For training purposes, I am trying to write my own smartpointer, imitating std::shared_ptr
. I have a static std::map<void *, int> ref_track
that keeps track whether there are still shared pointer referencing a certain block in memory.
My concept is this:
template <typename PType>
class shared_ptr
{
public:
shared_ptr()
: value_(nullptr), ptr_(nullptr)
{}
template <typename T>
explicit shared_ptr(T * ptr)
: shared_ptr()
{
reset(ptr);
}
template <typename T>
shared_ptr(shared_ptr<T> const & other)
: shared_ptr()
{
reset(other.get());
}
~shared_ptr()
{
reset();
}
void reset()
{
if(value_)
{
delete value_; // Segmentation fault here!
value_ = 0;
ptr_ = 0;
}
}
template <typename T>
void reset(T * ptr)
{
reset();
if(ptr)
{
value_ = new shared_ptr_internal::storage_impl<
T
>(ptr);
ptr_ = ptr;
}
}
PType * get() const
{
return ptr_;
}
typename shared_ptr_internal::ptr_trait<PType>::type operator *()
{
return *ptr_;
}
private:
shared_ptr_internal::storage_base * value_;
PType * ptr_;
};
When running my test suite, I noticed that
shared_ptr<int> a(new int(42));
a.reset(new int(13));
works fine, but
shared_ptr<int> a(new int(42));
a = shared_ptr<int>(new int(13));
leads to problems: *a
is 0
instead of 13
, and delete value_
crashes with a segmentation fault in the destructor of a
. I have marked the crash in the source code with a comment.
The used internal classes are
namespace shared_ptr_internal
{
typedef std::map<void *, int> ref_tracker;
typedef std::map<void *, int>::iterator ref_tracker_iterator;
typedef std::pair<void *, int> ref_tracker_entry;
static ref_tracker ref_track;
struct storage_base
{
virtual ~storage_base() {}
};
template <typename PType>
struct storage_impl : storage_base
{
storage_impl(PType * ptr)
: ptr_(ptr)
{
ref_tracker_iterator pos = ref_track.find(ptr);
if(pos == ref_track.end())
{
ref_track.insert(
ref_tracker_entry(ptr, 1)
);
}
else
{
++pos->second;
}
}
~storage_impl()
{
ref_tracker_iterator pos = ref_track.find(ptr_);
if(pos->second == 1)
{
ref_track.erase(pos);
delete ptr_;
}
else
{
--pos->second;
}
}
private:
PType * ptr_;
};
template <typename PType>
struct ptr_trait
{
typedef PType & type;
};
template <>
struct ptr_trait<void>
{
typedef void type;
};
}
Sorry for the bulk of source code, but I really do not know how I could narrow it down further. I would be grateful for any ideas what could be causing the segfault, and moreover why this does not happen when using reset manually.
Update
My (not-working) assignment operator:
template <typename T>
shared_ptr<PType> & operator =(shared_ptr<T> const & other)
{
if(this != &other)
{
value_ = nullptr;
ptr_ = nullptr;
reset(other.get());
}
return *this;
}
You're missing an assignment operator.
This means that in the following code:
a = shared_ptr<int>(new int(13));
a temporary shared pointer is created; then the default assignment operator simply copies the pointer to a
without releasing the old value or updating the reference count; then the temporary deletes the value, leaving a
with a dangling pointer.