[英]Const structure in doubly linked list is being modified in C
我正在处理双向链表,但出现了一个我没有解决的问题。 为了更好地说明问题,在我说出问题所在之前,这里是代码。
Dblist.h
# Ifndef CGI_DBLIST_H
# Define CGI_DBLIST_H
# Include "malloc.h"
/ * Structure represantative an element of the list. * /
typedef struct elem
{
int value;
struct elem * prev;
struct elem * next;
} Elem;
/ * Structure access to the list. * /
typedef struct
{
elem * first;
elem * last;
} dblist;
# ifdef __cplusplus
extern "C" {
# Endif
void Init (dblist * l); /* Initialize the list */
void pushback (dblist * s, int val); /* Add a value at end */
void PushFront (dblist * l, int val); /* Add a value at start */
int PopBack (dblist * l); /* Remove value at end */
int PopFront (dblist * l); /* Remove value at start */
void View (dblist l); /* Display whole list */
void ViewReverse (dblist l); /* Display all reversed */
void Clear (dblist * l); /* discard list */
dblist getInterval (dblist const * s);
#ifdef __cplusplus
}
#endif
#endif /* CGI_DBLIST_H */
Dblist.c
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include "dblist.h"
#include "malloc.h"
void Init (dblist * l)
{
l-> first = NULL;
l-> last = NULL;
}
void pushback (dblist * s, int val)
{
elem * n = malloc (sizeof (elem));
if (! n) exit (EXIT_FAILURE);
n-> value = val;
n-> prev = l-> last;
n-> next = NULL;
if (s-> last) s-> last-> next = n;
else s-> first = n;
l-> last = n;
}
void PushFront(dblist *l, int val)
{
elem *n = malloc(sizeof(elem));
if(!n) exit(EXIT_FAILURE);
n->value = val;
n->next = l->first;
n->prev = NULL;
if(l->first) l->first->prev = n;
else l->last = n;
l->first = n;
}
int PopBack(dblist *l)
{
int val;
elem *tmp = l->last;
if(!tmp) return -1;
val = tmp->value;
l->last = tmp->prev;
if(l->last) l->last->next = NULL;
else l->first = NULL;
free(tmp);
return val;
}
int popFront(dblist* l)
{
int val;
elem *tmp = l->first;
if(!tmp) return -1;
val = tmp->value;
l->first = tmp->next;
//if(l->first)l->first->prev = NULL;
//else l->last = NULL;
//free(tmp);
return val;
}
dblist getInterval (dblist const * s)
{
dblist* intervals = NULL;
memmove(&intervals, &l, sizeof(l));
if(intervals->first)intervals->first->prev = NULL;
else intervals->last = NULL;
return *intervals;
}
void View (dblist l)
{
elem *pelem = l.first;
while (Pelem)
{
printf ("% d \ n", pelem-> value);
pelem = pelem-> next;
}
}
void ViewReverse (dblist l)
{
elem* test = l.last;
while (test)
{
printf("% d \ n", test-> value);
test = test-> prev;
}
}
void Clear (dblist * l)
{
elem *tmp;
elem *pelem = l->first;
while(pelem)
{
tmp = pelem;
pelem = pelem->next;
free(tmp);
}
l->first = NULL;
l->last = NULL;
}
main.c
#include <stdlib.h>
#include <stdio.h>
#include "dblist.h"
int main ()
{
dblist pdbListe * = malloc (sizeof (dblist));
dblist interval;
Init (pdbListe);
printf ("Pushin In The gains list\n");
PushFront (pdbListe, 10);
Pushback (pdbListe, 20);
Pushback (pdbListe, 40);
PushFront (pdbListe, 23);
PushFront (pdbListe, 70);
PushFront (pdbListe, 54);
printf ("Viewing the list:\n");
View (pdbListe *);
puts ("--------------");
printf ("poping front capital gains from The Stack:\n");
printf ("% d\n", PopFront (pdbListe));
printf ("% d\n", PopFront (pdbListe));
/ / Printf ("% d\n", PopBack (pdbListe));
puts ("--------------");
printf ("Viewing the list after pop front:\n");
View (pdbListe *);
puts ("--------------");
printf ("this is pdbListe:% p\n", pdbListe);
printf ("this is interval:% p\n", & interval);
interval = getInterval (pdbListe);
printf ("Viewing the interval\n");
ViewReverse (interval);
printf ("first element is:% d\n", interval.first-> value);
printf ("last element is:% d\n", interval.last-> value);
puts ("--------------");
printf ("Reverse Viewing the list after pop front:\n");
ViewReverse (pdbListe *); // ISSUE HERE: it should print 6 elements not 4
puts ("--------------");
printf ("this is pdbListe:% p\n", pdbListe);
printf ("this is interval:% p\n", & interval);
printf ("sizeof pdbListe% d\n", sizeof (pdbListe));
printf ("sizeof interval% d\n", sizeof (interval));
printf ("Pushing back a value in The List:\n");
Pushback (pdbListe, 30);
printf ("Viewing the list after push back:\n");
View (pdbListe *);
puts ("--------------");
printf ("In The Front popping list:\n");
printf ("% d\n", PopFront (pdbListe));
printf ("% d\n", PopFront (pdbListe));
puts ("--------------");
printf ("Viewing the list after pop front:\n");
View (pdbListe *);
puts ("--------------");
printf ("Clearing the list\n");
Clear (pdbListe);
printf ("Freeing the list\n");
free (pdbListe);
system ("PAUSE");
return 0;
}
不在乎malloc.h,这是我用来确保内存usgae正确的小实用程序
所以问题是间隔变量来自getInterval(const dblist *)
,我希望在应用popBack
或popFront
时,它仅保留初始列表的一部分。
问题是,如果我对getInterval
进行修改,则会影响pdbListe
的值。 例如,尝试像这样修改getInterval
(尝试注释这些行,如下所示):
dblist getInterval(const dblist* l) {
dblist* intervals = NULL;
memmove(&intervals, &l, sizeof(l));
//if(intervals->first)intervals->first->prev = NULL;
//else intervals->last = NULL;
return *intervals;
}
从那里可以看到,不仅是getInterval
返回的结果(在本例中为main.c中的interval变量),而且还有pdbListe
变量,从本质上讲,永远不要修改它!
我该怎么办才能解决此问题? 我希望pdbListe
保持原样,并且永远不受getInterval
在做什么的影响。
如果希望getRange返回一个子列表,而原始列表保持不变,则需要修改终止列表的方式(将端点放到列表上)。 您将需要停止使用NULL作为结束标记,而改为使用与第一个/最后一个元素指针的比较。 例如:
void printList(dblist *list) {
for (elem *e = list->first; e != list->last; e = e->next) {
printf("%d ", e->value);
}
}
注意e != list->last
,而不是e != NULL
。
这样,您可以通过构造具有新起点/终点的dblist来创建子列表。 它不再需要对基础链接(下一个和上一个)进行任何修改。
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