c – 奇怪的错误 – 为什么编译器试图调用复制构造函数？

(118) std::map<int,layer> xs;
(119) xs.begin()->first; // error?!

图层是不可复制的可移动类型.

class layer : public observable
{
    layer(const layer&);
    layer& operator=(const layer&);
public:
    layer(int index = -1);
    layer(layer&& other);
    layer& operator=(layer&& other);
   //...
};

由于某种原因,第119行导致编译器尝试为std :: pair调用复制构造函数,为什么？

1>c:\program files (x86)\microsoft visual studio 10.0\vc\include\utility(131): error C2248: 'layer::layer' : cAnnot access private member declared in class 'layer'
1> ..\layer.h(55) : see declaration of 'layer::layer'
1> ..\layer.h(53) : see declaration of 'layer'
1> c:\program files (x86)\microsoft visual studio 10.0\vc\include\utility(129) : while compiling class template member function 'std::_Pair_base<_Ty1,_Ty2>::_Pair_base(const std::_Pair_base<_Ty1,_Ty2> &)'
1> with
1> [
1>     _Ty1=const int,1>     _Ty2=layer
1> ]
1> c:\program files (x86)\microsoft visual studio 10.0\vc\include\utility(174) : see reference to class template instantiation 'std::_Pair_base<_Ty1,_Ty2>' being compiled
1> with
1> [
1>     _Ty1=const int,1>     _Ty2=layer
1> ]
1> ..\stage.cpp(119) : see reference to class template instantiation 'std::pair<_Ty1,1>     _Ty2=layer
1> ]

我也试过以下,它也失败了.

(118) std::map<int,layer> xs;
(119) auto& t1 = *xs.begin();
(120) auto& t2 = t1.first; // error?!

这里发生了什么？