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std::search question...

Hi,

Can anybody help me with the following piece of code?

The purpose behind the code is to parse HTML files, strip out
the tags and return the text between tags. This is part of a larger
application which will perform "searches" for text values in a
directory of html files, trying to match only the non-tagged text
in the documents.

My approach was to customize a istreambuf_iter ator which would
iterate over the text characters in the HTML bodies, and ignore the html
tags. I did
this by deriving from the std istreambuf_iter ator, overriding the ++
operators to parse the underlying input to screen the data and return the
next valid character. The iteration seems to work ok on some simple
HTML test data - I've seen the tags stripped away and the text body get
through.

Now here's the clever part :). I thought I could use the std algorithm
search
directly, plug in the range of the stream and the range of the text string
to
look for, and there you go. Unfortunately, this doesn't work ! The search
always fails to find the search string.

Am I missing something about the search? I thought the requirements for
search only required the iterators be forward iterators - is there something
I've
misunderstood here ? I'd like to use this code, it strikes me as a nice
solution
utilizing the power of STL and the stream classes.

Many thanks for any help!

dave.

#include <algorithm>
#include <fstream>
#include <istream>
#include <iostream>
#include <algorithm>
#include <vector>
using namespace std;

class htmlstreambuf_i terator : public istreambuf_iter ator<char>
{
public:
htmlstreambuf_i terator( ifstream& stream)
:istreambuf_ite rator<char>(str eam),_intag(fal se)
{
// advance to first "legal" character in input.
while(
!istreambuf_ite rator<char>::eq ual(istreambuf_ iterator<char>( )) )
{

char c = istreambuf_iter ator<char>::ope rator*();
if ( c == '<' )
{
_intag=true;
incr();
break;

}
else if (!_intag )
{

break;

}

}

}
htmlstreambuf_i terator( )
:istreambuf_ite rator<char>(), _intag(false)
{}
htmlstreambuf_i terator( const htmlstreambuf_i terator& iter)
:istreambuf_ite rator<char>( iter), _intag(false)
{}

char operator*()
{
printf("operato r*() %c\n", istreambuf_iter ator<char>::ope rator*() );
return istreambuf_iter ator<char>::ope rator*();

}

const char* operator->()
{
return istreambuf_iter ator<char>::ope rator->();
}


htmlstreambuf_i terator& operator++()
{
incr();
return *this;
}
htmlstreambuf_i terator operator++(int)
{
htmlstreambuf_i terator tmp=*this;
incr();
return tmp;

}

bool equal( htmlstreambuf_i terator& rhs )
{
return istreambuf_iter ator<char>::equ al(rhs);
}

private:
void incr()
{
while( !istreambuf_ite rator<char>::eq ual(istreambuf_ iterator<char>( )) )
{
// advance and skip over any tag data to next legal character.
istreambuf_iter ator<char>::ope rator++();
char c = istreambuf_iter ator<char>::ope rator*();
if ( c == '<' && !_intag )
{
_intag = true;

}
else if ( c == '>' && _intag )
{
_intag = false;
}
else if (!_intag )
{
break;
}

}

}
private:

bool _intag;

};

int main(int argc, char* argv[])
{
ifstream input( argv[1] );

htmlstreambuf_i terator iter(input);
htmlstreambuf_i terator iterend;

string searchtext("D") ;
htmlstreambuf_i terator pos = search( iter, iterend,

searchtext.begi n(), searchtext.end( ) );

// pos always points to end of input. !!!
bool found = pos != iterend ;
return 0;
}

Jul 23 '05 #1
1 3062
Dave Townsend wrote:
My approach was to customize a istreambuf_iter ator which would
iterate over the text characters in the HTML bodies, and ignore the html tags. I did
this by deriving from the std istreambuf_iter ator, overriding the ++
operators to parse the underlying input to screen the data and return the next valid character.
You would be better off wrapping 'istreambuf_ite rator' into a custom
iterator class as this class is not intended to be derived from. In
particular, you did not override 'operator++()' but overload this
operator: 'std::istreambu f_iterator' does not have any virtual
functions
and thus you cannot override anything. A class using public inheritance
from 'std::istreambu f_iterator' is rather error prone and subject e.g.
to slicing (e.g. when being passed to a function taking an
'std::istreambu f_iterator' as argument).
Am I missing something about the search?
Yes, you do: 'std::search()' takes forward iterators, not input
iterators. However, 'std::istreambu f_iterator' is an input iterator.
It is not too hard to create a wrapper for 'std::istreambu f_iterator'
which allows forward iteration (in particular allows the possibility
to recover to some position).
I thought the requirements for
search only required the iterators be forward iterators - is there something I've misunderstood here?


Your misunderstandin g seems to be with 'std::istreambu f_iterator':
this is not a forward iterator. It is an input iterator. The key
difference is that input iterators only support single pass while
forward iterators allow multiple passed (plus the possibility to
write to the sequence which is not needed in this case).
--
<mailto:di***** ******@yahoo.co m> <http://www.dietmar-kuehl.de/>
<http://www.contendix.c om> - Software Development & Consulting

Jul 23 '05 #2

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