View Compilation Error

309818_0_5821.cpp
309818_0_5821.cpp(5) : error C2065: “a” : 未声明的标识符
309818_0_5821.cpp(5) : error C2593: “operator >>”不明确
        VC\include\istream(413): 可能是“std::basic_istream<_Elem,_Traits>::_Myt &std::basic_istream<_Elem,_Traits>::operator >>(std::basic_istream<_Elem,_Traits>::_Mysb *)”
        with
        [
            _Elem=char,
            _Traits=std::char_traits<char>
        ]
        VC\include\istream(394): 或       “std::basic_istream<_Elem,_Traits>::_Myt &std::basic_istream<_Elem,_Traits>::operator >>(void *& )”
        with
        [
            _Elem=char,
            _Traits=std::char_traits<char>
        ]
        VC\include\istream(376): 或       “std::basic_istream<_Elem,_Traits>::_Myt &std::basic_istream<_Elem,_Traits>::operator >>(long double &)”
        with
        [
            _Elem=char,
            _Traits=std::char_traits<char>
        ]
        VC\include\istream(358): 或       “std::basic_istream<_Elem,_Traits>::_Myt &std::basic_istream<_Elem,_Traits>::operator >>(double &)”
        with
        [
            _Elem=char,
            _Traits=std::char_traits<char>
        ]
        VC\include\istream(339): 或       “std::basic_istream<_Elem,_Traits>::_Myt &std::basic_istream<_Elem,_Traits>::operator >>(float &)”
        with
        [
            _Elem=char,
            _Traits=std::char_traits<char>
        ]
        VC\include\istream(320): 或       “std::basic_istream<_Elem,_Traits>::_Myt &std::basic_istream<_Elem,_Traits>::operator >>(unsigned __int64 &)”
        with
        [
            _Elem=char,
            _Traits=std::char_traits<char>
        ]
        VC\include\istream(301): 或       “std::basic_istream<_Elem,_Traits>::_Myt &std::basic_istream<_Elem,_Traits>::operator >>(__int64 &)”
        with
        [
            _Elem=char,
            _Traits=std::char_traits<char>
        ]
        VC\include\istream(281): 或       “std::basic_istream<_Elem,_Traits>::_Myt &std::basic_istream<_Elem,_Traits>::operator >>(unsigned long &)”
        with
        [
            _Elem=char,
            _Traits=std::char_traits<char>
        ]
        VC\include\istream(263): 或       “std::basic_istream<_Elem,_Traits>::_Myt &std::basic_istream<_Elem,_Traits>::operator >>(long &)”
        with
        [
            _Elem=char,
            _Traits=std::char_traits<char>
        ]
        VC\include\istream(245): 或       “std::basic_istream<_Elem,_Traits>::_Myt &std::basic_istream<_Elem,_Traits>::operator >>(unsigned int &)”
        with
        [
            _Elem=char,
            _Traits=std::char_traits<char>
        ]
        VC\include\istream(219): 或       “std::basic_istream<_Elem,_Traits>::_Myt &std::basic_istream<_Elem,_Traits>::operator >>(int &)”
        with
        [
            _Elem=char,
            _Traits=std::char_traits<char>
        ]
        VC\include\istream(200): 或       “std::basic_istream<_Elem,_Traits>::_Myt &std::basic_istream<_Elem,_Traits>::operator >>(unsigned short &)”
        with
        [
            _Elem=char,
            _Traits=std::char_traits<char>
        ]
        VC\include\istream(166): 或       “std::basic_istream<_Elem,_Traits>::_Myt &std::basic_istream<_Elem,_Traits>::operator >>(short &)”
        with
        [
            _Elem=char,
            _Traits=std::char_traits<char>
        ]
        VC\include\istream(147): 或       “std::basic_istream<_Elem,_Traits>::_Myt &std::basic_istream<_Elem,_Traits>::operator >>(std::_Bool &)”
        with
        [
            _Elem=char,
            _Traits=std::char_traits<char>
        ]
        VC\include\istream(141): 或       “std::basic_istream<_Elem,_Traits>::_Myt &std::basic_istream<_Elem,_Traits>::operator >>(std::ios_base &(__cdecl *)(std::ios_base &))”
        with
        [
            _Elem=char,
            _Traits=std::char_traits<char>
        ]
        VC\include\istream(135): 或       “std::basic_istream<_Elem,_Traits>::_Myt &std::basic_istream<_Elem,_Traits>::operator >>(std::basic_istream<_Elem,_Traits>::_Myios &(__cdecl *)(std::basic_istream<_Elem,_Traits>::_Myios &))”
        with
        [
            _Elem=char,
            _Traits=std::char_traits<char>
        ]
        VC\include\istream(130): 或       “std::basic_istream<_Elem,_Traits>::_Myt &std::basic_istream<_Elem,_Traits>::operator >>(std::basic_istream<_Elem,_Traits>::_Myt &(__cdecl *)(std::basic_istream<_Elem,_Traits>::_Myt &))”
        with
        [
            _Elem=char,
            _Traits=std::char_traits<char>
        ]
        VC\include\istream(983): 或       “std::basic_istream<_Elem,_Traits> &std::operator >><char,std::char_traits<char>>(std::basic_istream<_Elem,_Traits> &,short *)”
        with
        [
            _Elem=char,
            _Traits=std::char_traits<char>
        ]
        VC\include\istream(975): 或       “std::basic_istream<_Elem,_Traits> &std::operator >><char,std::char_traits<char>>(std::basic_istream<_Elem,_Traits> &,unsigned char &)”
        with
        [
            _Elem=char,
            _Traits=std::char_traits<char>
        ]
        VC\include\istream(967): 或       “std::basic_istream<_Elem,_Traits> &std::operator >><char,std::char_traits<char>>(std::basic_istream<_Elem,_Traits> &,unsigned char *)”
        with
        [
            _Elem=char,
            _Traits=std::char_traits<char>
        ]
        VC\include\istream(959): 或       “std::basic_istream<_Elem,_Traits> &std::operator >><char,std::char_traits<char>>(std::basic_istream<_Elem,_Traits> &,signed char &)”
        with
        [
            _Elem=char,
            _Traits=std::char_traits<char>
        ]
        VC\include\istream(951): 或       “std::basic_istream<_Elem,_Traits> &std::operator >><char,std::char_traits<char>>(std::basic_istream<_Elem,_Traits> &,signed char *)”
        with
        [
            _Elem=char,
            _Traits=std::char_traits<char>
        ]
        试图匹配参数列表“(std::istream, 'unknown-type')”时
309818_0_5821.cpp(5) : error C2065: “b” : 未声明的标识符
309818_0_5821.cpp(5) : fatal error C1903: 无法从以前的错误中恢复；正在停止编译