Helper functions for manipulating istreams (file, stdin, cin)

istream_helper_util.h

//Source code by Fabio Galuppo
//C++ MasterClass - https://www.linkedin.com/company/cppmasterclass - https://cppmasterclass.com.br/
//Fabio Galuppo - http://member.acm.org/~fabiogaluppo - [email protected]
//March 2025

#ifndef ISTREAM_HELPER_UTIL_H
#define ISTREAM_HELPER_UTIL_H

#include <functional>
#include <istream>
#include <optional>
#include <string>
#include <type_traits>
#include <utility>
#include <vector>

namespace istream_helper
{
    struct Util final
    {
        template<class CharT>
        [[nodiscard]] static inline bool check_error(std::basic_istream<CharT>& is) noexcept
        {
            if (is.fail() || is.bad()) return false;
            //else if (is.eof()) return true;
            return true;
        }

        template<class CharT, class UnaryOp>
        using result_type = std::optional<std::remove_reference_t<std::invoke_result_t<UnaryOp, std::basic_string<CharT>>>>;

        template<class CharT, class UnaryOp>
        [[nodiscard]] static inline result_type<CharT, UnaryOp> next(std::basic_istream<CharT>& is, UnaryOp&& unary_op) noexcept
        {
            std::basic_string<CharT> line;
            if (check_error(std::getline(is, line)))
                return std::forward<UnaryOp>(unary_op)(line);
            return std::nullopt;
        }

        template<class CharT>
        [[nodiscard]] static inline std::optional<std::basic_string<CharT>> next(std::basic_istream<CharT>& is) noexcept
        {
            return next<CharT>(is, std::identity{});
        }

        template<class CharT, class UnaryOp, class OutputIt>
        static inline OutputIt drain(std::basic_istream<CharT>& is, UnaryOp&& unary_op, OutputIt first) noexcept
        {
            while (auto opt = next(is, unary_op))
                *first++ = *opt;
            return first;
        }

        template<class CharT, class OutputIt>
        static inline OutputIt drain(std::basic_istream<CharT>& is, OutputIt first) noexcept
        {
            return drain<CharT>(is, std::identity{}, first);
        }

        template<class CharT, class UnaryOp>
        [[nodiscard]] static inline std::vector<result_type<CharT, UnaryOp>> drain(std::basic_istream<CharT>& is, UnaryOp&& unary_op) noexcept
        {
            std::vector<std::basic_string<CharT>> temp;
            drain(is, unary_op, std::back_inserter(temp));
            return temp;
        }

        template<class CharT>
        [[nodiscard]] static inline std::vector<std::basic_string<CharT>> drain(std::basic_istream<CharT>& is) noexcept
        {
            std::vector<std::basic_string<CharT>> temp;
            drain(is, std::back_inserter(temp));
            return temp;
        }
    };
}

#endif /* ISTREAM_HELPER_UTIL_H */