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
    fp = stdin;

What's the best way to do this with an ifstream instance? I've received a bit of code that has an ifstream as part of a class and I'd like to add code to do the equivalent, something like:

if ( filename == "-")
    logstream = cin;  // **how do I do this*?*
else
    logstream.open( filename.c_str() );

C. K. Young · Accepted Answer · 2010-01-29T04:28:36.307

25

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. :-)

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.

edited Jan 29 '10 at 04:28

answered Jan 29 '10 at 02:16

C. K. Young

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thanks... this only needs to be portable to linux and osx, so /dev/stdin it is! (sigh) – Mark Harrison Feb 04 '10 at 00:15
1

this illustrates how no one ever sat down and made a halfway decent set of c++ libs. (i would say boost if it didn't have so many versioning issues) – Erik Aronesty Jan 05 '12 at 18:17
1

FWIW, with C++11 instead of passing `noop()` to `reset` (and defining the struct), a simple (and funny looking) `[](...){}` does it. – akim Sep 27 '13 at 09:30
@akim Yep. Go lambdas! – C. K. Young Sep 27 '13 at 11:16

score 0 · Answer 2 · answered Jan 29 '10 at 15:52

0

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.

answered Jan 29 '10 at 15:52

Luc Hermitte

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I wrote a very similar answer an hour ago, but then deleted it after re-reading this part of the question: "I've received a bit of code that has an ifstream as part of a class". The question's title is misleading in that it may be read as "how do I redirect cin to read from a file", but it is, IIUC, the reverse: The OP already has an ifstream and would like to redirect it to cin. – Éric Malenfant Jan 29 '10 at 16:03
Indeed. We cannot change the streambuf of an ifstream like that. As in Chris Jester-Young's solution, the exact type of the stream used needs to be changed. – Luc Hermitte Jan 29 '10 at 16:56

C++: assign cin to an ifstream variable?

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