被取消引用指针是相等的标准nullptr未定义行为？ [英] Is dereferencing a pointer that's equal to nullptr undefined behavior by the standard?
问题描述
这是博客作者带来了关于空指针dereferecing讨论：
An blog author has brought up the discussion about null pointer dereferecing:
- http://www.viva64.com/en/b/0306/
我已经把一些相反的论点在这里：
I've put some counter arguments here:
- http://bit.ly/1L98GL4
他的理由是引用标准的主线是这样的：
His main line of reasoning quoting the standard is this:
在'和; podhd-> LINE6'前pression是在C语言中未定义行为
当podhd'是一个空指针。
The '&podhd->line6' expression is undefined behavior in the C language when 'podhd' is a null pointer.
C99标准说，有关的下列'和;'地址运算符
（6.5.3.2地址和间接运算符）：
The C99 standard says the following about the '&' address-of operator (6.5.3.2 "Address and indirection operators"):
一元和放大器的操作;运营商应是一个函数
标志，一个[]或目*运算符，或左值的结果
指定的对象不是一个位域，不与声明
寄存器存储类说明。
The operand of the unary & operator shall be either a function designator, the result of a [] or unary * operator, or an lvalue that designates an object that is not a bit-field and is not declared with the register storage-class specifier.
这位前pression'podhd-> LINE6显然不是一个功能标志，
一个[]或*运算的结果。它是一个左值前pression。然而，
当podhd指针为NULL，前pression没有指定的
因为6.3.2.3对象指针说：
The expression 'podhd->line6' is clearly not a function designator, the result of a [] or * operator. It is an lvalue expression. However, when the 'podhd' pointer is NULL, the expression does not designate an object since 6.3.2.3 "Pointers" says:
如果空指针常数转换为指针类型，
结果指针，叫做空指针，是保证比较
不等的指针的任何对象或功能。
If a null pointer constant is converted to a pointer type, the resulting pointer, called a null pointer, is guaranteed to compare unequal to a pointer to any object or function.
当，当它被评估一个左值没有指定一个对象，则
行为是不确定的（C99 6.3.2.1左值，数组和函数
代号）：
When "an lvalue does not designate an object when it is evaluated, the behavior is undefined" (C99 6.3.2.1 "Lvalues, arrays, and function designators"):
这是左值是与对象类型或不完全类型的前pression
非void;如果当它是一个左值没有指定一个对象
评估，这种行为是未定义。
An lvalue is an expression with an object type or an incomplete type other than void; if an lvalue does not designate an object when it is evaluated, the behavior is undefined.
所以，在短暂的同样的想法：
So, the same idea in brief:
当 - >上的指针被执行死刑，其计算结果为左值在哪里
没有对象存在，因此该行为是不确定的。
When -> was executed on the pointer, it evaluated to an lvalue where no object exists, and as a result the behavior is undefined.
此问题是纯粹的语言基础，我不要求对给定的系统是否允许一个与地址0在任何语言中是什么样的篡改。
This question is purely language based, I'm not asking regarding whether a given system allows one to tamper with what lies at address 0 in any language.
据我所看到的，有一个在非关联化的指针变量，其值等于 nullptr 没有限制，甚至想到一个指针的比较对 nullptr （或（无效*）0 ）常数可以在因为规定的段落某些情况下优化消失，但是这看起来像另一个的问题，它并不prevent解引用一个指针，其值等于 nullptr 。请注意，我检查其他SO问题和答案，我特别喜欢这套报价，以及标准报价以上，而我却没有的东西，显然从标准推断跌跌撞撞，如果一个指针 PTR 比较等于 nullptr ，取消引用这将是不确定的行为。
As far as I can see, there's no restriction in dereferencing a pointer variable whose value is equal to nullptr, even thought comparisons of a pointer against the nullptr (or (void *) 0) constant can vanish in optimizations in certain situations because of the stated paragraphs, but this looks like another issue, it doesn't prevent dereferencing a pointer whose value is equal to nullptr. Notice that I've checked other SO questions and answers, I particularly like this set of quotations, as well as the standard quotes above, and I didn't stumbled upon something that clearly infers from standard that if a pointer ptr compares equal to nullptr, dereferencing it would be undefined behavior.
目前大多数我所得到的是deferencing的恒的（或强制转换为任何指针类型）是什么，是UB，但没有说关于一个变量，是位等于从出现的值 nullptr 。
At most what I get is that deferencing the constant (or its cast to any pointer type) is what is UB, but nothing saying about a variable that's bit equal to the value that comes up from nullptr.
我想清楚地从持有的价值等于它的指针变量分离 nullptr 不变。但是，针对这两种情况下的答案是理想的。
I'd like to clearly separate the nullptr constant from a pointer variable that holds a value equals to it. But an answer that address both cases is ideal.
我不明白，优化可以快速在当不会遭受 nullptr 的比较等，并可以简单地剥离基于code。
I do realise that optimizations can quick in when there're comparisons against nullptr, etc and may simply strip code based on that.
如果得出的结论是，如果 PTR 等于 nullptr 的价值提领，绝对是UB，另一个问题如下：
If the conclusion is that, if ptr equals to the value of nullptr dereferencing it is definitely UB, another question follows:
做C和C ++标准，意味着在地址空间的特殊值必须存在只是为了重新present空值指针？
推荐答案
当你引用C，提领一空指针显然从这一标准报价（重点煤矿）未定义的行为：
As you quote C, dereferencing a null pointer is clearly undefined behavior from this Standard quote (emphasis mine):
（C11，6.5.3.2p4）如果一个无效值被分配到指针，的
一元*运算符的行为是未定义 0.102），
(C11, 6.5.3.2p4) "If an invalid value has been assigned to the pointer, the behavior of the unary * operator is undefined.102)"
102）：在由一元*运算符取消引用指针无效的值是一个空指针，对象类型正确对齐的地址指向，以及对象的地址其生命周期结束后。
102): "Among the invalid values for dereferencing a pointer by the unary * operator are a null pointer, an address inappropriately aligned for the type of object pointed to, and the address of an object after the end of its lifetime."
在C99相同的报价和C89 / C90相似。
Exact same quote in C99 and similar in C89 / C90.
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