error: invalid use of incomplete type 'struct word'|

发生错误的程序：

#include <stdio.h>
#include <string.h>
#include <algorithm>

int cmp(const void* x,const void* y)
{
struct word* xx=(word*)x;
struct word* yy=(word*)y;
return strcmp(xx->a,yy->a);
}

struct word
{
char a[26];
}w[1000];

int main()
{

……

}

错因：结构体的声明和定义在cmp函数之后