Re: auto_ptr<istream> problem

Mon, 23 Jun 2008 13:54:11 -0700 (PDT)
On 23 Jun, 03:51, Rolf Magnus <> wrote:

Your program below doesn't compile here on my compiler (and I wouldn't
expect it to), neither with the #define, nor without.

Thank you for pointing it out. In fact, I hadn't explicitly turned off
the "language extensions" in Visual Studio, and it was optimising away
the copy constructor without so much as a warning for my pointer

For auto_ptr, it implicitly converted to auto_ptr_ref, then
constructed from that, which is fair enough. In fact, my actual
pointer class does have an implicit conversion and pseudo-copy-
constructor similar to auto_ptr, so that wasn't my problem.

I found my problem in the full program, and I show it below in the
function loadResource. Rather than (as I hoped) using
auto_ptr<istream>::auto_ptr( istream * ), the return statement is in
fact calling auto_ptr_ref<istream>::auto_ptr_ref( void * ) then
auto_ptr( auto_ptr_ref<istream> & ). The pointer stored in the
auto_ptr_ref is statically cast to (auto_ptr<istream> *) when it is
really a (ifstream *) at heart. Cue problems.

istream_ptr loadResource( string const & name ) {
    return new fstream( name.c_str(), ios::binary );
    // Above line should be the following
    //return istream_ptr( new fstream( name.c_str(), ios::binary ) );

int main() {
    istream_ptr in = loadResource( "ptr_test.cpp" );
    char ch;
    in->read( &ch, 1 );
    return 0;

// ---------- Begin ptr_test.cpp ----------

#include <algorithm>
#include <istream>
#include <fstream>
#include <memory>
using namespace std;

// Ownership transfering pointer to input
stream //////////////////////

//#define USE_AUTO_PTR

#if defined(USE_AUTO_PTR)
typedef auto_ptr<istream> istream_ptr;

/* Not using copy constructor in example, so I won't bother writing
 * a standards compliant one here. Nor assignment operator. Instead
 * I'll make them private to be sure they're not generated.
template <class T>
struct ptr {
ptr( T * ptr ) : _ptr( ptr ) {}
//ptr( ptr<T> & other ) : _ptr( 0 ) { swap( other ); }
~ptr() { if ( _ptr ) delete _ptr; }
T * operator->() const { return _ptr; }
ptr( ptr<T> const & );
ptr<T> & operator=( ptr<T> const & );
//void swap( ptr<T> & other ) { std::swap( _ptr, other._ptr ); }
T * _ptr;
typedef ptr<istream> istream_ptr;


// Simple test
case ///////////////////////////////////////////////////

int main() {
istream_ptr in = new fstream( "ptr_test.cpp", ios::binary );


istream_ptr in(new fstream( "ptr_test.cpp", ios::binary ));

That avoids creating a temporary, for which a copy constructor would be
needed that takes a const istream_ptr as argument. std::auto_ptr doesn't
have that (and neither does your class, so it shouldn't compile either).

char ch;
in->read( &ch, 1 );
return 0;

// ---------- End ptr_test.cpp ----------

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