Re: auto_ptr definition

Barry <>
Tue, 27 Nov 2007 08:48:29 CST
<fih5ic$1ml$> <>
Nagrik wrote:

Hello group,

I came across this definition of auto_ptr at Wikipedia.

The auto_ptr class is declared in ISO/IEC 14882, section 20.4.5 as:

namespace std {

     template <class Y> struct auto_ptr_ref {};

     template<class X>
     class auto_ptr {
         typedef X element_type;

         // construct/copy/destroy:
         explicit auto_ptr(X* p =0) throw();
                            auto_ptr(auto_ptr&) throw();
         template<class Y> auto_ptr(auto_ptr<Y>&) throw();

         auto_ptr& operator=(auto_ptr&) throw();
         template<class Y> auto_ptr& operator=(auto_ptr<Y>&)
         auto_ptr& operator=(auto_ptr_ref<X> r)

         ~auto_ptr() throw();

         // members:
         X& operator*() const throw();
         X* operator->() const throw();
         X* get() const throw();
         X* release() throw();
         void reset(X* p =0) throw();

         // conversions:
                                    auto_ptr(auto_ptr_ref<X>) throw();
         template<class Y> operator auto_ptr_ref<Y>() throw();
         template<class Y> operator auto_ptr<Y>() throw();


I am confused about the syntex of third constructor in this


template<class Y> auto_ptr(auto_ptr<Y>&) throw();


This template constructor is used for conversion between different
pointers, when conversion exists.

auto_ptr<Derived> spD(new Derived);
auto_ptr<Base> spB(spD);

It looks like a constructor, which takes a parameter type
auto_ptr<Y>&, which in turn is another template class (empty struct)

*empty* here means implementation defined, I think.

named auto_ptr_ref and returns

template<class Y>

Can someone explain the whole syntex of this constructor. I
know the constructor are not supposed to return any value, then
why does this one return

template<class Y>

the conversion between auto_ptr_ref and auto_ptr is done transparent to
the user.


void f(auto_ptr<int> sp) {}

int main() {
   f(auto_ptr<int>(new int(10))); // temporary variable

temporary variable is rvalue, while auto_ptr copy ctor requires lvalue
(a reference, NOT const reference),
/operator auto_ptr_ref<X>()/ is called for conversion(meanwhile give up
ownership by calling release) to auto_ptr_ref, then constructor
auto_ptr(auto_ptr_ref<X>) is called.

the same thing happens when you write code like this:
auto_ptr<int> f() { return auto_ptr<int>(new int(10)); }

well, read some implementation, and have a debugger to see what's going on,
aside, VC8 has something wrong with auto_ptr_ref implementation, and use
/Za to disable extension, cause the extension even has something wrong
with the semantic of *explicit*

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