tr1::unique_ptr和SelectObject()

Question

我有一些管理异常安全的原始代码，如下所示：

void foo() {
    HDC hdc = //get an HDC
    HBITMAP hbitmap = //get an HBITMAP

    HGDIOBJ hbitmapOld = SelectObject(hdc, hbitmap);

    try {
        //do something that may throw an exception
    } catch (...) {
        SelectObject(hdc, hbitmapOld);
        throw;
    }
}

现在我想去掉try块，使用unique_ptr自动选择旧的位图。所以我写了这样的东西：

void foo() {
    //...

    //HGDIOBJ is defined as void*
    std::unique_ptr<void, std::function<HGDIOBJ(HGDIOBJ)>>
        hbitmapOld(SelectObject(hdc, hbitmap), std::bind(SelectObject, hdc, _1));
}

但是它不能编译。如何让它变得正确？

错误消息：

1>c:\program files (x86)\microsoft visual studio 10.0\vc\include\xxcallfun(7): error C2664: 'HGDIOBJ (HDC,HGDIOBJ)' : cannot convert parameter 2 from 'boost::arg<I>' to 'HGDIOBJ'
1>          with
1>          [
1>              I=1
1>          ]
1>          No user-defined-conversion operator available that can perform this conversion, or the operator cannot be called
1>          c:\program files (x86)\microsoft visual studio 10.0\vc\include\xxbind1(292) : see reference to function template instantiation '_Ret std::tr1::_Callable_fun<_Ty,_Indirect>::_ApplyX<_Ret,_Arg&,boost::arg<I>&>(_Arg0,_Arg1) const' being compiled
1>          with
1>          [
1>              _Ret=_Rx,
1>              _Ty=HGDIOBJ (__stdcall *const )(HDC,HGDIOBJ),
1>              _Indirect=false,
1>              _Arg=HDC,
1>              I=1,
1>              _Arg0=HDC &,
1>              _Arg1=boost::arg<1> &
1>          ]
1>          c:\program files (x86)\microsoft visual studio 10.0\vc\include\xxbind0(31) : see reference to function template instantiation '_Ret std::tr1::_Bind2<_Callable,_Arg0,_Arg1>::_ApplyX<_Rx,void&,std::tr1::_Nil&,std::tr1::_Nil&,std::tr1::_Nil&,std::tr1::_Nil&,std::tr1::_Nil&,std::tr1::_Nil&,std::tr1::_Nil&,std::tr1::_Nil&,std::tr1::_Nil&>(_Barg0,_Barg1,_Barg2,_Barg3,_Barg4,_Barg5,_Barg6,_Barg7,_Barg8,_Barg9)' being compiled
1>          with
1>          [
1>              _Ret=_Rx,
1>              _Callable=std::tr1::_Callable_fun<HGDIOBJ (__stdcall *const )(HDC,HGDIOBJ),false>,
1>              _Arg0=HDC,
1>              _Arg1=boost::arg<1>,
1>              _Barg0=HGDIOBJ &,
1>              _Barg1=std::tr1::_Nil &,
1>              _Barg2=std::tr1::_Nil &,
1>              _Barg3=std::tr1::_Nil &,
1>              _Barg4=std::tr1::_Nil &,
1>              _Barg5=std::tr1::_Nil &,
1>              _Barg6=std::tr1::_Nil &,
1>              _Barg7=std::tr1::_Nil &,
1>              _Barg8=std::tr1::_Nil &,
1>              _Barg9=std::tr1::_Nil &
1>          ]
1>          c:\program files (x86)\microsoft visual studio 10.0\vc\include\xxcallobj(13) : see reference to function template instantiation 'void *std::tr1::_Bind_base<_Ret,_BindN>::operator ()<_Arg0&>(_Carg0)' being compiled
1>          with
1>          [
1>              _Ret=HGDIOBJ ,
1>              _BindN=std::tr1::_Bind2<std::tr1::_Callable_fun<HGDIOBJ (__stdcall *const )(HDC,HGDIOBJ),false>,HDC,boost::arg<1>>,
1>              _Arg0=HGDIOBJ,
1>              _Carg0=HGDIOBJ &
1>          ]
1>          c:\program files (x86)\microsoft visual studio 10.0\vc\include\xxfunction(65) : see reference to function template instantiation '_Ret std::tr1::_Callable_obj<_Ty>::_ApplyX<_Rx,_Arg0&>(void)' being compiled
1>          with
1>          [
1>              _Ret=HGDIOBJ,
1>              _Ty=std::tr1::_Bind<HGDIOBJ,HGDIOBJ,std::tr1::_Bind2<std::tr1::_Callable_fun<HGDIOBJ (__stdcall *const )(HDC,HGDIOBJ),false>,HDC,boost::arg<1>>>,
1>              _Rx=HGDIOBJ,
1>              _Arg0=HGDIOBJ
1>          ]
