const调用运算符，调用绑定的非const成员函数

问题描述

以下代码在我尝试过的每个编译器(gcc 4.9.2，clang 3.6和VS 2015)上均可正常编译.但是VS 2013更新4消除了错误，我将在下面详细介绍.这是编译器中的错误吗?

The following code compiles fine on every compiler I tried (gcc 4.9.2, clang 3.6 and VS 2015). However VS 2013 update 4 craps out with the error I will detail below. Is this a bug in the compiler?

#include <iostream>
#include <functional>

template<typename Func>
struct Bar
{
    Func f_;
    Bar(Func f) : f_(f) { }
    void operator()() const
    {
        f_();
    }
};

template<typename T>
void Baz(T const& t)
{
    Bar<T> b(t);
    b();
}

struct Foo
{
    Foo()
    {
        auto r = std::bind(&Foo::DoFoo, this);
        Baz(r);
    }
    void DoFoo() { std::cout << "Doing Foo!\n"; }
};

int main()
{
    Foo f;
    return 0;
}

错误列表如下:

1>doodle.cpp(15): error C3848: expression having type 'const std::_Bind<true,void,std::_Pmf_wrap<void (__thiscall Foo::* )(void),void,Foo,>,Foo *const >' would lose some const-volatile qualifiers in order to call 'void std::_Bind<true,void,std::_Pmf_wrap<void (__thiscall Foo::* )(void),void,Foo,>,Foo *const >::operator ()<>(void)'
1>          doodle.cpp(14) : while compiling class template member function 'void Bar<T>::operator ()(void) const'
1>          with
1>          [
1>              T=std::_Bind<true,void,std::_Pmf_wrap<void (__thiscall Foo::* )(void),void,Foo,>,Foo *const >
1>          ]
1>          doodle.cpp(23) : see reference to function template    instantiation 'void Bar<T>::operator ()(void) const' being compiled
1>          with
1>          [
1>              T=std::_Bind<true,void,std::_Pmf_wrap<void (__thiscall Foo::* )(void),void,Foo,>,Foo *const >
1>          ]
1>          doodle.cpp(22) : see reference to class template instantiation 'Bar<T>' being compiled
1>          with
1>          [
1>              T=std::_Bind<true,void,std::_Pmf_wrap<void (__thiscall Foo::* )(void),void,Foo,>,Foo *const >
1>          ]
1>          doodle.cpp(31) : see reference to function template instantiation 'void Baz<std::_Bind<true,void,std::_Pmf_wrap<void (__thiscall Foo::* )(void),void,Foo,>,Foo *const >>(const T &)' being compiled
1>          with
1>          [
1>              T=std::_Bind<true,void,std::_Pmf_wrap<void (__thiscall Foo::* )(void),void,Foo,>,Foo *const >
1>          ]

我以为它希望DoFoo()是一个const成员函数，但是修复它并没有帮助.

I thought it wanted DoFoo() to be a const member function but fixing that didn't help.

推荐答案

这似乎是VS2013编译器错误，有2种解决方法:

This seems to be a VS2013 compiler bug and there are 2 ways to work around it:

使用std :: function而不是auto来存储对成员函数的调用

Use std::function instead of auto to store a call to a member function

std::function<void()> r = std::bind(&Foo::DoFoo, this);

或从Bar中的函数调用operator()中删除const

Or remove const from function call operator() in Bar

template<typename Func>
struct Bar
{
    Func f_;
    Bar(Func f) : f_(f) { }
    void operator()() // const
    {
        f_();
    }
};

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