recursion - Recursively Delete Even Items from Doubly Linked List C++ -

here same question asking: deleting nodes doubly link list c++

the difference is, want understand wrong code. don't want have accept different approach without understand why code doesn't work. here 2 versions of code know wrong both. both of them give me segmentation fault.

int removeeven(node *&head) { if(!head)               //base case: end of list reached     return 0; int count = removeeven(head->next);     //recurse end of list if(head->data % 2 != 0)     return count; else{     ++count;     if(head->next){         head->next->previous = head->previous;     }     if(head->previous){         head->previous->next = head->next;     } if(!(head->previous)){         node* temp = head;         head = head->next;         delete temp;     }     else         delete head; } return count; } 

this second 1 takes count = 0 default arg.

int removeeven(node *&head, int count) if(head && head->data % 2 != 0) //not null, not {     removeeven(head->next, count); } else if(head != null){      //not null, yes    ++count;     if(head->next)         head->next->previous = head->previous;     if(head->previous)         head->previous->next = head->next;     node* temp = head;     head = head->next;     delete temp;    removeeven(head, count); }     return count;       //base case: null } 

int removeeven(node *&head) {   if(!head)               //base case: end of list reached     return 0;   int count = removeeven(head->next);     //recurse end of list   if(head->data % 2 != 0)     return count;   else{     ++count;     // correct way!!! copy old pointer in temp     node *t = head;     if(head->next){       head->next->previous = head->previous;     }     if(head->previous){       // warning!!! here modifying head nullptr       head->previous->next = head->next;     }     // correct way!!! delete temp pointer     delete t;     // warning!!! here trying access nullptr in head     // if(!(head->previous)){     //   node* temp = head;     //   head = head->next;     //   delete temp;     // }     // else     //   delete head;   }   return count; } 

i have got hint of root cause valgrind , gdb

valgrind ./a.out  ==2729== memcheck, memory error detector ==2729== copyright (c) 2002-2013, , gnu gpl'd, julian seward et al. ==2729== using valgrind-3.10.0 , libvex; rerun -h copyright info ==2729== command: ./a.out ==2729==  [9] [8] [7] [6] [5] [4] [3] [2] [1] [0]  ==2729== invalid read of size 8 ==2729==    @ 0x4008f5: removeeven(node*&) (list1.cpp:26) ==2729==    0x40087a: removeeven(node*&) (list1.cpp:15) ==2729==    0x40087a: removeeven(node*&) (list1.cpp:15) ==2729==    0x40087a: removeeven(node*&) (list1.cpp:15) ==2729==    0x40087a: removeeven(node*&) (list1.cpp:15) ==2729==    0x40087a: removeeven(node*&) (list1.cpp:15) ==2729==    0x40087a: removeeven(node*&) (list1.cpp:15) ==2729==    0x40087a: removeeven(node*&) (list1.cpp:15) ==2729==    0x40087a: removeeven(node*&) (list1.cpp:15) ==2729==    0x40087a: removeeven(node*&) (list1.cpp:15) ==2729==    0x400afd: main (list1.cpp:81) ==2729==  address 0x10 not stack'd, malloc'd or (recently) free'd ==2729==  ==2729==  ==2729== process terminating default action of signal 11 (sigsegv) ==2729==  access not within mapped region @ address 0x10 ==2729==    @ 0x4008f5: removeeven(node*&) (list1.cpp:26) ==2729==    0x40087a: removeeven(node*&) (list1.cpp:15) ==2729==    0x40087a: removeeven(node*&) (list1.cpp:15) ==2729==    0x40087a: removeeven(node*&) (list1.cpp:15) ==2729==    0x40087a: removeeven(node*&) (list1.cpp:15) ==2729==    0x40087a: removeeven(node*&) (list1.cpp:15) ==2729==    0x40087a: removeeven(node*&) (list1.cpp:15) ==2729==    0x40087a: removeeven(node*&) (list1.cpp:15) ==2729==    0x40087a: removeeven(node*&) (list1.cpp:15) ==2729==    0x40087a: removeeven(node*&) (list1.cpp:15) ==2729==    0x400afd: main (list1.cpp:81) ==2729==  if believe happened result of stack ==2729==  overflow in program's main thread (unlikely ==2729==  possible), can try increase size of ==2729==  main thread stack using --main-stacksize= flag. ==2729==  main thread stack size used in run 8720384. ==2729==  ==2729== heap summary: ==2729==     in use @ exit: 240 bytes in 10 blocks ==2729==   total heap usage: 10 allocs, 0 frees, 240 bytes allocated ==2729==  ==2729== leak summary: ==2729==    lost: 24 bytes in 1 blocks ==2729==    indirectly lost: 0 bytes in 0 blocks ==2729==      possibly lost: 0 bytes in 0 blocks ==2729==    still reachable: 216 bytes in 9 blocks ==2729==         suppressed: 0 bytes in 0 blocks ==2729== rerun --leak-check=full see details of leaked memory ==2729==  ==2729== counts of detected , suppressed errors, rerun with: -v ==2729== error summary: 1 errors 1 contexts (suppressed: 0 0)  compilation segmentation fault @ fri jul 28 09:46:51