1>          c:\program files (x86)\microsoft visual studio 10.0\vc\include\xxfunction(64) : while compiling class template member function 'HGDIOBJ std::tr1::_Impl_no_alloc1<_Callable,_Rx,_Arg0>::_Do_call(_Arg0)'
1>          with
1>          [
1>              _Callable=_MyWrapper,
1>              _Rx=HGDIOBJ ,
1>              _Arg0=HGDIOBJ 
1>          ]
1>          c:\program files (x86)\microsoft visual studio 10.0\vc\include\xxfunction(386) : see reference to class template instantiation 'std::tr1::_Impl_no_alloc1<_Callable,_Rx,_Arg0>' being compiled
1>          with
1>          [
1>              _Callable=_MyWrapper,
1>              _Rx=HGDIOBJ ,
1>              _Arg0=HGDIOBJ 
1>          ]
1>          c:\program files (x86)\microsoft visual studio 10.0\vc\include\xxfunction(369) : see reference to function template instantiation 'void std::tr1::_Function_impl1<_Ret,_Arg0>::_Reset0o<_Myimpl,_Fty,std::allocator<_Ty>>(_Fty,_Alloc)' being compiled
1>          with
1>          [
1>              _Ret=HGDIOBJ ,
1>              _Arg0=HGDIOBJ ,
1>              _Fty=std::tr1::_Bind<HGDIOBJ,HGDIOBJ,std::tr1::_Bind2<std::tr1::_Callable_fun<HGDIOBJ (__stdcall *const )(HDC,HGDIOBJ),false>,HDC,boost::arg<1>>>,
1>              _Ty=std::tr1::_Function_impl1<HGDIOBJ ,HGDIOBJ >,
1>              _Alloc=std::allocator<std::tr1::_Function_impl1<HGDIOBJ ,HGDIOBJ >>
1>          ]
1>          c:\program files (x86)\microsoft visual studio 10.0\vc\include\functional(113) : see reference to function template instantiation 'void std::tr1::_Function_impl1<_Ret,_Arg0>::_Reset<_Fx>(_Fty)' being compiled
1>          with
1>          [
1>              _Ret=HGDIOBJ ,
1>              _Arg0=HGDIOBJ ,
1>              _Fx=std::tr1::_Bind<HGDIOBJ,HGDIOBJ,std::tr1::_Bind2<std::tr1::_Callable_fun<HGDIOBJ (__stdcall *const )(HDC,HGDIOBJ),false>,HDC,boost::arg<1>>>,
1>              _Fty=std::tr1::_Bind<HGDIOBJ,HGDIOBJ,std::tr1::_Bind2<std::tr1::_Callable_fun<HGDIOBJ (__stdcall *const )(HDC,HGDIOBJ),false>,HDC,boost::arg<1>>>
1>          ]
1>          r:\programming\windows\biota\library\orchid\src\pixmap.cpp(182) : see reference to function template instantiation 'std::tr1::function<_Fty>::function<std::tr1::_Bind<_Result_type,_Ret,_BindN>>(_Fx)' being compiled
1>          with
1>          [
1>              _Fty=HGDIOBJ (HGDIOBJ),
1>              _Result_type=HGDIOBJ,
1>              _Ret=HGDIOBJ,
1>              _BindN=std::tr1::_Bind2<std::tr1::_Callable_fun<HGDIOBJ (__stdcall *const )(HDC,HGDIOBJ),false>,HDC,boost::arg<1>>,
1>              _Fx=std::tr1::_Bind<HGDIOBJ,HGDIOBJ,std::tr1::_Bind2<std::tr1::_Callable_fun<HGDIOBJ (__stdcall *const )(HDC,HGDIOBJ),false>,HDC,boost::arg<1>>>
1>          ]
1>c:\program files (x86)\microsoft visual studio 10.0\vc\include\xxcallfun(7): error C2664: 'HGDIOBJ (HDC,HGDIOBJ)' : cannot convert parameter 1 from 'boost::arg<I>' to 'HDC'
1>          with
1>          [
1>              I=1
1>          ]
1>          No user-defined-conversion operator available that can perform this conversion, or the operator cannot be called
1>
1>Build FAILED.

Answer 1

这里您需要的是作用域保护习惯用法：http://www.drdobbs.com/cpp/184403758

Answer 2

我认为一个更好的解决方案是编写一个小类，它将在构造函数中完成工作，并在析构函数中执行回滚。当堆栈在异常期间展开时，总是为堆栈类调用析构函数。我认为即使你让你的unique_ptr代码正常工作，这也是一个比这更笨拙的解决方案。

例如，在我的一些代码中，我有一个scoped_noredraw类，它可以防止窗口在更新时刷新。如果函数以正常或异常方式返回，则始终在析构函数中重新打开窗口刷新。

Answer 3

谢谢你，AshleysBrain和Nemanja Trifunovic，他们提出了一种范围保护方法。

在我看来，unique_ptr是scope guard的一个更通用的实现(它应该能够做scope guard所做的事情，甚至更多)，而且我的方法的逻辑是正确的，所以在理论上它应该可以工作。

经过一些测试，我终于明白为什么它不能工作了。这是因为SelectObject()使用__stdcall约定，而微软在编写std::bind时选择忽略这种不方便的行为。:(