How to create std::thread with function pointer with a reference? - c++

Question

class taskq {
public:
int trigger(taskq &tq);
mutex mtx;
};
void func_wrapper(taskq &tq) {
cout<<endl;
}
int taskq::trigger(taskq &tq) {
thread thread(func_wrapper, tq);
return 0;
}
I tried to compile the above simple code but I am keep getting errors because of line thread thread(func_wrapper, tq);.
What's wrong with this code?
How could I fix it?
The error message is as below:
In file included from /usr/include/c++/4.9/thread:39:0,
from taskq.C:2:
/usr/include/c++/4.9/functional: In instantiation of ‘struct std::_Bind_simple<void (*(taskq))(taskq&)>’:
/usr/include/c++/4.9/thread:140:47: required from ‘std::thread::thread(_Callable&&, _Args&& ...) [with _Callable = void (&)(taskq&); _Args = {taskq&}]’
taskq.C:20:33: required from here
/usr/include/c++/4.9/functional:1665:61: error: no type named ‘type’ in ‘class std::result_of<void (*(taskq))(taskq&)>’
typedef typename result_of<_Callable(_Args...)>::type result_type;
^
/usr/include/c++/4.9/functional:1695:9: error: no type named ‘type’ in ‘class std::result_of<void (*(taskq))(taskq&)>’
_M_invoke(_Index_tuple<_Indices...>)
^
In file included from /usr/include/c++/4.9/functional:55:0,
from /usr/include/c++/4.9/thread:39,
from taskq.C:2:
/usr/include/c++/4.9/tuple: In instantiation of ‘constexpr std::_Head_base<_Idx, _Head, false>::_Head_base(_UHead&&) [with _UHead = taskq; <template-parameter-2-2> = void; long unsigned int _Idx = 1ul; _Head = taskq]’:
/usr/include/c++/4.9/tuple:271:42: required from ‘constexpr std::_Tuple_impl<_Idx, _Head, _Tail ...>::_Tuple_impl(std::_Tuple_impl<_Idx, _Head, _Tail ...>&&) [with long unsigned int _Idx = 1ul; _Head = taskq; _Tail = {}]’
/usr/include/c++/4.9/type_traits:900:43: required by substitution of ‘template<class _Tp, class _Arg, class> static std::true_type std::__do_is_direct_constructible_impl::__test(int) [with _Tp = std::_Tuple_impl<1ul, taskq>; _Arg = std::_Tuple_impl<1ul, taskq>&&; <template-parameter-1-3> = <missing>]’
/usr/include/c++/4.9/type_traits:912:43: required from ‘struct std::__is_direct_constructible_impl<std::_Tuple_impl<1ul, taskq>, std::_Tuple_impl<1ul, taskq>&&>’
/usr/include/c++/4.9/type_traits:134:12: required from ‘struct std::__and_<std::is_destructible<std::_Tuple_impl<1ul, taskq> >, std::__is_direct_constructible_impl<std::_Tuple_impl<1ul, taskq>, std::_Tuple_impl<1ul, taskq>&&> >’
/usr/include/c++/4.9/type_traits:916:12: required from ‘struct std::__is_direct_constructible_new_safe<std::_Tuple_impl<1ul, taskq>, std::_Tuple_impl<1ul, taskq>&&>’
/usr/include/c++/4.9/type_traits:994:12: [ skipping 7 instantiation contexts, use -ftemplate-backtrace-limit=0 to disable ]
/usr/include/c++/4.9/type_traits:1175:12: required from ‘struct std::is_nothrow_move_constructible<std::_Tuple_impl<1ul, taskq> >’
/usr/include/c++/4.9/type_traits:134:12: required from ‘struct std::__and_<std::is_nothrow_move_constructible<void (*)(taskq&)>, std::is_nothrow_move_constructible<std::_Tuple_impl<1ul, taskq> > >’
/usr/include/c++/4.9/tuple:267:7: required from ‘constexpr std::_Tuple_impl<_Idx, _Head, _Tail ...>::_Tuple_impl(std::_Tuple_impl<_Idx, _Head, _Tail ...>&&) [with long unsigned int _Idx = 0ul; _Head = void (*)(taskq&); _Tail = {taskq}]’
