在雙向鏈表中的Const結構正在修改中C

Question

我正在處理雙向鏈表，並且遇到了一個我沒有解決的問題。爲了更好地說明這一點，下面是代碼，然後再說明問題所在。

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; 
} 

的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，它是我用來確保正確內存usga的小工具

所以問題是間隔變量來自getInterval(const dblist *)，我希望它只保留一部分當popBack或popFront已被應用時的初始列表。

問題是如果我對getInterval進行修改，它會影響pdbListe的值。例如嘗試修改getInterval這樣的（嘗試如下圖所示評論那些行）：

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; 
} 

從那裏可以看出，不僅通過結果返回getInterval（在這種情況下main.c中的間隔可變）而且也是pdbListe變量，它本質上不應該被修改！

我能做些什麼來解決這個問題？我希望pdbListe保持原樣，不會受到getInterval正在做的事情的影響。

Answer 1

如果你想要getRange返回一個子列表，而原始列表保持未修改，那麼你需要修改你如何終止（放在端點上）你的列表。您將需要停止使用NULL作爲結束標記，並使用與第一個/最後一個元素指針的比較來代替。例如：

void printList(dblist *list) { 
    for (elem *e = list->first; e != list->last; e = e->next) { 
    printf("%d ", e->value); 
    } 
} 

注e != list->last，不e != NULL。

通過這種方式，您可以通過構建具有新的開始/結束點的dblist來創建子列表。它不再需要對底層鏈接進行任何修改（next和prev）。