warning message when calling a class member function inside another as a thread

0
votes

I have a class and it contains multiple methods. My requirement is to call one of the member function inside another as a thread. 

Class ApplicationManager
{
   ....
   ....
   void method();
   void test(std::string arg1, std::string& arg2);
};

void ApplicationManager::method()
{
   std::string arg, arg2;
   ....
   ....
   std::thread(&ApplicationManager::test, this, arg, arg2);
}

When I compile the above, I getting below warning message. Since it is too big, I am unable to get what it tries to say. Copied warning message below.

C:\Program Files (x86)\Microsoft Visual Studio\2017\Professional\VC\Tools\MSVC\14.16.27023\include\type_traits(16707566): warning C4239: nonstandard extension used: 'argument': conversion from '_Ty' to 'std::string &' with [ _Ty=std::basic_string,std::allocator> ] C:\Program Files (x86)\Microsoft Visual Studio\2017\Professional\VC\Tools\MSVC\14.16.27023\include\type_traits(16707566): note: A non-const reference may only be bound to an lvalue C:\Program Files (x86)\Microsoft Visual Studio\2017\Professional\VC\Tools\MSVC\14.16.27023\include\type_traits(16707566): note: see reference to function template instantiation 'void std::_Invoker_pmf_pointer::_Call<_Ty,ApplicationManager,std::basic_string,std::allocator>,std::basic_string,std::allocator>>(_Decayed,_Ty1 &&,std::basic_string,std::allocator> &&,std::basic_string,std::allocator> &&) noexcept(false)' being compiled with [ _Ty=void (__cdecl ApplicationManager::* )(std::string,std::string &), _Decayed=void (__cdecl ApplicationManager::* )(std::string,std::string &), _Ty1=ApplicationManager * ]

