Re: Are clang++ and libc++ compatible?

On Tue, Nov 12, 2013 at 1:21 PM, John Baldwin <jhb_at_freebsd.org> wrote:
> On Tuesday, November 12, 2013 1:11:04 pm Eitan Adler wrote:
>> On Tue, Nov 12, 2013 at 11:54 AM, Steve Kargl
>> <sgk_at_troutmask.apl.washington.edu> wrote:
>> > On Tue, Nov 12, 2013 at 04:38:17PM +0000, David Chisnall wrote:
>> >> On 12 Nov 2013, at 16:32, Steve Kargl <sgk_at_troutmask.apl.washington.edu>
> wrote:
>> >>
>> >>> Trying to build news/pan with clang++ dies with
>> >>>
>> >>> gmake[3]: Entering directory
> `/usr/ports/news/pan/work/pan-0.139/pan/general'
>> >>>  CXX    file-util.o
>> >>> In file included from file-util.cc:38:
>> >>> In file included from ./log.h:26:
>> >>> /usr/include/c++/v1/deque:907:49: error: invalid application of 'sizeof'
> to an
>> >>>      incomplete type 'value_type' (aka 'pan::Log::Entry')
>> >>>    static const difference_type __block_size = sizeof(value_type) < 256
> ? 4...
>> >>>
>> >>> Anyone know how to fix either clang++ or libc++?
>> >>
>> >> The error here does not appear to be in clang or libc++, but in the
>> >> use by the thing that you are compiling.
>> >> This is saying that you have tried to create a
> std::dequeu<pan::Log::Entry>,
>> >> but pan::Log::Entry is a forward declaration and so the template
>> >> instantiation fails.
>> >> The fix is to move the definition of pan::Log::Entry such that it
>> >> is visible at the time of its use.
>> >>
>> >
>> > I don't know C++, but it is at all like C, then the header files
>> > are normally placed at the top of a file before one's code.  In
>> > this case, the code in news/pan/work/pan-0.139/pan/general/log.h
>> > looks like (where I've striped comment to keep it short)
>> >
>> > #ifndef __Log_h__
>> > #define __Log_h__
>> >
>> > #include <ctime>
>> > #include <set>
>> > #include <string>
>> > #include <deque>
>> >
>> > namespace pan
>> > {
>> >   class Log
>> >   {
>> >     public:
>> >       enum Severity {
>> >         PAN_SEVERITY_INFO = 1,
>> >         PAN_SEVERITY_ERROR = 2,
>> >         PAN_SEVERITY_URGENT = (1<<10)
>> >       };
>> >
>> >       struct Entry {
>> >         time_t date;
>> >         Severity severity;
>> >         std::deque<Entry> messages;
>> >         std::string message;
>> >         bool is_child;
>> >         Entry() : is_child(false) { }
>> >       };
>> >
>> >       void add_entry(Entry& e, std::deque<Entry>& list);
>> >
>> >
>> > Are you saying that I need to move '#include <deque>' to
>> > the location above the 'void add_entry(...)' line?
>>
>> The problem here is that the code is trying to make a std::deque of
>> the type Entry before Entry is fully defined.
>> This is nearly identical to the problem in the simplified C code below:
>>
>> struct foo {
>> struct foo bar;
>> }
>
> Except it isn't.  It's declaring the head of a container.  This is more
> like:
>
>         struct foo {
>                 TAILQ_HEAD(, foo) messages;
>         };
>
> The problem is that unlike the queue macros (which are broken out into
> chunks), the compiler has to instantiate all of std::deque<> even though
> it will only use a portion of its definition.  I understand why this is
> a limitation of C++, but it's not quite as brain damaged as your
> example.

Yes, I'm aware of this difference.

-- 
Eitan Adler