Re: Making a smart pointer which works with incomplete types

Juha Nieminen <nospam@thanks.invalid>
Sun, 07 Sep 2008 15:45:22 GMT
Lance Diduck wrote:

Most shared_ptr implementations will use ECBO for the deleter. i.e.

template<class T,class D>

  In this case the user would have to always provide a deleter for the
smart pointer because it has no default value. This can be burdensome
and a nuisance. It should be made optional.

struct shared_ptr_imp:D{
T* data;
unsigned refcount;
void decrementReferenceCount()
            // decrement reference count and then:
            if(<refcount == 0>)
                this->D::operator()(data);//exapanded to see whats
really happening

  If D is a template class, that line instantiates the D::operator()
function. If that function calls the destructor of 'data', it will
require for T to be a complete type.

In the vast majority of cases, D is something like
template<class T>
struct DeleteMe{
   void operator()(T* d){delete d;}

  Which is exactly the problem: DeleteMe::operator() cannot properly
destroy 'd' if T is an incomplete type (it cannot call its destructor).

  That's exactly what the deleter function pointer trick solves: Rather
than performing a 'delete' in the destructor of the smart pointer, what
it does is that it simply calls the deleter function through the
pointer. The actual deleter function has been instantiated and the
pointer to it assigned to the deleter function pointer variable in the
constructor. The only thing the destructor refers to is this pointer
variable and nothing else. Thus it works with incomplete types (as long
as the constructor was instantiated in a context where the type was

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