C:\Program Files (x86)\Microsoft Visual Studio\2017\Professional\VC\Tools\MSVC\14.16.27023\include\thr/xthread(237): note: see reference to function template instantiation 'void std::invoke,std::allocator>,std::basic_string,std::allocator>>(_Callable &&,ApplicationManager &&,std::basic_string,std::allocator> &&,std::basic_string,std::allocator> &&) noexcept(false)' being compiled with [ _Callable=void (__cdecl ApplicationManager:: )(std::string,std::string &) ] C:\Program Files (x86)\Microsoft Visual Studio\2017\Professional\VC\Tools\MSVC\14.16.27023\include\thr/xthread(246): note: see reference to function template instantiation 'void std::_LaunchPad<_Target>::_Execute<0,1,2,3>(std::tuple,std::allocator>,std::basic_string,std::allocator>> &,std::integer_sequence<_Ty,0,1,2,3>)' being compiled with [ _Target=std::unique_ptr,std::allocator>,std::basic_string,std::allocator>>,std::default_delete,std::allocator>,std::basic_string,std::allocator>>>>, _Ty=size_t ] C:\Program Files (x86)\Microsoft Visual Studio\2017\Professional\VC\Tools\MSVC\14.16.27023\include\thr/xthread(245): note: see reference to function template instantiation 'void std::_LaunchPad<_Target>::_Execute<0,1,2,3>(std::tuple,std::allocator>,std::basic_string,std::allocator>> &,std::integer_sequence<_Ty,0,1,2,3>)' being compiled with [ _Target=std::unique_ptr,std::allocator>,std::basic_string,std::allocator>>,std::default_delete,std::allocator>,std::basic_string,std::allocator>>>>, _Ty=size_t ] C:\Program Files (x86)\Microsoft Visual Studio\2017\Professional\VC\Tools\MSVC\14.16.27023\include\thr/xthread(242): note: while compiling class template member function 'void std::_LaunchPad<_Target>::_Run(std::_LaunchPad<_Target> *) noexcept' with [ _Target=std::unique_ptr,std::allocator>,std::basic_string,std::allocator>>,std::default_delete,std::allocator>,std::basic_string,std::allocator>>>> ] C:\Program Files (x86)\Microsoft Visual Studio\2017\Professional\VC\Tools\MSVC\14.16.27023\include\thr/xthread(230): note: see reference to function template instantiation 'void std::_LaunchPad<_Target>::_Run(std::_LaunchPad<_Target> *) noexcept' being compiled with [ _Target=std::unique_ptr,std::allocator>,std::basic_string,std::allocator>>,std::default_delete,std::allocator>,std::basic_string,std::allocator>>>> ] C:\Program Files (x86)\Microsoft Visual Studio\2017\Professional\VC\Tools\MSVC\14.16.27023\include\thr/xthread(257): note: see reference to class template instantiation 'std::_LaunchPad<_Target>' being compiled with [ _Target=std::unique_ptr,std::allocator>,std::basic_string,std::allocator>>,std::default_delete,std::allocator>,std::basic_string,std::allocator>>>> ] C:\Program Files (x86)\Microsoft Visual Studio\2017\Professional\VC\Tools\MSVC\14.16.27023\include\thread(46): note: see reference to function template instantiation 'void std::_Launch,std::allocator>,std::basic_string,std::allocator>>,std::default_delete<_Ty>>>(_Thrd_t *,_Target &&)' being compiled with [ _Ty=std::tuple,std::allocator>,std::basic_string,std::allocator>>, _Target=std::unique_ptr,std::allocator>,std::basic_string,std::allocator>>,std::default_delete,std::allocator>,std::basic_string,std::allocator>>>> ] ....\Sources\ApplicationManager.cpp(3856): note: see reference to function template instantiation 'std::thread::thread(_Fn &&,ApplicationManager &&,std::string &,std::string &)' being compiled with [ _Fn=void (__cdecl ApplicationManager:: )(std::string,std::string &) ] C:\Program Files (x86)\Microsoft Visual Studio\2017\Professional\VC\Tools\MSVC\14.16.27023\include\filesystem(2392): note: see reference to class template instantiation 'std::chrono::time_point' being compiled C:\Program Files (x86)\Microsoft Visual Studio\2017\Professional\VC\Tools\MSVC\14.16.27023\include\type_traits(616): note: see reference to class template instantiation 'std::basic_string_view>' being compiled C:\Program Files (x86)\Microsoft Visual Studio\2017\Professional\VC\Tools\MSVC\14.16.27023\include\xstring(2124): note: see reference to class template instantiation 'std::is_convertible>>' being compiled with [ _StringViewIsh=const wchar_t * ] C:\Program Files (x86)\Microsoft Visual Studio\2017\Professional\VC\Tools\MSVC\14.16.27023\include\xstring(2122): note: see reference to variable template 'const bool conjunction_v > >,std::negation > >' being compiled C:\Program Files (x86)\Microsoft Visual Studio\2017\Professional\VC\Tools\MSVC\14.16.27023\include\xstring(2281): note: see reference to alias template instantiation 'std::basic_string,std::allocator>::_Is_string_view_ish<_StringViewIsh>' being compiled with [ _StringViewIsh=const wchar_t * ]*

1
c++multithreadingc++17stdthread
Cannot reproduce: godbolt.org/z/kK7Mth Please edit your post to include a minimal reproducible example. – aschepler
@aschepler I edited my post. my second argument is a reference. – Arun
@cigien can you point me the issue exactly :( – Arun
This appears to be an msvc extension. gcc and clang don't even compile this, when a member function argument is an lvalue reference. – cigien

1 Answers

1
votes

your function

test(std::string arg1, std::string& arg2);

takes a reference as a second argument but here 

std::thread(&ApplicationManager::test, this, arg, arg2);

it takes a value not a reference, hence you have to modify this to

std::thread(&ApplicationManager::test, this, arg, std::ref(arg2));

to wrap a reference to your variable in a reference wrapper object. see

Note that the compiler message you've posted is specific to MVC compiler but your code doesn't compile (before editing) with gcc or clang as cigien has mentioned.