Re: [Boost-users] shared ptr + const_cast woes

From: Stefan Tarrant (stefan.tarrant_at_[hidden])
Date: 2006-06-28 10:29:12

When you use get() all bets are off. You are basically taking 
responsibility for the ownership of the returned pointer yourself, which 
is generally a bad idea. Use a const_pointer_cast<T> instead.

Here is my re-worked example:

#include <vector>
#include <iostream>
#include <boost/shared_ptr.hpp>

int main()
{
   std::vector<boost::shared_ptr<int> > x;
   x.push_back(boost::shared_ptr<int>(new int));
   *x.front() = 4;
   std::cout << "use count: " << x.front().use_count() << std::endl;
   boost::shared_ptr<const int> y = x.front();
   std::cout << "use count: " << x.front().use_count() << std::endl;
   std::cout << "use count: " << y.use_count() << std::endl;
   boost::shared_ptr<int> z = boost::const_pointer_cast<int>(y);
   std::cout << "use count: " << z.use_count() << std::endl;
   std::cout << "use count: " << y.use_count() << std::endl;
   x.pop_back();
   std::cout << *y << std::endl;
   std::cout << "use count: " << z.use_count() << std::endl;
   std::cout << "use count: " << y.use_count() << std::endl;

   y.reset();
   std::cout << "use count: " << z.use_count() << std::endl;
   std::cout << "use count: " << y.use_count() << std::endl;
}

And the output:

use count: 1
use count: 2
use count: 2
use count: 3
use count: 3
4
use count: 2
use count: 2
use count: 1
use count: 0
Press any key to continue . . .

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Evan Drumwright wrote:
> Hi all,
> 
> I am having trouble with pointer ownership as a result of the const_cast 
> operator.  In particular, for the code below, when a const_cast is used to 
> create a const int pointer, the use count for x does not increase.  The 
> result of this is that when x.pop_back() is called, y is no longer valid 
> (though the use count for y remains 1).
> 
> The documentation for shared_ptr does not help here.  Any suggestions?
> 
> Thanks,
> Evan
> 
> 
> 
> #include <vector>
> #include <iostream>
> #include <boost/shared_ptr.hpp>
> 
> int main()
> {
> 	std::vector<boost::shared_ptr<int> > x;
> 	x.push_back(boost::shared_ptr<int>(new int));
> 	*x.front() = 4;
> 	std::cout << "use count: " << x.front().use_count() << std::endl;
> 	boost::shared_ptr<const int> y(const_cast<const int*>(x.front().get()));
> 	std::cout << "use count: " << x.front().use_count() << std::endl;
> 	std::cout << "use count: " << y.use_count() << std::endl;
> 	boost::shared_ptr<int> z(const_cast<int*>(y.get()));
> 	std::cout << "use count: " << z.use_count() << std::endl;
> 	std::cout << "use count: " << y.use_count() << std::endl;
> 	x.pop_back();
> 	std::cout << *y << std::endl;
> }
> 
> Output:
> use count: 1
> use count: 1
> use count: 1
> use count: 1
> use count: 1
> 0
> 
> 
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