Re: Design question related to std::auto_ptr

From:

Vladimir Jovic <vladaspams@gmail.com>

Newsgroups:

comp.lang.c++

Date:

Tue, 24 Mar 2009 08:36:41 +0100

Message-ID:

<gqa2i9$tla$1@news01.versatel.de>

James Kanze wrote:

On Mar 23, 11:10 am, Vladimir Jovic <vladasp...@gmail.com> wrote:

Alf P. Steinbach wrote:

[...]

Some folks have argued that the same technique must in
practice have to work nicely also for std::auto_ptr, at
least if also Foo::~Foo() is defined in the implementation
file, because how could any compiler manage to screw up
things then? But according to the standard's general rules
it's just UB to do it with std::auto_ptr. And one risks the
compiler warning about that UB, as above.

On my compiler (g++ 4.1.2), modifying the example to this:

//start-snip
#include<memory>

class Details;

class MyConcrete {
private:
std::auto_ptr<Details> details_;

};

int main() {
MyConcrete obj;
return 0;
}

class Details
{};

// end-snip

everything compiles fine (without warning). Am I causing UB in
any way?

Formally, yes. Put the definition of Details in another
translation unit, and g++ complains. It probably doesn't here
due to an artifact of the way it instantiates templates---I
think it parses the complete file before instantiating any
templates. And having seen the complete file, it knows the
structure of Details.

Sorry to be PITA, but I really don't understand this. I changed the
example to this:

// file fr.cpp
#include<memory>
#include "hy1.hpp"

class Details;

class MyConcrete {
private:
std::auto_ptr<Details> details_;
};

int main() {
   MyConcrete obj;
   return 0;
}

// file hy1.hpp
class Details
{
     public:
         Details(int a );
         ~Details();
         void inc();
         int *b;
};

// file hy1.cpp
#include "hy1.hpp"
Details::Details(int a ) :
     b(new int(a))
{
}
Details::~Details()
{
     delete(b);
}
void Details::inc()
{
     *b += 3;
}

As you can see, Details is in another translation unit, and the compiler
doesn't complain.
What do I have to do to make it complains?

PS Ignore missing guards and style