Re: I don't get why smart pointers are useful.

From:

"Alf P. Steinbach" <alfps@start.no>

Newsgroups:

comp.lang.c++

Date:

Mon, 14 Apr 2008 18:13:49 +0200

Message-ID:

<Q7SdndOQVeIiH57VnZ2dnUVZ_gudnZ2d@comnet>

* Simon L:

Pointer mismanagement is a big cause of bugs in software that I work
on. I decided to invest a little time into
having a look at auto_ptr and other smart pointers but I don't see how
they'll save me making the same mistakes.

A typical use and source of buggy pointers is in a collection of
objects. My containers of choice, eg stl::vector
don't like auto_ptr so I've had to use some MFC template classes.

In this test app I don't see how my pointers are suddenly safe or
smart. Yes they transfer ownership, yes they
auto delete but what I was hoping (expecting) was that manipulation of
old (ie ownership transferred or cleaned)
auto_ptrs would be handled.

Use of the pointer selection -> on my old auto_ptr blows up my app.,
gaining me very little as far as I can see.

Is it that my demo. app. is too trivial to see the benefits? If I have
to wrap all use of my object in if( something !=null) then I've
gained nothing.

Thanks

class cNew
{
public:
    cNew(int i, LPCTSTR pc)
    {
        iNew = i;
        strcpy(cText , pc);
    }
    char* GetCNew(){return cNew;}

This should not have compiled. You can't return a type. You must return a value.

One therefore suspects that you're not posting actual code.

    int iNew;
    char cText[256];
};

CList<auto_ptr<sNew> , auto_ptr<sNew>& > m_Arr;

void CSTLTestDlg::OnBnClickedOk()
{

    auto_ptr<sNew> aNew ( new cNew(1, CString("Hello World")));

    AfxMessageBox( aNew->GetCNew() , MB_OKCANCEL );

    m_Arr.AddTail( aNew );
    //ownership of cNew passes to an auto_ptr in my List

    AfxMessageBox( m_Arr.GetHead()->GetCNew() , MB_YESNO );
    //seems to contain valid data

    char* pcData = aNew->GetCNew();
    //pcData is non null eg random memory

    int iData = aNew->iNew;
    //exception

    m_Arr.RemoveAll();

    OnOK();

}

Hirr hirr! :-)

Well, enough about that joke, but seriously, your 'm_Arr' should be

   std::vector<std::string> m_Arr;

and that's it.

Except for efficiency. std::string is a rather inefficient mutable string
class. But mostly serves well enough, and in above example would do fine.

Then the OnBnClickedOk code would be

   void CSTLTestDlg::OnBnClickedOK()
   {
       std::string const aNew = "Hello World";
       AfxMessageBox( aNew.c_str(), MB_OK );

       m_Arr.push_back( aNew );
       AfxMessageBox( m_arr[0].c_str(), MB_OK );

       char const* pcData = aNew.data();
       m_arr.clear();
   }

Cheers, & hth.,

- Alf