c++ - Initializing reference to istream -


i trying write program can process either stdin or file specified on command line.

i'm doing trying initialize reference istream either refer cin or ifstream, using conditional.

(similar techniques described here , here)

but when try ifstream, seem error basic_istream move-constructor declared protected.

istream& reftocin  ( cin );                      // ok const istream& reftofile = ifstream(args[1]);    // ok  const istream& instream ( fileisprovided()? ifstream(args[1]) : cin ); // 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); // either file or cin

can reasonably done way? there alternative i'm overlooking?

the problem code following:

your left-hand side of ternary operator temporary (rvalue). however, right hand-side lvalue (cin lvalue). result, compiler trying create temporary out of cin, , fails because of copy constructor being not available.

as sultions - can replace rdbuf() of cin rdbuf() of file, , use cin everywhere.

here's ultimate solution op came with:

ifstream file; std::streambuf* old_cin_buf = cin.rdbuf(); // store old value if (fileisprovided()) {     file.open(args[1]);     old_cin_buf = cin.rdbuf(file.rdbuf()); // replace readbuffer on cin.     // store previous value well. } // use cin operations now.  either use file or stdin appropriate. ... // restore original value, in case changed using file. cin.rdbuf(old_cin_buf); // better done before file object here goes out of scope

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