Use member function in std::packaged_task with std::thread (not std::async)

0
votes

I'm trying to get a std::future from std::thread and it seems that the only way (?) to do so is to use std::packaged_task. Well, in addition, the function I want to call in std::thread is a class member function. I could find this thread Use member function in std::packaged_task but they use std::async instead of std::thread. I tried to get something compiling based on that post but when I try to compile this code here:

#include <iostream>
#include <future>
#include <thread>

class test
{
    int fthread(int b)
    {
        return a + b;
    }

    int a;
public:
    test(int a_) : a(a_) {}

    int run(int b)
    {
        std::packaged_task<int(int)>
            task(std::bind(&test::fthread, this));

        //std::thread a(std::move(std::ref(task)), 5, 1);
        std::thread th(std::move(std::ref(task)), b);//<--Edit typo...

        std::future<int> fut = task.get_future();

        return fut.get();
    }
};

int main()
{
    int a = 5, b = 1;
    test t(a);

    int c = t.run(b);

    if (c != (a+b))
        return 1;
    return 0;
}

I get the following error on MSVC 2013:

error C2064: term does not evaluate to a function taking 1 arguments
class does not define an 'operator()' or a user defined conversion operator to a pointer-to-function or reference-to-function that takes appropriate number of arguments

1>          C:\MesProgs\Microsoft Visual Studio 12.0\VC\include\functional(1137) : see reference to function template instantiation 'int std::_Bind<true,int,std::_Pmf_wrap<int (__cdecl test::* )(int),int,test,int>,test *const >::_Do_call<_Ty,0x00>(std::tuple<_Ty &>,std::_Arg_idx<0x00>)' being compiled
1>          with
1>          [
1>              _Ty=int
1>          ]
1>          C:\MesProgs\Microsoft Visual Studio 12.0\VC\include\functional(1137) : see reference to function template instantiation 'int std::_Bind<true,int,std::_Pmf_wrap<int (__cdecl test::* )(int),int,test,int>,test *const >::_Do_call<_Ty,0x00>(std::tuple<_Ty &>,std::_Arg_idx<0x00>)' being compiled
1>          with
1>          [
1>              _Ty=int
1>          ]
1>          C:\MesProgs\Microsoft Visual Studio 12.0\VC\include\xrefwrap(283) : see reference to function template instantiation 'int std::_Bind<true,int,std::_Pmf_wrap<int (__cdecl test::* )(int),int,test,int>,test *const >::operator ()<_Ty>(_Ty &&)' being compiled
1>          with
1>          [
1>              _Ty=int
1>          ]
1>          C:\MesProgs\Microsoft Visual Studio 12.0\VC\include\xrefwrap(283) : see reference to function template instantiation 'int std::_Bind<true,int,std::_Pmf_wrap<int (__cdecl test::* )(int),int,test,int>,test *const >::operator ()<_Ty>(_Ty &&)' being compiled
1>          with
1>          [
1>              _Ty=int
1>          ]
1>          C:\MesProgs\Microsoft Visual Studio 12.0\VC\include\functional(228) : see reference to function template instantiation '_Ret std::_Callable_obj<std::_Bind<true,_Ret,std::_Pmf_wrap<int (__cdecl test::* )(int),int,test,int>,test *const >,false>::_ApplyX<_Rx,_Ty>(_Ty &&)' being compiled
1>          with
1>          [
1>              _Ret=int
1>  ,            _Rx=int
1>  ,            _Ty=int
1>          ]
1>          C:\MesProgs\Microsoft Visual Studio 12.0\VC\include\functional(228) : see reference to function template instantiation '_Ret std::_Callable_obj<std::_Bind<true,_Ret,std::_Pmf_wrap<int (__cdecl test::* )(int),int,test,int>,test *const >,false>::_ApplyX<_Rx,_Ty>(_Ty &&)' being compiled
1>          with
1>          [
1>              _Ret=int
1>  ,            _Rx=int
1>  ,            _Ty=int
1>          ]
1>          C:\MesProgs\Microsoft Visual Studio 12.0\VC\include\functional(226) : while compiling class template member function 'int std::_Func_impl<_MyWrapper,_Alloc,_Ret,int>::_Do_call(int &&)'
1>          with
1>          [
1>              _Alloc=std::allocator<std::_Func_class<int,int>>
1>  ,            _Ret=int
1>          ]
1>          C:\MesProgs\Microsoft Visual Studio 12.0\VC\include\functional(495) : see reference to class template instantiation 'std::_Func_impl<_MyWrapper,_Alloc,_Ret,int>' being compiled
1>          with
1>          [
1>              _Alloc=std::allocator<std::_Func_class<int,int>>
1>  ,            _Ret=int
1>          ]
1>          C:\MesProgs\Microsoft Visual Studio 12.0\VC\include\functional(396) : see reference to function template instantiation 'void std::_Func_class<_Ret,int>::_Do_alloc<_Myimpl,_Ty,_Alloc>(_Fty &&,_Alloc)' being compiled
1>          with
1>          [
1>              _Ret=int
1>  ,            _Ty=std::_Bind<true,int,std::_Pmf_wrap<int (__cdecl test::* )(int),int,test,int>,test *const >
1>  ,            _Alloc=std::allocator<std::_Func_class<int,int>>
1>  ,            _Fty=std::_Bind<true,int,std::_Pmf_wrap<int (__cdecl test::* )(int),int,test,int>,test *const >
1>          ]
1>          C:\MesProgs\Microsoft Visual Studio 12.0\VC\include\functional(396) : see reference to function template instantiation 'void std::_Func_class<_Ret,int>::_Do_alloc<_Myimpl,_Ty,_Alloc>(_Fty &&,_Alloc)' being compiled
1>          with
1>          [
1>              _Ret=int
1>  ,            _Ty=std::_Bind<true,int,std::_Pmf_wrap<int (__cdecl test::* )(int),int,test,int>,test *const >
