defaulting ifstream variable to standard input

This is a discussion on defaulting ifstream variable to standard input within the C++ Programming forums, part of the General Programming Boards category; Hi peeps Can anyone tell me what must be a fairly standard incantation? About a decade ago I wrote Code: ...

  1. #1
    Registered Luser risby's Avatar
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    defaulting ifstream variable to standard input

    Hi peeps

    Can anyone tell me what must be a fairly standard incantation?

    About a decade ago I wrote
    Code:
        ifstream fin;
    
        if (NULL == inputStream)
        {
            fin.attach(0);
            inputStream = "stdin";
        }
        else
        {
            fin.open(inputStream, ios::in);
        }
    inputStream is NULL unless a filename has been specified on the command line. The attach() function that was available with that compiler allowed me to use this program as part of a pipeline.

    Code:
    who -a | sortByDate
    or
    Code:
    sortByDate uuu.txt
    I haven't really used C++ since then and the attach() function seems to have been an extension that isn't available today. I can't simply do
    Code:
    fin = cin
    can I?
    ===
    Don't grumble about what you can't have;
    be grateful you don't get what you deserve.

  2. #2
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    Same as here: http://www.tek-tips.com/viewthread.c...1448313&page=1

    I don't think there is a way to assign cin to an ifstream.
    Maybe you could open the file then pass either the file or cin to a function (which takes an istream& parameter) that does all the reading?

    Something like this:
    Code:
    void ReadStuff( std::istream&  fin );
    
    int main( int argc, char* argv[] )
    {
    ...
        if ( argc > 1 )
       {
          std::ifstream fin( argv[1] );
          ReadStuff( fin );
       }
       else
       {
          ReadStuff( cin );
       }
    ...
    }
    Last edited by cpjust; 02-05-2008 at 02:35 PM.

  3. #3
    Code Goddess Prelude's Avatar
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    Do you need any ifstream specific functionality? You can use an istream reference:
    Code:
    istream& in;
    
    if ( inputStream == NULL )
      in = cin;
    else
      in = ifstream ( inputStream );
    My best code is written with the delete key.

  4. #4
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    Quote Originally Posted by Prelude View Post
    Do you need any ifstream specific functionality? You can use an istream reference:
    Code:
    istream& in;
    
    if ( inputStream == NULL )
      in = cin;
    else
      in = ifstream ( inputStream );
    Since in is a reference, you need to initialize it when you declare it.
    Also, I don't think you can assign an fstream to an istream like that. Comeau says "error: initial value of reference to non-const must be an lvalue" if I do:
    Code:
    istream& in = ifstream( "file.txt" );

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    Quote Originally Posted by Prelude View Post
    Do you need any ifstream specific functionality? You can use an istream reference:
    Code:
    istream& in;
    
    if ( inputStream == NULL )
      in = cin;
    else
      in = ifstream ( inputStream );
    You must mean:
    Code:
    istream& in = ( inputStream == NULL )? cin : ifstream ( inputStream );
    'Cause references can't be assigned.
    It is too clear and so it is hard to see.
    A dunce once searched for fire with a lighted lantern.
    Had he known what fire was,
    He could have cooked his rice much sooner.

  6. #6
    Captain Crash brewbuck's Avatar
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    Quote Originally Posted by King Mir View Post
    You must mean:
    Code:
    istream& in = ( inputStream == NULL )? cin : ifstream ( inputStream );
    'Cause references can't be assigned.
    That's no good either, because ifstream(inputStream) is a temporary so you are creating a reference to a temporary.

    Without using a pointer there is no real elegant solution to this problem. One method is to break all your code out to a separate function and pass the reference to it:

    Code:
    void code(std::istream &input_stream); // actually processing goes here
    
    if(using_cin)
    {
        code(std::cin);
    }
    else
    {
        std::ifstream in(filename);
        code(in);
    }

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    Didn't notice that.

    Well you could do this:
    Code:
    ifstream file_stream;
    if(inputStream)
        file_stream.open(inputStream);
    istream& in = inputStream? file_stream: cin;

    The other way is to change the buffer of a file stream to point to standard input. But that's confusing and error prone.
    Last edited by King Mir; 02-06-2008 at 07:09 AM. Reason: Fixed a bug in the code.
    It is too clear and so it is hard to see.
    A dunce once searched for fire with a lighted lantern.
    Had he known what fire was,
    He could have cooked his rice much sooner.

  8. #8
    Captain Crash brewbuck's Avatar
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    Quote Originally Posted by King Mir View Post
    Didn't notice that.

    Well you could do this:
    Code:
    ifstream file_stream;
    if(!inputStream)
        file_stream.open(inputStream);
    istream& in = inputStream? file_stream: cin;
    I forgot about ifstream:pen(). Yes, that would be one way to do it.

  9. #9
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    Quote Originally Posted by cpjust View Post
    I don't think there is a way to assign cin to an ifstream.
    There is but it's not worth it.

    Here an link to an example.
    The linked code redirects an output stream to a file, but similar code can be used to redirect an input stream to act as a ifstream.
    It is too clear and so it is hard to see.
    A dunce once searched for fire with a lighted lantern.
    Had he known what fire was,
    He could have cooked his rice much sooner.

  10. #10
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    Quote Originally Posted by King Mir View Post
    There is but it's not worth it.

    Here an link to an example.
    The linked code redirects an output stream to a file, but similar code can be used to redirect an input stream to act as a ifstream.
    Oh yeah, I think I tried that once, but I gave up since it didn't look too safe.

  11. #11
    Registered Luser risby's Avatar
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    Quote Originally Posted by King Mir View Post
    Code:
    ifstream file_stream;
    if(!inputStream)
        file_stream.open(inputStream);
    istream& in = inputStream? file_stream: cin;
    That looks good. I've tried it and it works fine with stdin but fails to read anything from a file. I'll play around with it though, thanks.
    ===
    Don't grumble about what you can't have;
    be grateful you don't get what you deserve.

  12. #12
    Registered Luser risby's Avatar
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    Quote Originally Posted by cpjust View Post
    Can I just ask why you posted this? It makes me think I've done something wrong.

    And, btw, what has happened to Tek-Tips? It's been down for the last couple of hours.
    ===
    Don't grumble about what you can't have;
    be grateful you don't get what you deserve.

  13. #13
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    Quote Originally Posted by risby View Post
    Can I just ask why you posted this? It makes me think I've done something wrong.

    And, btw, what has happened to Tek-Tips? It's been down for the last couple of hours.
    Just in case anyone else has the same question, they can check both places for the answer.
    Tek-Tips goes up & down more than the stock market. Wait a couple hours and it'll usually be back up.

  14. #14
    C++ Developer XSquared's Avatar
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    Code:
       std::ifstream foo(argv[1]);
       std::istream fin(std::cin.rdbuf());
    
       if(foo)
       {
          fin.rdbuf(foo.rdbuf());
       }
    Naturally I didn't feel inspired enough to read all the links for you, since I already slaved away for long hours under a blistering sun pressing the search button after typing four whole words! - Quzah

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  15. #15
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    Quote Originally Posted by XSquared View Post
    Code:
       std::ifstream foo(argv[1]);
       std::istream fin(std::cin.rdbuf());
    
       if(foo)
       {
          fin.rdbuf(foo.rdbuf());
       }
    That works. Thanks all.
    ===
    Don't grumble about what you can't have;
    be grateful you don't get what you deserve.

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