[Bug c++/44499] New: No default constructor available

[Bug c++/44499] New: No default constructor available

[piotr dot wyderski at gmail dot com]

The following code:

    class C {

    };

    class D : public C {

    };

    const D g_d;


fails to compile on trunk (rev. 160489) with the following message:

$ g++ test.cpp
test.cpp:11:9: error: uninitialized const 'g_d' [-fpermissive]
test.cpp:6:11: note: 'const class D' has no user-provided default constructor


-- 
           Summary: No default constructor available
           Product: gcc
           Version: 4.6.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
        AssignedTo: unassigned at gcc dot gnu dot org
        ReportedBy: piotr dot wyderski at gmail dot com
 GCC build triplet: i686-pc-cygwin
  GCC host triplet: i686-pc-cygwin
GCC target triplet: i686-pc-cygwin


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=44499

[Bug c++/44499] No default constructor available

[redi at gcc dot gnu dot org]

------- Comment #1 from redi at gcc dot gnu dot org  2010-06-11 09:26 -------
gcc is correct, accepting the code previously was a bug that was fixed recently

You need to provide an initializer for g_d


-- 

redi at gcc dot gnu dot org changed:

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


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=44499

[Bug c++/44499] No default constructor available

[redi at gcc dot gnu dot org]

------- Comment #2 from redi at gcc dot gnu dot org  2010-06-11 10:27 -------
A question: apart from quoting chapter and verse from the standard (8.5
[dcl.init], para 9 in C++03, para 6 in C++0x,) how could the diagnostic have
been any clearer?

It indicates you can use -fpermissive to relax the warning, and it includes a
note telling you the type has no user-provided default constructor, which is
true.  Why would you assume this is a bug, when a developer has gone to the
trouble of writing the note?


-- 


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=44499

[Bug c++/44499] No default constructor available

[manu at gcc dot gnu dot org]

------- Comment #3 from manu at gcc dot gnu dot org  2010-06-11 10:53 -------
(In reply to comment #1)
> gcc is correct, accepting the code previously was a bug that was fixed recently
> 
> You need to provide an initializer for g_d

This sort of changes should be documented in the changes.html page or in
porting_to.html


-- 

manu at gcc dot gnu dot org changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |manu at gcc dot gnu dot org


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=44499

[Bug c++/44499] No default constructor available

[piotr dot wyderski at gmail dot com]

------- Comment #4 from piotr dot wyderski at gmail dot com  2010-06-11 11:01 -------
(In reply to comment #2)
> A question: apart from quoting chapter and verse from the standard (8.5
> [dcl.init], para 9 in C++03, para 6 in C++0x,) how could the diagnostic have
> been any clearer?
> 
> It indicates you can use -fpermissive to relax the warning, and it includes a
> note telling you the type has no user-provided default constructor, which is
> true.  Why would you assume this is a bug, when a developer has gone to the
> trouble of writing the note?

All the compilers I am aware of accept the aforementioned construction,
so I blindly assumed that 4.6 is wrong in issuing a warning. The note's
content was considered irrelevant, since no error was expected. But if
the behaviour is OK, then it is OK no matter what a surprise it turns out to
be.


-- 


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=44499

[Bug c++/44499] No default constructor available

[manu at gcc dot gnu dot org]

-- 

manu at gcc dot gnu dot org changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
         AssignedTo|unassigned at gcc dot gnu   |manu at gcc dot gnu dot org
                   |dot org                     |
             Status|UNCONFIRMED                 |ASSIGNED
     Ever Confirmed|0                           |1
   Last reconfirmed|0000-00-00 00:00:00         |2010-06-11 11:11:57
               date|                            |


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=44499

[Bug c++/44499] No default constructor available

[manu at gcc dot gnu dot org]

------- Comment #5 from manu at gcc dot gnu dot org  2010-06-11 11:11 -------
Comeau C/C++ 4.3.10.1 (Oct  6 2008 11:28:09) for ONLINE_EVALUATION_BETA2
Copyright 1988-2008 Comeau Computing.  All rights reserved.
MODE:strict errors C++ C++0x_extensions

"ComeauTest.c", line 9: error: const variable "g_d" requires an initializer --
class
          "D" has no explicitly declared default constructor
      const D g_d;
                 ^

"ComeauTest.c", line 9: warning: variable "g_d" was declared but never
referenced
      const D g_d;
              ^

1 error detected in the compilation of "ComeauTest.c".


I think the error message of comeau is better (but our note is better). I will
write a patch.


-- 

manu at gcc dot gnu dot org changed:

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


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=44499

[Bug c++/44499] No default constructor available

[manu at gcc dot gnu dot org]

------- Comment #6 from manu at gcc dot gnu dot org  2010-06-11 11:13 -------
LLVM is still using GCC in their demo, not Clang. So I cannot test their
output.


-- 


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=44499

[Bug c++/44499] No default constructor available

[manu at gcc dot gnu dot org]

------- Comment #7 from manu at gcc dot gnu dot org  2010-07-02 08:09 -------
Could someone test what clang says here? Their diagnostics are generally better
than g++.


-- 


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=44499

[Bug c++/44499] No default constructor available

[pluto at agmk dot net]

------- Comment #8 from pluto at agmk dot net  2010-07-02 08:12 -------
(In reply to comment #7)
> Could someone test what clang says here? Their diagnostics are generally better
> than g++.
> 

$ clang++ pr44499.cpp -c
pr44499.cpp:5:9: error: default initialization of an object of const type 'D
const' requires a user-provided default constructor
const D g_d;
        ^
1 error generated.


-- 


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=44499

[Bug c++/44499] No default constructor available

[manu at gcc dot gnu dot org]

------- Comment #9 from manu at gcc dot gnu dot org  2010-07-02 09:15 -------
Thanks Pawel,

which diagnostic do you prefer? 

I would favor clang's but I would still keep the note that points to the class
definition.


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http://gcc.gnu.org/bugzilla/show_bug.cgi?id=44499