Question

我有这个MCVE

#include <iostream>
#include <thread>

class Foo
{
    void bar_i() { std::cout << "hello" << std::endl; }

public:
    void bar()
    {
        // I don't know why I used std::ref(*this) instead of this, I wrote this some time ago
        std::thread t(&Foo::bar_i, std::ref(*this));
        t.join();
    }
};

int main()
{
    Foo f;
    f.bar();
}

构建于VS 2013 Update 3和Coliru之上，但不构建在具有此错误列表的VS 2015上：

1>c:\program files (x86)\microsoft visual studio 14.0\vc\include\thr\xthread(238): error C2893: Failed to specialize function template 'unknown-type std::invoke(_Callable &&,_Types &&...)'
1>  c:\program files (x86)\microsoft visual studio 14.0\vc\include\thr\xthread(238): note: With the following template arguments:
1>  c:\program files (x86)\microsoft visual studio 14.0\vc\include\thr\xthread(238): note: '_Callable=void (__thiscall Foo::* )(void)'
1>  c:\program files (x86)\microsoft visual studio 14.0\vc\include\thr\xthread(238): note: '_Types={std::reference_wrapper<Foo>}'
1>  c:\program files (x86)\microsoft visual studio 14.0\vc\include\thr\xthread(247): note: see reference to function template instantiation 'void std::_LaunchPad<_Target>::_Execute<0,1>(std::tuple<void (__thiscall Foo::* )(void),std::reference_wrapper<Foo>> &,std::integer_sequence<_Ty,0,1>)' being compiled
1>          with
1>          [
1>              _Target=std::unique_ptr<std::tuple<void (__thiscall Foo::* )(void),std::reference_wrapper<Foo>>,std::default_delete<std::tuple<void (__thiscall Foo::* )(void),std::reference_wrapper<Foo>>>>,
1>              _Ty=size_t
1>          ]
1>  c:\program files (x86)\microsoft visual studio 14.0\vc\include\thr\xthread(247): note: see reference to function template instantiation 'void std::_LaunchPad<_Target>::_Execute<0,1>(std::tuple<void (__thiscall Foo::* )(void),std::reference_wrapper<Foo>> &,std::integer_sequence<_Ty,0,1>)' being compiled
1>          with
1>          [
1>              _Target=std::unique_ptr<std::tuple<void (__thiscall Foo::* )(void),std::reference_wrapper<Foo>>,std::default_delete<std::tuple<void (__thiscall Foo::* )(void),std::reference_wrapper<Foo>>>>,
1>              _Ty=size_t
1>          ]
1>  c:\program files (x86)\microsoft visual studio 14.0\vc\include\thr\xthread(242): note: while compiling class template member function 'void std::_LaunchPad<_Target>::_Run(std::_LaunchPad<_Target> *) noexcept'
1>          with
1>          [
1>              _Target=std::unique_ptr<std::tuple<void (__thiscall Foo::* )(void),std::reference_wrapper<Foo>>,std::default_delete<std::tuple<void (__thiscall Foo::* )(void),std::reference_wrapper<Foo>>>>
1>          ]
1>  c:\program files (x86)\microsoft visual studio 14.0\vc\include\thr\xthread(230): note: see reference to function template instantiation 'void std::_LaunchPad<_Target>::_Run(std::_LaunchPad<_Target> *) noexcept' being compiled
1>          with
1>          [
1>              _Target=std::unique_ptr<std::tuple<void (__thiscall Foo::* )(void),std::reference_wrapper<Foo>>,std::default_delete<std::tuple<void (__thiscall Foo::* )(void),std::reference_wrapper<Foo>>>>
1>          ]
1>  c:\program files (x86)\microsoft visual studio 14.0\vc\include\thr\xthread(256): note: see reference to class template instantiation 'std::_LaunchPad<_Target>' being compiled
1>          with
1>          [
1>              _Target=std::unique_ptr<std::tuple<void (__thiscall Foo::* )(void),std::reference_wrapper<Foo>>,std::default_delete<std::tuple<void (__thiscall Foo::* )(void),std::reference_wrapper<Foo>>>>
1>          ]
1>  c:\program files (x86)\microsoft visual studio 14.0\vc\include\thread(52): note: see reference to function template instantiation 'void std::_Launch<std::unique_ptr<std::tuple<void (__thiscall Foo::* )(void),std::reference_wrapper<Foo>>,std::default_delete<std::tuple<void (__thiscall Foo::* )(void),std::reference_wrapper<Foo>>>>>(_Thrd_t *,_Target &&)' being compiled
1>          with
1>          [
1>              _Target=std::unique_ptr<std::tuple<void (__thiscall Foo::* )(void),std::reference_wrapper<Foo>>,std::default_delete<std::tuple<void (__thiscall Foo::* )(void),std::reference_wrapper<Foo>>>>
1>          ]
1>  d:\libraries\c++ active\tests\tests\test.cpp(13): note: see reference to function template instantiation 'std::thread::thread<void(__thiscall Foo::* )(void),std::reference_wrapper<Foo>,void>(_Fn &&,std::reference_wrapper<Foo> &&)' being compiled
1>          with
1>          [
1>              _Fn=void (__thiscall Foo::* )(void)
1>          ]

如果不是std::ref，则此代码将编译。什么似乎是问题？

Answer 1

据我所知，20.9.2中的规则会导致您的代码等同于

auto f = &Foo::bar_i;
decay<decltype(std::ref(*this))>::type t1 = std::ref(*this); // still reference_wrapper
((*t1).*f)();

这实际上是一个错误。

只需使用std::thread t(&Foo::bar_i, this);，因为std::thread中存在的指向成员函数的机制知道在执行调用时取消引用第一个参数。

根据我正在查看的n4527草案，std::reference_wrapper使用的std::bind的特殊处理（参见20.9.10.3）不适用于{{1} }。 std::thread使用的特殊衰减逻辑也不会导致std::tuple成为正常参考。

Visual C ++ 14 - 无法构建一个基于Visual C ++ 12构建的简单线程程序

1 个答案: