Re: [Boost-users] What would happen if boost::shared_ptr is reset to an address in the stack

From: David Abrahams (dave_at_[hidden])
Date: 2007-10-16 16:50:29

on Tue Oct 16 2007, "Peng Yu" <pengyu.ut-AT-gmail.com> wrote:

> On 10/16/07, David Abrahams <dave_at_[hidden]> wrote:
>>
>> on Tue Oct 16 2007, "Peng Yu" <pengyu.ut-AT-gmail.com> wrote:
>>
>> > Hi,
>> >
>> > The following program runs well in my system. However, if I change 1
>> > to 0, it will terminate at the line with "delete". Does this mean that
>> > boost::shared_ptr can tell if the pointer is from the stack or the
>> > heap and does different things based on where the pointer is from?
>>
>> No.  The termination is just an expression of the fact that you invoked
>> undefined behavior by passing an illegal argument to delete.
>>
>> >
>> > Thanks,
>> > Peng
>> >
>> > #include <iostream>
>> > #include <boost/shared_ptr.hpp>
>> >
>> > int main() {
>> > #if 1//try change this to 0
>> >   int a[100];
>> >   boost::shared_ptr<int> p(a);
>> >   std::cout << std::endl;
>> > #else
>> >   int a[100];
>> >   delete a; // would terminate here
>> >   std::cout << std::endl;
>> > #endif
>> > }
>
> I don't think my question was completely answered. Do you means
> reseting shared_ptr with an array from the stack is not defined? 

Yes, that's true unless you also supply a custom deleter that has
legal semantics. "delete a" is illegal unless a is dynamically
allocated.

However, it's not what I mean.  I mean that "delete a" is undefined
when a is a stack array.  It's even not legal when it's a heap array
allocated via "a = new int[SIZE]"; for that you'd need "delete[] a"
(which also doesn't work when a is a stack array).

-- 
Dave Abrahams
Boost Consulting
http://www.boost-consulting.com