std::counted_iterator already exists, does it not fulfill your needs?
------- Original Message -------
On Monday, April 18th, 2022 at 3:19 PM, Frederick Virchanza Gotham via Std-Proposals <std-proposals@lists.isocpp.org> wrote:


We can read from "cin" or any file (fstream) as though it were a container in volatile memory, using istream_iterator. But we can't put a limit on how many char's to take from the istream.

Sometimes there is an alternative function available to use, e.g. 'copy_n' instead of 'copy', but sometimes there isn't.

I propose a new template class istream_iterator_limited(N) that will stop reading from the istream after N characters.

- - - - 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 the function 'boost::algorithm::hex'.
Unfortunately I couldn't do this because 'hex' takes two iterators [begin,end), and there's no alternative function called "hex_n". So I had to read the file into a string object first with 'copy_n', and then pass the string into the 'hex' function.

- - - - 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,32u),
istream_iterator_limited<char>(), g_some_global_container.begin() );

- - - - HOW I CODED IT:

#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(1u)
{
/* The initialiser list above does all the construction work */
}

istream_iterator_limited(void) /* This gives the End-Of-Stream iterator */
: Base() , m_limit(0u), m_current_count(1u)
{
/* 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;
}
};

- - - - HOW I TESTED IT:

#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;
}