C++: assign cin to an ifstream variable?

Question

You know the common stdio idiom that stdin is specified by a filename of \"-\", e.g.

if ((strcmp(fname, \"-\"))
    fp = fopen(fname);
else
             


        

Answer 1

• Save the initial streambuf of cin into a variable
• Change the streambuf of cin to the one from the file.
• Do what you need to do
• And don't forget to restore cin streambuf before closing your file -- RAII may help.

Answer 2

cin is not an ifstream, but if you can use istream instead, then you're in to win. Otherwise, if you're prepared to be non-portable, just open /dev/stdin or /dev/fd/0 or whatever. :-)

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If you do want to be portable, and can make your program use istream, here's one way to do it:

struct noop {
    void operator()(...) const {}
};

// ...

shared_ptr<istream> input;
if (filename == "-")
    input.reset(&cin, noop());
else
    input.reset(new ifstream(filename.c_str()));

The noop is to specify a deleter that does nothing in the cin case, because, well, cin is not meant to be deleted.