1>  ,            _Alloc=std::allocator<std::_Func_class<int,int>>
1>  ,            _Fty=std::_Bind<true,int,std::_Pmf_wrap<int (__cdecl test::* )(int),int,test,int>,test *const >
1>          ]
1>          C:\MesProgs\Microsoft Visual Studio 12.0\VC\include\functional(385) : see reference to function template instantiation 'void std::_Func_class<_Ret,int>::_Reset_alloc<_Ty,std::allocator<std::_Func_class<_Ret,int>>>(_Fty &&,_Alloc)' being compiled
1>          with
1>          [
1>              _Ret=int
1>  ,            _Ty=std::_Bind<true,int,std::_Pmf_wrap<int (__cdecl test::* )(int),int,test,int>,test *const >
1>  ,            _Fty=std::_Bind<true,int,std::_Pmf_wrap<int (__cdecl test::* )(int),int,test,int>,test *const >
1>  ,            _Alloc=std::allocator<std::_Func_class<int,int>>
1>          ]
1>          C:\MesProgs\Microsoft Visual Studio 12.0\VC\include\functional(385) : see reference to function template instantiation 'void std::_Func_class<_Ret,int>::_Reset_alloc<_Ty,std::allocator<std::_Func_class<_Ret,int>>>(_Fty &&,_Alloc)' being compiled
1>          with
1>          [
1>              _Ret=int
1>  ,            _Ty=std::_Bind<true,int,std::_Pmf_wrap<int (__cdecl test::* )(int),int,test,int>,test *const >
1>  ,            _Fty=std::_Bind<true,int,std::_Pmf_wrap<int (__cdecl test::* )(int),int,test,int>,test *const >
1>  ,            _Alloc=std::allocator<std::_Func_class<int,int>>
1>          ]
1>          C:\MesProgs\Microsoft Visual Studio 12.0\VC\include\functional(671) : see reference to function template instantiation 'void std::_Func_class<_Ret,int>::_Reset<_Ty>(_Fty &&)' being compiled
1>          with
1>          [
1>              _Ret=int
1>  ,            _Ty=std::_Bind<true,int,std::_Pmf_wrap<int (__cdecl test::* )(int),int,test,int>,test *const >
1>  ,            _Fty=std::_Bind<true,int,std::_Pmf_wrap<int (__cdecl test::* )(int),int,test,int>,test *const >
1>          ]
1>          C:\MesProgs\Microsoft Visual Studio 12.0\VC\include\functional(671) : see reference to function template instantiation 'void std::_Func_class<_Ret,int>::_Reset<_Ty>(_Fty &&)' being compiled
1>          with
1>          [
1>              _Ret=int
1>  ,            _Ty=std::_Bind<true,int,std::_Pmf_wrap<int (__cdecl test::* )(int),int,test,int>,test *const >
1>  ,            _Fty=std::_Bind<true,int,std::_Pmf_wrap<int (__cdecl test::* )(int),int,test,int>,test *const >
1>          ]
1>          C:\MesProgs\Microsoft Visual Studio 12.0\VC\include\future(588) : see reference to function template instantiation 'std::function<_Ret (int)>::function<_Ty>(_Fx &&)' being compiled
1>          with
1>          [
1>              _Ret=int
1>  ,            _Ty=std::_Bind<true,int,std::_Pmf_wrap<int (__cdecl test::* )(int),int,test,int>,test *const >
1>  ,            _Fx=std::_Bind<true,int,std::_Pmf_wrap<int (__cdecl test::* )(int),int,test,int>,test *const >
1>          ]
1>          C:\MesProgs\Microsoft Visual Studio 12.0\VC\include\future(588) : see reference to function template instantiation 'std::function<_Ret (int)>::function<_Ty>(_Fx &&)' being compiled
1>          with
1>          [
1>              _Ret=int
1>  ,            _Ty=std::_Bind<true,int,std::_Pmf_wrap<int (__cdecl test::* )(int),int,test,int>,test *const >
1>  ,            _Fx=std::_Bind<true,int,std::_Pmf_wrap<int (__cdecl test::* )(int),int,test,int>,test *const >
1>          ]
1>          C:\MesProgs\Microsoft Visual Studio 12.0\VC\include\future(1742) : see reference to function template instantiation 'std::_Packaged_state<_Ret (int)>::_Packaged_state<_Ty>(_Fty2 &&)' being compiled
1>          with
1>          [
1>              _Ret=int
1>  ,            _Ty=std::_Bind<true,int,std::_Pmf_wrap<int (__cdecl test::* )(int),int,test,int>,test *const >
1>  ,            _Fty2=std::_Bind<true,int,std::_Pmf_wrap<int (__cdecl test::* )(int),int,test,int>,test *const >
1>          ]
1>          C:\MesProgs\Microsoft Visual Studio 12.0\VC\include\future(1744) : see reference to function template instantiation 'std::_Packaged_state<_Ret (int)>::_Packaged_state<_Ty>(_Fty2 &&)' being compiled
1>          with
1>          [
1>              _Ret=int
1>  ,            _Ty=std::_Bind<true,int,std::_Pmf_wrap<int (__cdecl test::* )(int),int,test,int>,test *const >
1>  ,            _Fty2=std::_Bind<true,int,std::_Pmf_wrap<int (__cdecl test::* )(int),int,test,int>,test *const >
1>          ]
1>          main.cpp(51) : see reference to function template instantiation 'std::packaged_task<int (int)>::packaged_task<std::_Bind<true,int,std::_Pmf_wrap<int (__cdecl test::* )(int),int,test,int>,test *const >>(_Fty2 &&)' being compiled
1>          with
1>          [
1>              _Fty2=std::_Bind<true,int,std::_Pmf_wrap<int (__cdecl test::* )(int),int,test,int>,test *const >
1>          ]
1>          main.cpp(51) : see reference to function template instantiation 'std::packaged_task<int (int)>::packaged_task<std::_Bind<true,int,std::_Pmf_wrap<int (__cdecl test::* )(int),int,test,int>,test *const >>(_Fty2 &&)' being compiled
1>          with
1>          [
1>              _Fty2=std::_Bind<true,int,std::_Pmf_wrap<int (__cdecl test::* )(int),int,test,int>,test *const >
1>          ]

