[Bug c++/38265] New: STL treats explicit constructors as converting constructors

[Bug c++/38265] New: STL treats explicit constructors as converting constructors

[konto dot dydaktyczne at gmail dot com]

When some STL containers are created, explicit constructors for contained
objects are treated as converting constructors. The keyword "explicit" is
ignored, and no error message is issued; see the code.

#include <vector>
#include <deque>

class X {
        public:
                explicit X(int) {}
};

int main() {
        int a[1] = {};
        std::vector<X> v(a, a + 1);
        std::deque<X> d(a, a + 1);
}


-- 
           Summary: STL treats explicit constructors as converting
                    constructors
           Product: gcc
           Version: 4.1.2
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
        AssignedTo: unassigned at gcc dot gnu dot org
        ReportedBy: konto dot dydaktyczne at gmail dot com
 GCC build triplet: i686-pc-linux-gnu
  GCC host triplet: i686-pc-linux-gnu
GCC target triplet: i686-pc-linux-gnu


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=38265

[Bug c++/38265] STL treats explicit constructors as converting constructors

[cfairles at gcc dot gnu dot org]

------- Comment #1 from cfairles at gcc dot gnu dot org  2008-11-25 19:34 -------
GCC 4.4.0 also accepts this code as does Comeau 4.3.10.1.


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http://gcc.gnu.org/bugzilla/show_bug.cgi?id=38265

[Bug libstdc++/38265] STL treats explicit constructors as converting constructors

[paolo dot carlini at oracle dot com]

------- Comment #2 from paolo dot carlini at oracle dot com  2008-11-25 19:48 -------
Unless I'm badly mistaken, this behaviour, dating back to the original HP/SGI
STL and as such very difficult to change now, is a small extension due to the
use in the containers of _Construct instead of calling allocator::construct
directly.  All in all, at the moment I don't think we have good reasons to
change it.


-- 

paolo dot carlini at oracle dot com changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
          Component|c++                         |libstdc++


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=38265

[Bug libstdc++/38265] STL treats explicit constructors as converting constructors

[konto dot dydaktyczne at gmail dot com]

------- Comment #3 from konto dot dydaktyczne at gmail dot com  2008-11-28 10:07 -------
Let me add an std::list and an std::set to my code; see below. Both additions
produce errors. So,

           [sequence containers]
      std::vector -> no error signaled, "explicit" ignored
      std::list   ->    error signaled, because of "explicit"
      std::deque  -> no error signaled, "explicit" ignored  

           [associative containers]
      std::set    ->    error signaled, because of "explicit"



#include <vector>
#include <deque>
#include <list>
#include <set>

class X {
        public:
                explicit X(int) {}
                int operator<(const X&) const {
                        return 0;
                }
};

int main() {
        int a[1] = {};

        std::vector<X> v(a, a + 1);
        std::deque<X> d(a, a + 1);

        std::list<X> l(a, a + 1);
        std::set<X> s(a, a + 1);
}


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http://gcc.gnu.org/bugzilla/show_bug.cgi?id=38265

[Bug libstdc++/38265] STL treats explicit constructors as converting constructors

[paolo dot carlini at oracle dot com]

------- Comment #4 from paolo dot carlini at oracle dot com  2008-11-28 10:50 -------
Yes, this is obvious, just grep for _Construct.


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http://gcc.gnu.org/bugzilla/show_bug.cgi?id=38265

[Bug libstdc++/38265] STL treats explicit constructors as converting constructors

[paolo dot carlini at oracle dot com]

------- Comment #5 from paolo dot carlini at oracle dot com  2008-11-28 11:03 -------
But I see now that al long time ago list & co also used _Construct, this is
indeed an inconsistency... Let's see what we can do here...


-- 

paolo dot carlini at oracle dot com changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
         AssignedTo|unassigned at gcc dot gnu   |paolo dot carlini at oracle
                   |dot org                     |dot com
             Status|UNCONFIRMED                 |ASSIGNED
     Ever Confirmed|0                           |1
   Last reconfirmed|0000-00-00 00:00:00         |2008-11-28 11:03:38
               date|                            |


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=38265

[Bug libstdc++/38265] STL treats explicit constructors as converting constructors

[paolo dot carlini at oracle dot com]

------- Comment #6 from paolo dot carlini at oracle dot com  2008-11-28 11:17 -------
Hummm, the issue seems different than I remembered it. Will see..


-- 


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=38265

[Bug libstdc++/38265] STL treats explicit constructors as converting constructors

[konto dot dydaktyczne at gmail dot com]

------- Comment #7 from konto dot dydaktyczne at gmail dot com  2008-11-28 11:23 -------
(In reply to comment #1)
> GCC 4.4.0 also accepts this code as does Comeau 4.3.10.1.

Microsoft (R) 32-bit C/C++ Optimizing Compiler Version 15.00.21022.08 for 80x86
does not accept this code.


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http://gcc.gnu.org/bugzilla/show_bug.cgi?id=38265

[Bug libstdc++/38265] STL treats explicit constructors as converting constructors

[paolo dot carlini at oracle dot com]

------- Comment #8 from paolo dot carlini at oracle dot com  2008-11-28 11:30 -------
The issue is different because if I remove all uses of _Construct and implement
the various uninitialized_* per the letter of the Standard, nothing changes...


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http://gcc.gnu.org/bugzilla/show_bug.cgi?id=38265

[Bug libstdc++/38265] STL treats explicit constructors as converting constructors

[paolo dot carlini at oracle dot com]

------- Comment #9 from paolo dot carlini at oracle dot com  2008-11-28 11:40 -------
... and I'm coming to the conclusion that this is not a bug in our library.
Consider the std::vector case: I cannot see anything wrong with using
std::uninitialized_copy in the implementation of the constructor. In turn, the
former does, per the Standard:

   ::new(static_cast<void*>(&*__cur)) typename
      iterator_traits<_ForwardIterator>::value_type(*__first);

thus, the explicit constructor can be used.


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http://gcc.gnu.org/bugzilla/show_bug.cgi?id=38265

[Bug libstdc++/38265] STL treats explicit constructors as converting constructors

[konto dot dydaktyczne at gmail dot com]

------- Comment #10 from konto dot dydaktyczne at gmail dot com  2008-11-29 16:36 -------
(In reply to comment #9)

The Standard does not require that std::uninitialized_copy be invoked when
constructing an std::vector or std::deque.

Moreover, an explicit constructor constructs objects only where the
direct-initialization syntax or where casts are explicitly used. None of these
cases occurs in the code above.


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http://gcc.gnu.org/bugzilla/show_bug.cgi?id=38265

[Bug libstdc++/38265] STL treats explicit constructors as converting constructors

[konto dot dydaktyczne at gmail dot com]

------- Comment #11 from konto dot dydaktyczne at gmail dot com  2008-11-29 16:40 -------
(In reply to comment #10)

> an explicit constructor constructs objects only where the
> direct-initialization syntax or where casts are explicitly used. None of these
> cases occurs in the code above.

I mean the code in the bug description.


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http://gcc.gnu.org/bugzilla/show_bug.cgi?id=38265

[Bug libstdc++/38265] STL treats explicit constructors as converting constructors

[paolo dot carlini at oracle dot com]

------- Comment #12 from paolo dot carlini at oracle dot com  2008-11-29 17:05 -------
Subject: Re:  STL treats explicit constructors as converting constructors


> The Standard does not require that std::uninitialized_copy be  
> invoked when
> constructing an std::vector or...

Sure, but certainly doesn't forbid it, and I maintain is the natural  
choice in this case.




>


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http://gcc.gnu.org/bugzilla/show_bug.cgi?id=38265