偶然发现在C中sizeof(void)是合法的，于是，对它的作用产生了疑问。查阅资料在GNU文档中发现如下解释：
In GNU C, addition and subtraction operations are supported on pointers to void and on pointers to functions. This is done by treating the size of a void or of a function as 1. A consequence of this is that sizeof is also allowed on void and on function types, and returns 1.
测试代码如下：
#include <stdio.h>
int main(int argc, char* argv[]) {
printf("sizeof(void): %lu\n", sizeof(void));
printf("sizeof(function): %lu\n", sizeof(main));
return ;
}
C语言中输出如下：
sizeof(void): 1
sizeof(function): 1
C++中sizeof(void)是不合法的，编译错误如下：
void_size.c: In function ‘int main(int, char**)’:
void_size.c:15: error: invalid application of ‘sizeof’ to a void type
void_size.c:16: error: ISO C++ forbids applying ‘sizeof’ to an expression of function type
References
1.http://gcc.gnu.org/onlinedocs/gcc-4.4.6/gcc/Pointer-Arith.html