istream_iterator_limited

From: Frederick Gotham (cauldwell.thomas_at_[hidden])
Date: 2019-10-31 08:42:22

[MODERATOR: I tried to post this a week ago, but for some reason the
word SPAM was prefixed to the subject line and so it didn't show up on
the GMane server. I don't think many people actually saw it.]

- - - - WHY I NEEDED THIS:

I wanted to open a file using "ifstream", and then I wanted to read 32
bytes from the file and feed them directly into boost::algorithm::hex.
Unfortunately I couldn't do this because 'hex' takes two iterators
[begin,end), and there's no alternative function "hex_n". So I had to
read the file into a string first, and then pass the string to 'hex'.

- - - - THE IDEA I CAME UP WITH:

Open the file using "ifstream", and then use a special iterator that
will stop reading from the file after 32 bytes, with syntax like this:

    ifstream logfile("log.txt");

    hex( istream_iterator_limited<char>(logfile,32),
istream_iterator_limited<char>(), g_some_global_container.begin() );

- - - - SAMPLE CODE:

#include <cstddef>  /* size_t */
#include <iterator> /* istream_iterator */

template <typename T>
class istream_iterator_limited : public std::istream_iterator<T> {
protected:
        std::size_t const m_limit;
        std::size_t m_current_count;

public:
        /* Shorter names */
        typedef std::istream_iterator<T> Base;
        typedef istream_iterator_limited<T> This;

        istream_iterator_limited(typename Base::istream_type
&obj_stream, size_t const arg_limit)
                : Base(obj_stream), m_limit(arg_limit), m_current_count
(1)
        {
                /* The initialiser list above does all the construction
work */

        }

        istream_iterator_limited(void)  /* This gives the End-Of-Stream
iterator */
                : Base() , m_limit(0), m_current_count(1)
        {
            /* The initialiser list above does all the construction work
*/
        }

        This &operator++(void)
        {
                if ( m_limit <= m_current_count )
                {
                        Base::operator=( Base() );  /* Becomes End-Of-
Stream iterator */
                }
                else
                {
                    ++m_current_count;
                    Base::operator++();
                }

                return *this;
        }
};

#include <algorithm> /* copy */
#include <string>  /* string */
#include <sstream>  /* istringstream */
#include <iostream>  /* cout, endl */
#include <iterator>  /* back_inserter */

auto main(void) -> int
{
    std::istringstream iss("123456789");

    std::string str;

    /* Let's use copy to read 5 char's into our string */
    std::copy( istream_iterator_limited<char>(iss,5),
istream_iterator_limited<char>(), std::back_inserter(str) );

    std::cout << str << std::endl;

    str.clear();

    /* Get another 3 */
    std::copy( istream_iterator_limited<char>(iss,3),
istream_iterator_limited<char>(), std::back_inserter(str) );

    std::cout << str << std::endl;
}