c++ - tr1::unique_ptr 和 SelectObject()

Question

我有一些像这样管理异常安全的原始代码:

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;
    }
}

现在我想摆脱 try block 并使用 unique_ptr 自动选择旧位图。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));
}

但是它不编译。如何正确处理？

错误信息:

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

我认为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，总是会为堆栈类调用析构函数。我认为即使您的 unique_ptr 代码可以正常工作，它也是一个比这更尴尬的解决方案。

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。如果函数正常返回或异常返回，窗口刷新总是在析构函数中重新打开。