[GCC-XML] wrong artificial op=()

[Axel Naumann]

Hi Roman,

> This code compiles fine with gccxml, but const_container class
> description doesn't contain artificial default constructor. At the
> same time, it contains default copy constructor.
>
> I am not C++ expert, but I think the class should have default
> constructor.

I believe you're not testing what you think you are. Check this code
out, e.g. with GCC 4.3.1, slightly modified from your code:

$ cat gccxmlml.cxx
struct const_item{ const int values[10]; };
void test_const_item() { const_item x;}
struct const_container{ const const_item items[10]; };
void test_const_container() { const_container x;}

$ g++ -c gccxmlml.cxx
gccxmlml.cxx: In function 'void test_const_item()':
gccxmlml.cxx:2: error: structure 'x' with uninitialized const members
gccxmlml.cxx: In function 'void test_const_container()':
gccxmlml.cxx:4: error: structure 'x' with uninitialized const members

> I think the class should have default constructor.

GCC disagrees, and for a good reason. The standard states that an
implicitly implemented default constructor will make the program
ill-formed if a user provided default constructor with empty initializer
list and empty body would make the program ill-formed. As you probably
know a const member _has_ to be initialized during construction, it must
be part of the constructor's initializer list:

$ cat gccxmlml1.cxx
struct Go {const int i; Go(): i(42) {} };
struct NoGo {const int i; NoGo() {} };

$ g++ -c gccxmlml1.cxx
gccxmlml1.cxx: In constructor 'NoGo::NoGo()':
gccxmlml1.cxx:2: error: uninitialized member 'NoGo::i' with 'const' type
'const int'

For obvious reasons (but check the standard 12.8.7 if you want to see
the proof) this restriction does not exist for the copy constructor.

Summary: I believe the standard forbids the default constructor that you
want to see, and that GCCXML with Brad's patch generates the proper
constructors and assignment operators.

Cheers, Axel.


On 2008-10-04 21:01, Roman Yakovenko wrote:
> On Wed, Oct 1, 2008 at 6:35 PM, Axel Naumann <Axel.Naumann at cern.ch> wrote:
>> Hi Brad,
>>
>> On 2008-10-01 17:15, Brad King wrote:
>>>> the XML file contains an artificial operator=() for the classes S, T, V, and W.
>>>> This assignment operator should not exist according to the standard
>>>> [class.copy], 12.8.12. Both GCC 4.3 and MSVC 9.0 will refuse to compile code
>>>> that requires the implicit definition of these operators:
>>> [snip]
>>>> Could these operators be suppressed, please?
>>> I've come up with a hack that pretends these artifical methods are
>>> called, suppresses any resulting error messages, and then discards the
>>> methods if errors occurred.  It seems to work for the examples you provided.
> 
> Brad, I think there is a problem with these changes.
> Consider next use case:
> 
>     struct const_item{ const int values[10]; };
>     void test_const_item( const_item x = const_item() );
>     struct const_container{ const const_item items[10]; };
>     void test_const_container( const_container x = const_container() );
> 
> This code compiles fine with gccxml, but const_container class
> description doesn't contain artificial default constructor. At the
> same time, it contains default copy constructor.
> 
> I am not C++ expert, but I think the class should have default constructor.
>