A Sample auto_ptr implementation

From:
Ankur Arora <ankurarora81@gmail.com>
Newsgroups:
comp.lang.c++
Date:
Mon, 13 Oct 2008 04:58:59 -0700 (PDT)
Message-ID:
<b8b78794-226c-49fa-8eea-849adbb90459@m3g2000hsc.googlegroups.com>
Hi All,

I'm building a sample application that uses a custom auto_ptr
implementation.
The program crashes with the following output:-

Output (Debug Assertion failed)
----------

         release called
         copy constructor
         aap: ptr2= 10
         aap: ptr3= 20
         aap: ptr3= 10
         aap: exiting app
         Deleting pointee...10
         Deleting pointee...-572662307 (...problem ?)

Code
--------

#include<iostream>

using namespace std;

// MyAutoPtr Interface
template <class T>
class MyAutoPtr
{
    public:

        explicit MyAutoPtr(T* ptr=0);

        ~MyAutoPtr();

        template <class U>
        MyAutoPtr(MyAutoPtr<U> *rhs);

        //copy constructor member template, initialize this member pointer
with any other compatible auto_ptr
        template <class U>
        MyAutoPtr(MyAutoPtr<U>& rhs);

        //assignment operator
        template <class U>
        MyAutoPtr<T>& operator=(MyAutoPtr<U>& rhs);

        //relinquish ownership
        T* release();

        T* get() const;

        //reset the auto_ptr, delete pointee, assume ownership of p
        void reset(T* ptr=0);

        T& operator*() const;
        T* operator->() const;

    private:

        T* pointee;
        //template <class U>
        //friend class MyAutoPtr<U>;
};

//Implementation

template<typename T>
inline MyAutoPtr<T>::MyAutoPtr(T* ptr):pointee(ptr)
{ }

template <class T>
inline MyAutoPtr<T>::~MyAutoPtr()
{
    if(pointee)
    {
        cout<<"\n\t Deleting pointee..."<<*pointee;
        delete pointee;
    }
}

template <class T>
template <class U>
inline MyAutoPtr<T>::MyAutoPtr(MyAutoPtr<U>& rhs) :
pointee(rhs.release())
{
    cout<<"\n\t copy constructor";
}

template<class T>
template<class U>
MyAutoPtr<T>& MyAutoPtr<T>::operator=(MyAutoPtr<U>& rhs)
{
    cout<<"\n\t Inside operator=";
    if(this!=&rhs)
    {
        cout<<"\n\t Inside operator=, calling reset";
        reset(rhs.release());
    }
    return *this;
}

template <class T>
T* MyAutoPtr<T>::get() const
{
    return pointee;
}

template <class T>
inline T& MyAutoPtr<T>::operator *() const
{
    return *pointee;
}

template<class T>
inline T* MyAutoPtr<T>::operator->() const
{ return pointee; }

template<class T>
T* MyAutoPtr<T>::release()
{
    cout<<"\n\t release called";
    T* oldp=pointee;
    pointee=0;

    return oldp;
}

template<class T>
void MyAutoPtr<T>::reset(T* p)
{
    cout<<"\n\t reset called";
    if(pointee!=p)
    {
        delete pointee;
        //pointee=0;
    }

    pointee=p;
}

void main()
{
    MyAutoPtr<int> intp(new int(10));
    MyAutoPtr<int> intp2(intp);
    MyAutoPtr<int> intp3(new int(20));

    //cout<<"ptr1= "<<*intp;
    cout<<"\n\t aap: ptr2= "<<*intp2;
    cout<<"\n\t aap: ptr3= "<<*intp3;

    intp3 = intp2; //
==============================> 1

    cout<<"\n\t aap: ptr3= "<<*intp3;

    cout<<"\n\t aap: exiting app";
}

It seems, on debugging, that class's custom operator= is somehow not
been called at 1 and hence a problem while destructing. (since delete
is being called twice on the same pointer)
---------------------------------------------------------------------------------------------------------

Q1. Any problems with the above code ?
Q2. What is causing operator= not to be called and how to fix it ?
Q3. Following statements (in the class interface) cause compiler
errors, if not commented out

                private:
        T* pointee;
        //template <class U>
        //friend class MyAutoPtr<U>;

        MSDN defines this error as:-

Compiler Error C3772
Error Message
"name" : invalid friend template declaration

It is invalid to declare a friend of a class template specialization.
You cannot declare an explicit or partial specialization of a class
template and in the same statement declare a friend of that
specialization. The name placeholder identifies the invalid
declaration.
To correct this error
-Do not declare a friend of a class template specialization.
-If appropriate for your application, declare a friend of the class
template, or declare a friend of a particular partial or explicit
specialization.

Why such error, if this is not a specialization? or is it a
specialization somehow?
---------------------------------------------------------------------------------------------------------

Any help will be highly appreciated!

Thanks!

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