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From: Andreas Huber (ahd6974-spamgroupstrap_at_[hidden])
Date: 2004-10-23 19:03:04

Hi Colin

colin.rafferty_at_[hidden] wrote:
>>> No, the constructor is implicit by design. Do you have a case where
>>> the implicit constructor causes problems?
>> Sure, this is the same problem as for shared_ptr<>.
>> extern void foo(const intrusive_ptr<Bar>&);
>> void baz()
>> {
>> Bar* bar = new Bar;
>> foo(bar); // oops!
>> delete bar;
>> }

For this to work, intrusive_ptr_add_ref and intrusive_ptr_release
functions accepting a Bar * (or a base class pointer) must exist. That
is, someone already decided that *all* heap-allocated Bar objects will
be intrusively reference-counted. Creating a heap-allocated Bar object
without immediately passing the pointer to the intrusive_ptr constructor
is dangerous. Is there a reason why you do that?

I can't speak for Peter but I believe he made intrusive_ptr constructor
non-explicit because there is no room for abuse as long as you follow
the best practices he describes. The same isn't true for heap-allocated
objects pointed to by a shared_ptr:

extern void foo( const shared_ptr< Bar > & );

void baz()
  shared_ptr< Bar > pBar = new Bar();
  foo( pBar.get() ); // compiler error here because of explicit ctor

With a non-explicit ctor, the last line would compile just fine but
eventually lead to a double-delete. With intrusive_ptr this isn't a
problem at all, hence the non-explicit constructor there.


Andreas Huber
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