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#include <iostream>
#include <vector>
#include <fstream>
 
using namespace std;
bool FileIsProvided() { return true; }
 
int main(int argc, char* argv[])
{
	vector<string> args{argv+1, argv + argc};
	istream& refToCIN  ( cin );                // This is OK
	const istream& refToFile = ifstream(args[1]);    // This is OK
 
	istream& inStream ( FileIsProvided()? ifstream(args[1]) : cin );
	// This causes error:
	// std::basic_istream<char,std::char_traits<char>>::basic_istream' : 
	// cannot access protected member declared in class std::basic_istream<char,std::char_traits<char>>
 
	// ProcessStream(inStream); // This could either be a file or cin
	return 0;
}

In file included from /usr/include/c++/5/iostream:40:0,
                 from prog.cpp:1:
/usr/include/c++/5/istream: In function 'int main(int, char**)':
/usr/include/c++/5/istream:613:7: error: 'std::basic_istream<_CharT, _Traits>::basic_istream(std::basic_istream<_CharT, _Traits>&&) [with _CharT = char; _Traits = std::char_traits<char>]' is protected
       basic_istream(basic_istream&& __rhs)
       ^
prog.cpp:14:60: error: within this context
  istream& inStream ( FileIsProvided()? ifstream(args[1]) : cin );
                                                            ^
In file included from /usr/include/c++/5/iostream:40:0,
                 from prog.cpp:1:
/usr/include/c++/5/istream:611:7: error: 'std::basic_istream<_CharT, _Traits>::basic_istream(const std::basic_istream<_CharT, _Traits>&) [with _CharT = char; _Traits = std::char_traits<char>]' is protected
       basic_istream(const basic_istream&) = delete;
       ^
prog.cpp:14:60: error: within this context
  istream& inStream ( FileIsProvided()? ifstream(args[1]) : cin );
                                                            ^
prog.cpp:14:60: error: use of deleted function 'std::basic_istream<_CharT, _Traits>::basic_istream(const std::basic_istream<_CharT, _Traits>&) [with _CharT = char; _Traits = std::char_traits<char>]'
In file included from /usr/include/c++/5/iostream:40:0,
                 from prog.cpp:1:
/usr/include/c++/5/istream:611:7: note: declared here
       basic_istream(const basic_istream&) = delete;
       ^
prog.cpp:14:64: error: invalid initialization of non-const reference of type 'std::istream& {aka std::basic_istream<char>&}' from an rvalue of type 'std::basic_istream<char>'
  istream& inStream ( FileIsProvided()? ifstream(args[1]) : cin );
                                                                ^

Standard output is empty