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

Question

Why can I not construct a integer member variable from an istream as in this code below? Is there a way?

#include <iostream>     // std::cout
#include <fstream>      // std::ifstream


class a {
public:
   a(std::istream& strm) : N(strm >> N) {}  // error on this line***

   void get_number(std::istream& strm) {
      int a;
      strm >> a;
      std::cout << "you retrieved: " << a << " from stream\n";
   }

private:
   const int N;
};


int main() {
   std::ifstream ifs ("test.txt", std::ifstream::in);
   a(ifs);

   // this works
   int n;
   ifs >> n;
   std::cout << n;
   return 0;
}

Here is the error message from 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 which takes a right-hand operand of type 'const int' (or there is no acceptable conversion)
1>        c:\program files (x86)\microsoft visual studio 9.0\vc\include\istream(1144): could be 'std::basic_istream<_Elem,_Traits> &std::operator >><std::char_traits<char>>(std::basic_istream<_Elem,_Traits> &,signed char *)' [found using argument-dependent lookup]
1>        with
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>        with
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>        with
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>        with
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>        with
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>        with
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>        with
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>        with
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>        with
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>        with
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>        with
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>        with
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>        with
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>        with
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>        with
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>        with
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>        with
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>        with
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>        with
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>        with
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>        with
1>        [
1>            _Elem=char,
1>            _Traits=std::char_traits<char>
1>        ]
1>        while trying to match the argument list '(std::istream, const int)'
1>c:\cpp\stream_test\stream_test\main.cpp(7) : fatal error C1903: unable to recover from previous error(s); stopping compilation
1>Build log was saved at "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 ==========

Answer 1

Because you can't read into a const variable. Overloads of operator>> take lvalue references to non-const variables as their second parameters and const int can't bind to int&. Additionaly, operator>> returns a reference to the left hand operand, and you can't use that to initilize an integer.

You can write a function:

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

and use it as

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

But I'd rethink whether you really want a const member variable. It'll make your class non-coypable.

Answer 2

The type of expression

strm >> N 

is std::istream

while N is defined as having type int

const int N;

So this mem-initializer

N( strm >> N )

is invalid

If data member N would not be a constant you could write

N( ( strm >> N, N ) )

However when N is a constant then this expression strm >> N does not mean an initializarion. It is an attempt to change a constant object.