Re: c++ conversion files
From: Jonathan Turkanis (technews_at_kangaroologic.com)
Date: 12/06/04
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Date: Mon, 6 Dec 2004 13:04:29 -0700
<jcoffin@taeus.com> wrote in message
news:1102360105.134738.14420@z14g2000cwz.googlegroups.com...
> Hi Johathan,
Hi.,
> To me, your code
Which code?
> looks far more impenetratable than self-explanatory.
> If I had to guess at what it did, I'd have guessed at it treating
> UNIX-style new-lines as the default. It associates Windows-style
> new-lines with the input, and nothing in particular with the output, so
> from reading the code, I'd have guessed this was supposd to do
> Windows->UNIX conversion.
I assume you mean this:
> using namespace std;
> using namespace boost::io;
>
> ifstream in("src", ios::binary | ios::in);
> ofstream out("dest", ios::binary | ios::out);
>
> filtering_istream fin;
> fin.push(newline_filter(newline::windows));
> fin.push(in);
> boost::io::copy(fin, out);
> I've reread your code a number of times, and I'm _utterly_ lost as to
> what you think would give the reader even the faintest hint that it
> does anything related to MacOS at all.
Well,
- filtering_istream is an input stream which can zero or have one or more
filters added to it.
- newline_filter is a filter which, quoting from the documentation I cited,
"converts between the text file formats used by various operating systems. Its
sole constructor takes an integral flag parameter used to specify the source and
target formats "
- newline::windows is an integral flag, which is "Useful for converting data to
the Windows format"
Maybe you would be happier if the constant newline::windows were renamed
newline::convert_to_windows_format or newline::convert_to_crlf? Perhaps that's
not a bad idea, but I like shorter names.
> By contrast, looking at:
>
> FILE *fin = fopen("src", "rb");
> FILE *fout = fopen("dest", "wb");
> int ch;
>
> while ( EOF!=(ch=getc(fin))) {
> putc(ch, fout);
> if (ch == '\r')
> putc('\n', fout);
> }
>
> The actual conversion seems (to me) almost impossible to miss. The only
> part open to any question at all is whether the reader realizes what
> OSes are associated with CR-only vs. CR/LF line-ends. I'd have to
> guess, however, that any programmer who cares to read Mac text files
> under Windows would immediately recognize what it does.
First, your code is hard-wired to use FILEs. My library works for any types
modeling the concepts Source and Sink.
http://home.comcast.net/~jturkanis/iostreams/libs/io/doc/?path=5.1
Second, converting line endings is common enough that it's worth encapsulating.
> As far as extensibilty goes, consider transferring a document from text
> format into a word processor. For the sake of discussion, assume we
> want to convert each paragraph into (in essence) one long line so the
> word processor can change line breaks as needed for the margins being
> used. If there are two or more new-lines in a row, one is retained to
> mark the end of the paragraph, but all other new-lines are deleted.
Right, newline filter doesn't do this, and doesn't pretend to.
> Now, I don't understand your code well enough to say it can't do this
> job. OTOH, if it takes more than about a minute to figure out how to
> get your code to do it, doing so is more trouble than directly writing
> the code to do so.
Basically, you can take whatever code you would write in your "while (
EOF!=(ch=getc(fin)))" loop, replace getc, putc, etc. with their generic
counterparts boost::io::get, boost::io::put, etc., stick the code into the body
of a filter member function, and Voila! --you have a reusable component.
E.g., for converting from mac to windows, based on your code:
using namespace boost::io;
struct convert_to_windows : boost::io::output_filter {
template<typename Sink>
void put(Sink& sink, char c)
{
boost::io::put(Sink, c);
if (ch == '\r')
boost::io::put(Sink, '\n');
}
};
For the word-processor example, assuming all newlines have been converted to
'\n' using the (chainable) newline_filter, we can write:
struct remove_excess_newlines : boost::io::output_filter {
remove_excess_newlines() : last_char_was_newline_(false) { }
template<typename Sink>
void put(Sink& sink, char c)
{
bool current_char_is_newline = c == '\n';
if (!current_char_is_newline || !last_char_was_newline_)
boost::io::put(Sink, c);
last_char_was_newline_ = current_char_is_newline;
}
bool last_char_was_newline_;
};
This is untested, but I'm sure something equally trivial will do.
> Later,
> Jerry.
Jonathan
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