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From: Richard Damon <rdamon@BeltronicsInspection.com>
Newsgroups: comp.std.c++
Subject: Re: why doesn't delete NULL the pointer?
Date: Mon,  8 Apr 2002 17:10:33 GMT
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David Schwartz <davids@webmaster.com> wrote:

>Ken Alverson wrote:
>
>> I still feel my analogy is valid, but this really isn't going anywhere, so
>> I'll drop the issue.
>> 
>> Ken
>
>	So what do you do if the value of the pointer is still needed?!
>
>Consider the following code:
>
>Mutex mutex_table[256];
>
>void LockMutex(void *p)
>{
> mutex_table[reinterpret_cast<unsigned int>(p)%256].Lock();
>}
>
>void UnlockMutex(void *p)
>{
> mutex_table[reinterpret_cast<unsigned int>(p)%256].Unlock();
>}
>
>void ConditionalLockedDelete(Foo *p)
>{
> LockMutex(p);
> if(p->ShouldDelete()) delete p;
> UnlockMutex(p);
>}

correct would be (need to change def of (Un)LockMutex
{
  int hash = HashPointer(p);
  LockMutex(hash);
  if(p->ShouldDelete()) delete p;
  UnlockMutex(hash);
}

This doesn't reaccess the pointer after deleting, and hopefully will encourage
you to use a better hash function. A large portion of your mutex_table is not
used as the bottom bits of p are often the same to enforce alignment
requirements, and if Foo is large some implementations may page align p and your
table will only have one usable mutex.
>
>	How would this work if 'delete' NULLed the pointer? Here's another
>case:
>
>class Foo
>{
> protected:
> Foo *brother;
> bool can_delete;
> public:
> static bool Detach(Foo *a)
> {
>  if(can_delete) { delete a; return true; }
>  return false;
> }
> void BrotherNowGone(void) { brother=NULL; }
> Foo *GetBrother() { return brother; }
>}
>
>void DetachAndDeLink(Foo *a, Foo *b)
>{
  Foo *temp = a;
> bool did_delete=a->Detach();
> if(did_delete && (b->GetBrother()==a)) b->BrotherNowGone();
use temp instead of a now.
>}
>
>	How would this work if 'delete' NULLed the pointer?
>
>	DS
>
>---
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-- 
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