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

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

[redi at gcc dot gnu.org]

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

--- Comment #13 from Jonathan Wakely <redi at gcc dot gnu.org> ---
The C++11 standard clarifies that this is intended to work. The range
constructors require the type to be EmplaceConstructible into the container
from the iterator's value_type, which means calling
allocator_traits<A>::construct with the iterator's value type. For
std::allocator that is a call to placement new using direct-initialization
which means explicit constructors are viable.

If you don't want the explicit constructor to be viable then you need to use a
custom allocator which does something different.

(In reply to konto.dydaktyczne from comment #3)
>            [sequence containers]
>       std::vector -> no error signaled, "explicit" ignored
>       std::list   ->    error signaled, because of "explicit"

This now compiles in C++11 mode.

>       std::deque  -> no error signaled, "explicit" ignored  
> 
>            [associative containers]
>       std::set    ->    error signaled, because of "explicit"

This still fails, at a different place, but I think it should compile.

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

[gromer at google dot com]

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

Geoff Romer <gromer at google dot com> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |gromer at google dot com

--- Comment #14 from Geoff Romer <gromer at google dot com> ---
> The C++11 standard clarifies that this is intended to work. The range
> constructors require the type to be EmplaceConstructible into the container
> from the iterator's value_type

But see [sequence.reqmts]/p3: "i and j denote iterators satisfying input
iterator requirements and refer to elements implicitly convertible to
value_type". So this is not "intended to work" per se; a conforming library
could disallow it. However, the notes on LWG 536 say "Some support, not
universal, for respecting the explicit qualifier", so it looks like the
standard intentionally permits this as a conforming extension.

In principle, I think "perfect initialization" is what's called for here: the
range ctor should be explicit if and only if it uses an explicit constructor
for value_type. However, I doubt it's worth the trouble (OTOH, it definitely
will be worth the trouble when/if we get single-argument range constructors).

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

[redi at gcc dot gnu.org]

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

Jonathan Wakely <redi at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|ASSIGNED                    |RESOLVED
         Resolution|---                         |INVALID

--- Comment #15 from Jonathan Wakely <redi at gcc dot gnu.org> ---
(In reply to Geoff Romer from comment #14)
> But see [sequence.reqmts]/p3: "i and j denote iterators satisfying input
> iterator requirements and refer to elements implicitly convertible to
> value_type". So this is not "intended to work" per se; a conforming library
> could disallow it. However, the notes on LWG 536 say "Some support, not
> universal, for respecting the explicit qualifier", so it looks like the
> standard intentionally permits this as a conforming extension.

Which makes the original testcase invalid, so it's a bug in the user's code,
not a bug in libstdc++ to accept it.

> In principle, I think "perfect initialization" is what's called for here:
> the range ctor should be explicit if and only if it uses an explicit
> constructor for value_type.

I don't think that would be a good idea. I don't think it follows that X(int)
being explicit should mean deque<X>(int*, int*) should be explicit. By that
logic shared_ptr(unique_ptr<T>&&) should be explicit, because unique_ptr(T*) is
explicit.

int* is not int, and deque<X> is not X.

[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

[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.


-- 


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

[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

[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

[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 #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..


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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 #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

[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);
}


-- 


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