example ] Results of most expressions are undefined for singular values; the only exceptions are destroying an iterator that holds a singular value, the assignment of a non-singular value to an iterator that holds a singular value, and, for iterators that satisfy the DefaultConstructible requirements, using a value-initialized iterator as the source of a copy or move operation. [ Note: This guarantee is not offered for default initialization, although the distinction only matters for types with trivial default constructors such as pointers or aggregates holding pointers. — end note ] In these cases the singular value is overwritten the same way as any other value. Dereferenceable values are always non-singular.

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