Re: [Boost-users] Use boost::shared_ptr with boost::adaptors

Subject: Re: [Boost-users] Use boost::shared_ptr with boost::adaptors
From: tolik levchik (endight_at_[hidden])
Date: 2012-04-29 05:32:44

Thank you for explanation.

2012/4/29 Jeffrey Lee Hellrung, Jr. <jeffrey.hellrung_at_[hidden]>

> On Sat, Apr 28, 2012 at 7:10 AM, tolik levchik <endight_at_[hidden]> wrote:
>
>> The following code gives me segfault. But if `foo_ptr` is `foo*` it works
>> fine.
>>
>>     #include <vector>
>>     #include <boost/range/adaptor/indirected.hpp>
>>     #include <boost/range/adaptor/transformed.hpp>
>>     #include <boost/range/algorithm/for_each.hpp>
>>     #include <boost/bind.hpp>
>>     #include <boost/shared_ptr.hpp>
>>
>>     struct foo{
>>         foo(int _i):
>>             i(_i){}
>>         virtual void bar() const{
>>              std::cout << "foo::bar::i " << i << std::endl;
>>         }
>>         int i;
>>     };
>>
>>     typedef boost::shared_ptr<foo> foo_ptr;
>>     //typedef foo* foo_ptr;
>>     foo_ptr trasform(int i){
>>         return foo_ptr(new foo(i));
>>     }
>>     int main(){
>>         std::vector<int> vec;
>>         vec.push_back(1);
>>
>>         using namespace boost::adaptors; using boost::bind;
>>         boost::for_each(vec
>>                         | transformed(bind(&trasform, _1))
>>                         | indirected,
>>      bind(&foo::bar, _1));
>>     }
>> Is it expected behavior of adaptors ?
>>
>
> I think so. Here's my guess what's going on. Consider what the dereference
> function might look like for an iterator to the range
>
> vec | transformed(bind(&transform, _1)) |  indirected
>
> foo& dereference()
> {
>     int& i = *vec_it; // dereference iterator to vec
>     foo_ptr = transform(i); // pass i through bind(&transform, _1)
>     foo& x = *foo_ptr; // pass foo_ptr through indirected
>     return x;
> }
>
> Sadly, the only foo_ptr "backing" the foo& goes out of scope once the foo&
> is returned, so that foo object is deleted and you get a dangling reference.
>
> I don't think there's an obvious alternate implementation that will allow
> you to get away with the above. When you start chaining together range
> adaptors, especially transforms, you have to consider whether the argument
> to the transformation function needs to persist for the result to be remain
> valid.
>
> HTH,
>
> - Jeff
>
>
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