Re: [Boost-users] thread/call_once compile error on trunk on my code

Subject: Re: [Boost-users] thread/call_once compile error on trunk on my code
From: Vicente J. Botet Escriba (vicente.botet_at_[hidden])
Date: 2013-04-21 05:18:16

Le 20/04/13 17:03, Olaf Peter a écrit :
> Hello,
>
> g++ (GCC) 4.6.3 20120306 (Red Hat 4.6.3-2)
>
> and maybe the last changes
> ...
> breaks my code now which is basically:
>
> namespace {
>
> struct id_string
> {
> static boost::once_flag flag;
> typedef std::map<int, std::string> map_type;
> ...
>
> static void do_init(token_id_string& self)
> {
> insert(self.m_map)(x, "y")(...)
> ;
> }
>
> std::string const& operator()(id::type id) const
> {
> boost::call_once(flag, &id_string::do_init, *this);
>
> map_iterator s = m_map.find(id);
> ...
> }
> };
>
Hi,

the error
/usr/local/include/boost/thread/detail/invoke.hpp:406:65: error: invalid 
initialization of non-const reference of type '{anonymous}::id_string&' 
from an rvalue of type '{anonymous}::id_string'

says you that the parameter (const) can not be used where a non-const is 
expected.

Shouldn't the operator be declared non-const

std::string const& operator()(id::type id) ;

In addition if you want to pass a reference you would need to use 
boost::ref().

The following compile well without the patch

struct id_string
{
static boost::once_flag flag;
static void do_init(id_string & )
{}
void operator()()
{
boost::call_once(flag, &id_string::do_init, boost::ref(*this));
}
};

Note that the following compiles

struct id_string
{
static boost::once_flag flag;
static void do_init(id_string & )
{}
void operator()()
{
std::bind(&id_string::do_init, std::ref(*this))();
}
void operator()(int) const
{
std::bind(&id_string::do_init, *this)();
}
};

I would expect that the preceding fails as *this is a 'id_string const&' 
and do_init expects an 'id_string &'.
This would explain why with the patch it worked as the patch was calling 
to std::bind.

But if make use of the class

{
id_string id;
//id();
id(1);
}

I get the following error

gcc.compile.c++ 
/tmp/bin.v2/boost/bin.v2/libs/thread/test/call_once_p.test/gcc-4.6.1.0x/debug/threading-multi/sync/mutual_exclusion/once/call_once/call_once_pass.o
In file included from /usr/include/c++/4.6/functional:56:0,
from ../../../boost/thread/detail/invoke.hpp:34,
from ../../../boost/thread/pthread/once_atomic.hpp:17,
from ../../../boost/thread/once.hpp:20,
from sync/mutual_exclusion/once/call_once/call_once_pass.cpp:26:
/usr/include/c++/4.6/tuple: In constructor ‘constexpr 
std::_Head_base<_Idx, _Head, false>::_Head_base(const _Head&) [with 
unsigned int _Idx = 0u, _Head = id_string]’:
/usr/include/c++/4.6/tuple:162:44: instantiated from ‘constexpr 
std::_Tuple_impl<_Idx, _Head, _Tail ...>::_Tuple_impl(const _Head&, 
const _Tail& ...) [with unsigned int _Idx = 0u, _Head = id_string, _Tail 
= {}]’
/usr/include/c++/4.6/tuple:423:24: instantiated from ‘constexpr 
std::tuple<_T1>::tuple(const _T1&) [with _T1 = id_string]’
/usr/include/c++/4.6/functional:1186:70: instantiated from 
‘std::_Bind<_Functor(_Bound_args ...)>::_Bind(_Functor&&, _Args&& ...) 
[with _Args = {const id_string&}, _Functor = void (*)(id_string&), 
_Bound_args = {id_string}]’
/usr/include/c++/4.6/functional:1450:41: instantiated from ‘typename 
std::_Bind_helper<_Functor, _ArgTypes>::type std::bind(_Functor&&, 
_ArgTypes&& ...) [with _Functor = void (*)(id_string&), _ArgTypes = 
{const id_string&}, typename std::_Bind_helper<_Functor, 
_ArgTypes>::type = std::_Bind<void (*(id_string))(id_string&)>]’
sync/mutual_exclusion/once/call_once/call_once_pass.cpp:206:43: 
instantiated from here
/usr/include/c++/4.6/tuple:97:25: erreur: use of deleted function 
‘id_string::id_string(const id_string&)’
sync/mutual_exclusion/once/call_once/call_once_pass.cpp:194:8: erreur: 
‘id_string::id_string(const id_string&)’ is implicitly deleted because 
the default definition would be ill-formed:
sync/mutual_exclusion/once/call_once/call_once_pass.cpp:194:8: erreur: 
use of deleted function ‘boost::once_flag::once_flag(const 
boost::once_flag&)’
../../../boost/thread/pthread/once_atomic.hpp:59:5: erreur: declared here

Could you tell me if works for you after rolling back the patch and 
adding the suggested changes on your code?

Best,
Vicente