/usr/include/c++/4.9/functional:1727:41: required from ‘typename std::_Bind_simple_helper<_Func, _BoundArgs>::__type std::__bind_simple(_Callable&&, _Args&& ...) [with _Callable = void (&)(taskq&); _Args = {taskq&}; typename std::_Bind_simple_helper<_Func, _BoundArgs>::__type = std::_Bind_simple<void (*(taskq))(taskq&)>]’
/usr/include/c++/4.9/thread:140:47: required from ‘std::thread::thread(_Callable&&, _Args&& ...) [with _Callable = void (&)(taskq&); _Args = {taskq&}]’
taskq.C:20:33: required from here
/usr/include/c++/4.9/tuple:140:42: error: use of deleted function ‘taskq::taskq(taskq&&)’
: _M_head_impl(std::forward<_UHead>(__h)) { }
^
taskq.C:9:7: note: ‘taskq::taskq(taskq&&)’ is implicitly deleted because the default definition would be ill-formed:
class taskq {
^
taskq.C:9:7: error: use of deleted function ‘std::mutex::mutex(const std::mutex&)’
In file included from taskq.C:3:0:
/usr/include/c++/4.9/mutex:129:5: note: declared here
mutex(const mutex&) = delete;
^
In file included from /usr/include/c++/4.9/functional:55:0,
from /usr/include/c++/4.9/thread:39,
from taskq.C:2:
/usr/include/c++/4.9/tuple: In instantiation of ‘constexpr std::_Head_base<_Idx, _Head, false>::_Head_base(const _Head&) [with long unsigned int _Idx = 1ul; _Head = taskq]’:
/usr/include/c++/4.9/tuple:255:44: recursively required from ‘constexpr std::_Tuple_impl<_Idx, _Head, _Tail ...>::_Tuple_impl(const _Head&, const _Tail& ...) [with long unsigned int _Idx = 1ul; _Head = taskq; _Tail = {}]’
/usr/include/c++/4.9/tuple:255:44: required from ‘constexpr std::_Tuple_impl<_Idx, _Head, _Tail ...>::_Tuple_impl(const _Head&, const _Tail& ...) [with long unsigned int _Idx = 0ul; _Head = void (*)(taskq&); _Tail = {taskq}]’
/usr/include/c++/4.9/tuple:531:30: required from ‘constexpr std::tuple<_T1, _T2>::tuple(const _T1&, const _T2&) [with _T1 = void (*)(taskq&); _T2 = taskq]’
/usr/include/c++/4.9/functional:1678:74: required from ‘std::_Bind_simple<_Callable(_Args ...)>::_Bind_simple(_Callable&&, _Args2&& ...) [with _Args2 = {taskq&}; <template-parameter-2-2> = void; _Callable = void (*)(taskq&); _Args = {taskq}]’
/usr/include/c++/4.9/functional:1727:41: required from ‘typename std::_Bind_simple_helper<_Func, _BoundArgs>::__type std::__bind_simple(_Callable&&, _Args&& ...) [with _Callable = void (&)(taskq&); _Args = {taskq&}; typename std::_Bind_simple_helper<_Func, _BoundArgs>::__type = std::_Bind_simple<void (*(taskq))(taskq&)>]’
/usr/include/c++/4.9/thread:140:47: required from ‘std::thread::thread(_Callable&&, _Args&& ...) [with _Callable = void (&)(taskq&); _Args = {taskq&}]’
taskq.C:20:33: required from here
/usr/include/c++/4.9/tuple:134:25: error: use of deleted function ‘taskq::taskq(const taskq&)’
: _M_head_impl(__h) { }
^
taskq.C:9:7: note: ‘taskq::taskq(const taskq&)’ is implicitly deleted because the default definition would be ill-formed:
class taskq {
^
taskq.C:9:7: error: use of deleted function ‘std::mutex::mutex(const std::mutex&)’
In file included from taskq.C:3:0:
/usr/include/c++/4.9/mutex:129:5: note: declared here
mutex(const mutex&) = delete;
^
make: *** [taskq.o] Error 1

Answer 1

thread thread(func_wrapper, std::ref(tq));
Note that you need to make sure the reference is still valid while you are executing func_wrapper (since I'm guessing such function uses tq).

Answer 2

std::thread thread([&tq]() {
func_wrapper(tq);
});