[SPAM] istream_iterator_limited

From: Frederick Gotham (cauldwell.thomas_at_[hidden])
Date: 2019-10-28 08:22:22

- - - - 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;
}