I'd appreciate any help! Thank you

1
multithreadingc++11stlpackaged-task
You may also want to look at std::promise as a way of using futures. – Casey
thank you, I'll have a look at it. – laobrasuca

1 Answers

2
votes

You have a few issues:

  • You don't specify a placeholder for the expected argument in your bind. It should be std::bind(&test::fthread, this, std::placeholders::_1), or better yet [this](int i){ return fthread(i); }.

  • You're trying to move a reference_wrapper into std::thread when you should be simply moving the packaged_task directly: std::thread a(std::move(task), ... or passing a reference_wrapper by value: std::thread a(std::ref(task), ..... If you move the packaged_task into the thread, be sure to extract its future before moving.

  • You pass two arguments - 5 and 1 - into the std::thread constructor when the packaged_task only takes one argument. From examination of the test in main, you want that argument to be b: std::thread a(std::move(task), b);

  • You don't join or detach the thread, so its destructor will call std::terminate.

Addressing all of these results in:

int run(int b)
{
    std::packaged_task<int(int)>
            task(std::bind(&test::fthread, this, std::placeholders::_1));
    std::future<int> fut = task.get_future();

    std::thread(std::move(task), b).join();

    return fut.get();
}

which is semantically identical to:

int run(int b)
{
    return std::async(std::launch::async, [this,b]{ return fthread(b); }).get();
}

Demo at Coliru