c++ - '--p' 在 for(p=&(*L).elem[(*L).length-1];p>=q;--p) 中如何工作？

Question

我正在尝试调试以下代码:

Status ListInsert_Sq(SqLIst *L, int i, LElemType_Sq e)
{
    LElemType_Sq *newbase;
    LElemType_Sq *p, *q;

    if(i<1 || i>(*L).length+1)
        return ERROR;                   
    if((*L).length >= (*L).listsize)
    {
        newbase = (LElemType_Sq*)realloc(LElemType_Sq *)realloc((*L).elem, ((*L).listsiz+LISTINCREMENT)*sizeof(LElemType_Sq));
        if(!newbase)
            exit(OVERFLOW);

        (*L).elem = newbase;
        (*L).listsize += LISTINCREMENT; 
     } 

     q = &(*L).elem[i-1];
     for(p=&(*L).elem[(*L).length-1];p>=q;--p)
        *(p+1) = *p;

    *q = e;
    (*L).length++;

    return OK;
} 

这里使用--p正确吗？ 具体来说，我想知道列表的最后一个元素是否会被前一个元素移动或覆盖？

谢谢!

Answer 1

重写后，代码的猜测意图如下:

<小时/>

#include <stdlib.h>
#include <string.h>
#include "listmeuk.h"

Status ListInsert_Sq(SqLIst *lp, int index, LElemType_Sq elem)
{
    if(index<1 || index>lp->length+1) return ERROR; // this assumes 1-based indexing
    index--; // transform to zero-based indexing

    if(lp->length >= lp->listsize)
    {
        LElemType_Sq *newbase;
        newbase = realloc(lp->elem, (lp->listsiz+LISTINCREMENT) *sizeof *newbase);
        if(!newbase) exit(OVERFLOW);

        lp->elem = newbase;
        lp->listsize += LISTINCREMENT; 
     } 

    if(index < lp->length) memmove(&(lp->elem[index+1]), &(lp->elem[index]), (lp->length-index) * sizeof lp->elem[0] );

    lp->elem[index] = elem;
    lp->length++;

    return OK; 

}