Re: vector of pimpl's / weird behavior

From:
 James Kanze <james.kanze@gmail.com>
Newsgroups:
comp.lang.c++
Date:
Mon, 01 Oct 2007 08:47:01 -0000
Message-ID:
<1191228421.645632.206830@k79g2000hse.googlegroups.com>
On Sep 28, 9:49 pm, er <erwann.rog...@gmail.com> wrote:

the code below generates the following behavior. cld someone please
help understand it?
1) clean project + build project generates build errors (see bottom)
2) build a second time, errors disappears, run: ok
3) uncomment [1] and comment [2], no problem
4) uncomment [3] and [4], also no problem

#ifndef A_IMPL_H_
#define A_IMPL_H_
#include <memory>
class A_impl{
        public:
                A_impl();
                virtual ~A_impl(){};
                virtual std::auto_ptr<A_impl> clone()const=0;
        private:
                A_impl& operator=(const A_impl&);};
#endif /*A_IMPL_H_*/

#ifndef A_H_
#define A_H_
#include <memory>
//#include "A_impl.h" // [1]
class A_impl;//[2]
class A{
        public:
                A(const A_impl& impl_);
                A(const A& o);
                A& operator=(const A& rhs);
        private:
                A();
                std::auto_ptr<A_impl> pimpl;};
#endif /*A_H_*/


The line with std::auto_ptr<> is undefined behavior (with the
code as written). According to the standard, you can only
instantiate a template in the standard library over a completely
defined type.

#ifndef AS_H_
#define AS_H_
#include <vector>
#include "A.h"
class As{
        public:
                As(
                        const std::vector<A>& sub_collec_
                );
        private:
                std::vector<A> sub_collec;//[3]};
#endif /*AS_H_*/
#include "A.h"
#include "A_impl.h"
A::A(const A_impl& impl_):pimpl(impl_.clone()){};
A::A(const A& o):pimpl((o.pimpl)->clone()){};
A& A::operator=(const A& rhs){
        if(&rhs!=this){
                pimpl=rhs.pimpl->clone();
        };
        return *this;};

#include "As.h"
As::As(
        const std::vector<A>& sub_collec_
): sub_collec(sub_collec_){};//[4]

instantiated from 'void std::_Destroy(_Tp*) [with _Tp = A]'
testing_vector_pimpl A.h line 6 1191008360119 128231
/usr/lib/gcc/x86_64-linux-gnu/4.1.2/../../../../include/c++/4.1.2/bits/
stl_construct.h instantiated from 'void
std::__destroy_aux(_ForwardIterator, _ForwardIterator, __false_type)
[with _ForwardIterator = A*]' testing_vector_pimpl line 122
1191008360120 128232
/usr/lib/gcc/x86_64-linux-gnu/4.1.2/../../../../include/c++/4.1.2/bits/
stl_construct.h instantiated from 'void
std::_Destroy(_ForwardIterator, _ForwardIterator, std::allocator<_T2>)
[with _ForwardIterator = A*, _Tp = A]' testing_vector_pimpl =

   line 182

1191008360120 128234
/usr/lib/gcc/x86_64-linux-gnu/4.1.2/../../../../include/c++/4.1.2/bits/
stl_construct.h instantiated from 'void
std::_Destroy(_ForwardIterator, _ForwardIterator) [with
_ForwardIterator = A*]' testing_vector_pimpl line 155 =

 1191008360120

128233
/usr/lib/gcc/x86_64-linux-gnu/4.1.2/../../../../include/c++/4.1.2/bits/
stl_vector.h instantiated from 'std::vector<_Tp, _Alloc>::~vector()
[with _Tp = A, _Alloc = std::allocator<A>]' testing_vec=

tor_pimpl line

272 1191008360120 128235


I don't know exactly why the rebuild eliminates the error with
g++. With some other compilers, however, template
instantiations are stored in a repository; if the template is
successfully instantiated in another translation unit, then the
rebuild will work.

But it doesn't really matter. You have undefined behavior, and
you should fix it.

--
James Kanze (GABI Software) email:james.kanze@gmail.com
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