询问一段代码的合法性

slackware12

终于整理清楚了思路。 不知道对不对。

void *p = ∑
int *pp = &*p;
/*
* 能通过编译， 理由如下：
* 6.5.3.2 Address and indirection operators
*    4 The unary * operator denotes indirection. If the operand points to a function, the result is a function designator; if it points to an object, the result is an lvalue designating the object. If the operand has type ‘‘pointer to type’’, the result has type ‘‘type’’ .     ===> 根据此条规则(大虾也说过了), *p是合法的, *p 的类型就是void.
*
*    1 The operand of the unary & operator shall be either a function designator, the result of a [] or unary * operator .      ==>  '&' 的操作数可以是[]运算符 或 *运算符构成的表达式, 16楼提到了下面的规则:
* 6.3.2.2 void
*    1 The (nonexistent) value of a void expression (an expression that has type void) shall not be used in any way.
*         我个人理解: &*p 表达式中, '&'操作符并没有使用到*p的值, 所以并没有违反上面那条规则.(不知道我这种理解是否正确)
*   
*  int *pp = ...; 由于赋值运算符'='的右操作数是(void *)型, 根据下条规则, (void *)指针可以转换成任意一种类型的指针. 因此可以直接赋值.
* 6.5.16.1 Simple assignment
*    1 One of the following shall hold)
         -- one operand is a pointer to an object or incomplete type and the other is a pointer to a qualified or unqualified version of void, and the type pointed to by the left has all the qualifiers of the type pointed to by the right;
*
*/




int a[] = {1, 2, 3};
printf("%p\n", &2[a]);
/*
* 2[a] 等价于a[2], 则printf 打印的是a[2]的地址.
*/

非常感谢各位大虾提供的思路. 头一次拜读了那么多页ISO C文档。

starwing83

6.5.3.2.3 The unary & operator yields the address of its operand. If the operand has type ‘‘type’’,
the result has type ‘‘pointer to type’’. 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, 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. Otherwise, the result is
a pointer to the object or function designated by its operand.

下次看标准得认真呐~~

starwing83

回复  starwing83


    从来没看过c99标准， 弱弱的问一下， 这个东西，在什么场合比较重要。。
peidright 发表于 2010-06-19 19:44

    面试的时候可以起到装B的作用。

克拉玛依

你对各种地址的意思不是很明白的

KBTiller

面试的时候可以起到装B的作用。
starwing83 发表于 2010-06-20 05:11

   
有时候恰恰相反，面视官“二胡”时起反作用

KBTiller

回复 21# slackware12


    if it points to an object, the result is an lvalue designating the object. If the operand has type ‘‘pointer to type’’, the result has type ‘‘type’’ .     ===> 根据此条规则(大虾也说过了), *p是合法的, *p 的类型就是void.


    我觉得“if it points to an object”似乎是一个前提条件
    我个人认为“*p 的类型就是void”不成立
    换句话说，我感觉“If the operand has type ‘‘pointer to type’’, the result has type ‘‘type’’”只在points to an object/function的前提下成立

slackware12

又学到很多东西了。
ISO C文档是揭露本质的好东西啊。

liexusong

不要为这些没什么用的语法问题纠结吧! 多想想其他有用的问题, 好像设计和算法吧!

pandaiam

这贴要认真学习一下啊。

ypyf3000

void*类型指针是不能解引用的