Re: Implementation of shared_ptr
Marcel M?ller wrote:
fungus wrote:
This is a question about the implementation of shared_ptr:
Does shared_ptr allocate a little control block to hold the reference
count?
Yes.
In fact, boost::shared_ptr allocates a block which is rather larger
than one reference count (because it has to support, among other things,
a deleter function and weak pointers, and it must be thread-safe, so it
requires a mutex).
Something like that:
class ref_count
{ friend void intrusive_ptr_add_ref(ref_count*);
friend void intrusive_ptr_release(ref_count*);
private:
unsigned count;
protected:
ref_count : count(0) {}
virtual ~ref_count() {}
public:
bool ref_is_managed() { return ref_count != 0; }
bool ref_is_unique() { return ref_count == 1; }
// Only a return value of true is thread-safe.
};
void intrusive_ptr_add_ref(ref_count* ref)
{ interlocked_increment(&ref->count);
}
void intrusive_ptr_release(ref_count* ref)
{ interlocked_decrement(&ref->count);
if (!ref->is_managed())
delete ref;
}
Now all you have to do for an intrusive reference count is to inherit
from ref_count.
Your ref_count class lacks a proper copy constructor and assignment
operator, which means that it will fail spectacularly if objects
inherited from it are copied/assigned around (if the reference count in
the source and target are different).
This is an extremely typical mistake with intrusive reference
counting. It's trivial to fix, though.
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...
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