c++ - Visual C++ 14 - 无法构建基于 Visual C++ 12 的简单线程程序

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();
}

它基于 VS 2013 Update 3 和 Coliru , 但不建立在 VS 2015 上并带有此错误列表:

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，这段代码可以编译。似乎是什么问题？

Answer 1

据我所知，20.9.2 中的规则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);，因为指向成员函数的机制存在于 std::thread 中知道在执行调用时取消引用第一个参数。

根据稿件n4527我在看，std::bind(参见 20.9.10.3)使用的 std::reference_wrapper 的特殊处理不适用于 std::线程。 std::tuple 使用的特殊衰减逻辑也不是，这导致 reference_wrapper 成为普通引用。