Function unget

3rd/nlohmann_json/single_include/nlohmann/json.hpp:7994–8018

! @brief unget current character (read it again on next get) We implement unget by setting variable next_unget to true. The input is not changed - we just simulate ungetting by modifying chars_read_total, chars_read_current_line, and token_string. The next call to get() will behave as if the unget character is read again. */

    behave as if the unget character is read again.
    */
    void unget()
    {
        next_unget = true;

        --position.chars_read_total;

        // in case we "unget" a newline, we have to also decrement the lines_read
        if (position.chars_read_current_line == 0)
        {
            if (position.lines_read > 0)
            {
                --position.lines_read;
            }
        }
        else
        {
            --position.chars_read_current_line;
        }

        if (JSON_HEDLEY_LIKELY(current != std::char_traits<char_type>::eof()))
        {
            JSON_ASSERT(!token_string.empty());
            token_string.pop_back();
        }
    }

    /// add a character to token_buffer
    void add(char_int_type c)