c++ - C2679 compiler error constructing const int member variable from std::istream -

why can not construct integer member variable istream in code below? there way?

#include <iostream>     // std::cout #include <fstream>      // std::ifstream   class { public:    a(std::istream& strm) : n(strm >> n) {}  // error on line***     void get_number(std::istream& strm) {       int a;       strm >> a;       std::cout << "you retrieved: " << << " stream\n";    }  private:    const int n; };   int main() {    std::ifstream ifs ("test.txt", std::ifstream::in);    a(ifs);     // works    int n;    ifs >> n;    std::cout << n;    return 0; } 

here error message vs2008:

1>------ build started: project: stream_test, configuration: debug win32 ------ 1>compiling... 1>main.cpp 1>c:\cpp\stream_test\stream_test\main.cpp(7) : error c2679: binary '>>' : no operator found takes right-hand operand of type 'const int' (or there no acceptable conversion) 1>        c:\program files (x86)\microsoft visual studio 9.0\vc\include\istream(1144): 'std::basic_istream<_elem,_traits> &std::operator >><std::char_traits<char>>(std::basic_istream<_elem,_traits> &,signed char *)' [found using argument-dependent lookup] 1>        1>        [ 1>            _elem=char, 1>            _traits=std::char_traits<char> 1>        ] 1>        c:\program files (x86)\microsoft visual studio 9.0\vc\include\istream(1146): or       'std::basic_istream<_elem,_traits> &std::operator >><std::char_traits<char>>(std::basic_istream<_elem,_traits> &,signed char &)' [found using argument-dependent lookup] 1>        1>        [ 1>            _elem=char, 1>            _traits=std::char_traits<char> 1>        ] 1>        c:\program files (x86)\microsoft visual studio 9.0\vc\include\istream(1148): or       'std::basic_istream<_elem,_traits> &std::operator >><std::char_traits<char>>(std::basic_istream<_elem,_traits> &,unsigned char *)' [found using argument-dependent lookup] 1>        1>        [ 1>            _elem=char, 1>            _traits=std::char_traits<char> 1>        ] 1>        c:\program files (x86)\microsoft visual studio 9.0\vc\include\istream(1150): or       'std::basic_istream<_elem,_traits> &std::operator >><std::char_traits<char>>(std::basic_istream<_elem,_traits> &,unsigned char &)' [found using argument-dependent lookup] 1>        1>        [ 1>            _elem=char, 1>            _traits=std::char_traits<char> 1>        ] 1>        c:\program files (x86)\microsoft visual studio 9.0\vc\include\istream(155): or       'std::basic_istream<_elem,_traits> &std::basic_istream<_elem,_traits>::operator >>(std::basic_istream<_elem,_traits> &(__cdecl *)(std::basic_istream<_elem,_traits> &))' 1>        1>        [ 1>            _elem=char, 1>            _traits=std::char_traits<char> 1>        ] 1>        c:\program files (x86)\microsoft visual studio 9.0\vc\include\istream(161): or       'std::basic_istream<_elem,_traits> &std::basic_istream<_elem,_traits>::operator >>(std::basic_ios<_elem,_traits> &(__cdecl *)(std::basic_ios<_elem,_traits> &))' 1>        1>        [ 1>            _elem=char, 1>            _traits=std::char_traits<char> 1>        ] 1>        c:\program files (x86)\microsoft visual studio 9.0\vc\include\istream(168): or       'std::basic_istream<_elem,_traits> &std::basic_istream<_elem,_traits>::operator >>(std::ios_base &(__cdecl *)(std::ios_base &))' 1>        1>        [ 1>            _elem=char, 1>            _traits=std::char_traits<char> 1>        ] 1>        c:\program files (x86)\microsoft visual studio 9.0\vc\include\istream(175): or       'std::basic_istream<_elem,_traits> &std::basic_istream<_elem,_traits>::operator >>(std::_bool &)' 1>        1>        [ 1>            _elem=char, 1>            _traits=std::char_traits<char> 1>        ] 1>        c:\program files (x86)\microsoft visual studio 9.0\vc\include\istream(194): or       'std::basic_istream<_elem,_traits> &std::basic_istream<_elem,_traits>::operator >>(short &)' 1>        1>        [ 1>            _elem=char, 1>            _traits=std::char_traits<char> 1>        ] 1>        c:\program files (x86)\microsoft visual studio 9.0\vc\include\istream(228): or       'std::basic_istream<_elem,_traits> &std::basic_istream<_elem,_traits>::operator >>(unsigned short &)' 1>        1>        [ 1>            _elem=char, 1>            _traits=std::char_traits<char> 1>        ] 1>        c:\program files (x86)\microsoft visual studio 9.0\vc\include\istream(247): or       'std::basic_istream<_elem,_traits> &std::basic_istream<_elem,_traits>::operator >>(int &)' 1>        1>        [ 1>            _elem=char, 1>            _traits=std::char_traits<char> 1>        ] 1>        c:\program files (x86)\microsoft visual studio 9.0\vc\include\istream(273): or       'std::basic_istream<_elem,_traits> &std::basic_istream<_elem,_traits>::operator >>(unsigned int &)' 1>        1>        [ 1>            _elem=char, 1>            _traits=std::char_traits<char> 1>        ] 1>        c:\program files (x86)\microsoft visual studio 9.0\vc\include\istream(291): or       'std::basic_istream<_elem,_traits> &std::basic_istream<_elem,_traits>::operator >>(long &)' 1>        1>        [ 1>            _elem=char, 1>            _traits=std::char_traits<char> 1>        ] 1>        c:\program files (x86)\microsoft visual studio 9.0\vc\include\istream(309): or       'std::basic_istream<_elem,_traits> &std::basic_istream<_elem,_traits>::operator >>(__w64 unsigned long &)' 1>        1>        [ 1>            _elem=char, 1>            _traits=std::char_traits<char> 1>        ] 1>        c:\program files (x86)\microsoft visual studio 9.0\vc\include\istream(329): or       'std::basic_istream<_elem,_traits> &std::basic_istream<_elem,_traits>::operator >>(__int64 &)' 1>        1>        [ 1>            _elem=char, 1>            _traits=std::char_traits<char> 1>        ] 1>        c:\program files (x86)\microsoft visual studio 9.0\vc\include\istream(348): or       'std::basic_istream<_elem,_traits> &std::basic_istream<_elem,_traits>::operator >>(unsigned __int64 &)' 1>        1>        [ 1>            _elem=char, 1>            _traits=std::char_traits<char> 1>        ] 1>        c:\program files (x86)\microsoft visual studio 9.0\vc\include\istream(367): or       'std::basic_istream<_elem,_traits> &std::basic_istream<_elem,_traits>::operator >>(float &)' 1>        1>        [ 1>            _elem=char, 1>            _traits=std::char_traits<char> 1>        ] 1>        c:\program files (x86)\microsoft visual studio 9.0\vc\include\istream(386): or       'std::basic_istream<_elem,_traits> &std::basic_istream<_elem,_traits>::operator >>(double &)' 1>        1>        [ 1>            _elem=char, 1>            _traits=std::char_traits<char> 1>        ] 1>        c:\program files (x86)\microsoft visual studio 9.0\vc\include\istream(404): or       'std::basic_istream<_elem,_traits> &std::basic_istream<_elem,_traits>::operator >>(long double &)' 1>        1>        [ 1>            _elem=char, 1>            _traits=std::char_traits<char> 1>        ] 1>        c:\program files (x86)\microsoft visual studio 9.0\vc\include\istream(422): or       'std::basic_istream<_elem,_traits> &std::basic_istream<_elem,_traits>::operator >>(void *&)' 1>        1>        [ 1>            _elem=char, 1>            _traits=std::char_traits<char> 1>        ] 1>        c:\program files (x86)\microsoft visual studio 9.0\vc\include\istream(441): or       'std::basic_istream<_elem,_traits> &std::basic_istream<_elem,_traits>::operator >>(std::basic_streambuf<_elem,_traits> *)' 1>        1>        [ 1>            _elem=char, 1>            _traits=std::char_traits<char> 1>        ] 1>        while trying match argument list '(std::istream, const int)' 1>c:\cpp\stream_test\stream_test\main.cpp(7) : fatal error c1903: unable recover previous error(s); stopping compilation 1>build log saved @ "file://c:\cpp\stream_test\stream_test\debug\buildlog.htm" 1>stream_test - 2 error(s), 0 warning(s) ========== build: 0 succeeded, 1 failed, 0 up-to-date, 0 skipped ========== 

because can't read const variable. overloads of operator>> take lvalue references non-const variables second parameters , const int can't bind int&. additionaly, operator>> returns reference left hand operand, , can't use initilize integer.

you can write function:

int get_from(std::istream& is) {     int i;     if (is >> i) {         return i;     } else {         // error, might wanna throw here     } } 

and use

a(std::istream& strm) : n(get_from(strm)) {} 

but i'd rethink whether want const member variable. it'll make class non-coypable.