std::thread does not accept std::move

std::thread does not accept std::move by James K Edwards

Closed

as Fixed

Type:	Bug
ID:	737812
Opened:	4/19/2012 8:58:36 PM
Access Restriction:	Public
Moderator Decision:	Sent to Engineering Team for consideration

Workaround(s)

view

User(s) can reproduce this bug

This line fails to compile.

std::thread th(&ThFun, std::move(prms));

std::bind requires copyable, but std::thread does not.

Details (expand)

Visual Studio 11 Beta

copy and paste this and compile.

#include <thread>
#include <cassert>
#include <iostream>
#include <string>
#include <future>
#include <utility>

void ThFun(std::promise<std::string>& prms)
{
    std::string hello = "Hello From Future!\n";
    prms.set_value(hello);
}

int main()
{
    std::promise<std::string> prms;
    auto ftr = prms.get_future();
    std::thread th(&ThFun, std::move(prms));
    std::cout << "Hello from Main\n";
    std::string str = ftr.get();
    std::cout << str << std::endl;
    th.join();

    return 0;
}

English

Windows 8

English

1>------ Build started: Project: PromisesAndFutures, Configuration: Debug Win32 ------
1>Build started 4/20/2012 9:27:43 AM.
1>InitializeBuildStatus:
1> Creating "Debug\PromisesAndFutures.unsuccessfulbuild" because "AlwaysCreate" was specified.
1>ClCompile:
1> Main.cpp
1>c:\program files (x86)\microsoft visual studio 11.0\vc\include\tuple(119): error C2248: 'std::promise<_Ty>::promise' : cannot access private member declared in class 'std::promise<_Ty>'
1>         with
1>         [
1>             _Ty=std::string
1>         ]
1>         c:\program files (x86)\microsoft visual studio 11.0\vc\include\future(1575) : see declaration of 'std::promise<_Ty>::promise'
1>         with
1>         [
1>             _Ty=std::string
1>         ]
1>         c:\program files (x86)\microsoft visual studio 11.0\vc\include\tuple(486) : see reference to function template instantiation 'std::_Tuple_val<_Ty>::_Tuple_val<const _Ty&>(_Other)' being compiled
1>         with
1>         [
1>             _Ty=std::promise<std::string>,
1>             _Other=const std::promise<std::string> &
1>         ]
1>         c:\program files (x86)\microsoft visual studio 11.0\vc\include\tuple(486) : while compiling class template member function 'std::tuple<<unnamed-symbol>>::tuple(const std::tuple<<unnamed-symbol>> &)'
1>         with
1>         [
1>             <unnamed-symbol>=std::promise<std::string>
1>         ]
1>         c:\program files (x86)\microsoft visual studio 11.0\vc\include\functional(1140) : see reference to class template instantiation 'std::tuple<<unnamed-symbol>>' being compiled
1>         with
1>         [
1>             <unnamed-symbol>=std::promise<std::string>
1>         ]
1>         c:\program files (x86)\microsoft visual studio 11.0\vc\include\thread(50) : see reference to class template instantiation 'std::_Bind<_Forced,_Ret,_Fun,_V0_t,_V1_t,_V2_t,_V3_t,_V4_t,_V5_t,<unnamed-symbol>>' being compiled
1>         with
1>         [
1>             _Forced=true,
1>             _Ret=void,
1>             _Fun=void (__cdecl *const )(std::promise<std::string> &),
1>             _V0_t=std::promise<std::string>,
1>             _V1_t=std::_Nil,
1>             _V2_t=std::_Nil,
1>             _V3_t=std::_Nil,
1>             _V4_t=std::_Nil,
1>             _V5_t=std::_Nil,
1>             <unnamed-symbol>=std::_Nil
1>         ]
1>         c:\users\jim\documents\visual studio 11\projects\promisesandfutures\promisesandfutures\main.cpp(18) : see reference to function template instantiation 'std::thread::thread<void(__cdecl *)(std::promise<_Ty> &),std::promise<_Ty>>(_Fn,_V0_t &&)' being compiled
1>         with
1>         [
1>             _Ty=std::string,
1>             _Fn=void (__cdecl *)(std::promise<std::string> &),
1>             _V0_t=std::promise<std::string>
1>         ]
1>
1>Build FAILED.
1>
1>Time Elapsed 00:00:01.69
========== Build: 0 succeeded, 1 failed, 0 up-to-date, 0 skipped ==========

1 succeeded, 0 failed, 0 up-to-date, 0 skipped

0 attachments

Hi,

Thanks for reporting this bug. I've resolved it as Fixed because this compiles with my current build of VC11 (and you'll observe this in VC11 RTM).

However, this area is fragile, and we didn't intentionally fix this. The Connect bug http://connect.microsoft.com/VisualStudio/feedback/details/729886/std-thread-constructor-doesnt-handle-movable-object (internally DevDiv#377755) tracks the ultimate problem here: we used std::bind() to implement std::thread, but the Standard actually forbids that because they have different semantics. In particular, bound arguments are supposed to be presented as lvalues to bind() functors (so they can be invoked repeatedly), but as rvalues to thread functors.

Your code compiles because you're moving prms into the thread, and then having ThFun() take an lvalue reference. Very similar code, taking the promise by value or by rvalue reference, will not compile with VC11 (but should, according to C++11).

In the future, we will attempt to fix this, by reimplementing std::thread so it doesn't use std::bind(). However, we won't have enough time to do this for VC11 RTM.

If you have any further questions, feel free to E-mail me at stl@microsoft.com .

Stephan T. Lavavej
Visual C++ Libraries Developer