求sizeof(f)大小的疑问

KBTiller

好像不能对函数名进行sizeof（）运算。
haoyuejushi 发表于 2011-08-24 16:35

    函数名确实没有这个运算

KBTiller

C99标准规定，函数、不能确定类型的表达式以及位域（bit-field）成员不能被计算sizeof值;
sizeof是在编译时确定的，不是运行时确定；
...
davidfoxhu 发表于 2011-08-24 17:12

对VLA，sizeof不是编译时确定的

manULinux

#define   sizeof(x)  ({   typeof(x)   _x; (char   *)((&_x+1))   -   (char   *)(&_x);})

是一个宏，不是函数。   

哥哥们呀 还看汇编，唉。。。

zylthinking

#define   sizeof(x)  ({   typeof(x)   _x; (char   *)((&_x+1))   -   (char   *)(&_x);})

是一个宏， ...
manULinux 发表于 2011-08-24 20:22

    typeod(f) 是什么呢

幻の上帝

Undefined behavior.
ISO C99：
4/2 If a ‘‘shall’’ or ‘‘shall not’’ requirement that appears outside of a constraint is violated, the behavior is undefined. Undefined behavior is otherwise indicated in this International Standard by the words ‘‘undefined behavior’’ or by the omission of any explicit definition of behavior. There is no difference in emphasis among these three; they all describe ‘‘behavior that is undefined’’.
6.3.2.1/4
A function designator is an expression that has function type. Except when it is the operand of the sizeof operator 54) or the unary & operator, a function designator with type ‘‘function returning type’’ is converted to an expression that has type ‘‘pointer to function returning type’’.
54) Because this conversion does not occur, the operand of the sizeof operator remains a function designator and violates the constraint in 6.5.3.4.
6.5.3.4/2 The sizeof operator shall not be applied to an expression that has function type or an incomplete type, to the parenthesized name of such a type, or to an lvalue that designates a bit-field object.
PS.C89也一样。
PS2.C++直接ill-formed了所以报错。
PS3.至今买不起。

幻の上帝

#define   sizeof(x)  ({   typeof(x)   _x; (char   *)((&_x+1))   -   (char   *)(&_x);})

是一个宏， ...
manULinux 发表于 2011-08-24 20:22

原来关键字是宏……

manULinux

回复 14# zylthinking


    重新温习一下C吧 。

manULinux

回复 16# 幻の上帝


    我没说他不是关键字，实现方式不是宏么？

zylthinking

回复  zylthinking


    重新温习一下C吧 。
manULinux 发表于 2011-08-25 08:13

授我以鱼， 不要授我以渔

变异老鼠

这是 gcc 的扩展：
http://gcc.gnu.org/onlinedocs/gc ... .html#Pointer-Arith