I am solving a problem currently. Here is the related code:
q3main.cc:
#include <iostream>
#include <fstream>
#include <string>
#include <cctype>
using namespace std;
#include "q3stringliteral.h"

void processLine(const std::string& line, std::ostream& out) {
    out << line << " : ";
    StringLiteral parser;
    bool valid_string_found = false;
    _Enable {
        try {
            for (char c : line) {
                parser.next(c);
            }
            parser.next(EOF); // Signal end of line
            if (!valid_string_found) {
                out << "no" << endl;
            }
        } catch (StringLiteral::Match& m) {
            valid_string_found = true;
            out << m.str << " yes" << endl;
            out << "  " << static_cast<int>(m.prefix) << " " << m.length << " \"";
            for (size_t i = 1; i < m.str.length() - 1; ++i) {
                char c = m.str[i];
                if (c == '\\') {
                    ++i;  // Move to the next character
                    if (i < m.str.length() - 1) {
                        switch (m.str[i]) {
                            case 'n': case 't': case 'r': case 'a': case 'b': case 'f': case 'v':
                                out << 'X';
                                break;
                            default:
                                out << m.str[i];
                        }
                    }
                } else if (std::isprint(c)) {
                    out << c;
                } else {
                    out << 'X';
                }
            }
            out << '"';
            if (m.str.length() < line.length()) {
                out << " -- extraneous characters \"" << line.substr(m.str.length()) << '"';
            }
            out << endl;
        } catch (StringLiteral::Error&) {
            if (!valid_string_found) {
                out << "no" << endl;
            }
        }
    }
}

int main(int argc, char* argv[]) {
    std::istream* infile = &std::cin;  // default value
    std::ostream* outfile = &std::cout;  // default value

    try {
        switch (argc) {
            case 2:
                infile = new std::ifstream(argv[1]);
                if (!*infile) {
                    throw std::runtime_error("Could not open input file");
                }
                break;
            case 1:  // defaults
                break;
            default:  // wrong number of options
                throw std::runtime_error("Invalid number of arguments");
        }
    } catch (const std::exception& e) {
        std::cerr << "Error: " << e.what() << std::endl;
        std::cerr << "Usage: " << argv[0] << " [ infile ]" << std::endl;
        exit(EXIT_FAILURE);
    }

    *infile >> std::noskipws;  // turn off white space skipping during input

    std::string line;
    while (std::getline(*infile, line)) {
        if (line.empty()) {
            *outfile << "Warning! Blank line." << std::endl;
        } else {
            processLine(line, *outfile);
        }
    }

    if (infile != &std::cin) delete infile;  // close file, do not delete cin!
}



q3stringliteral.cc:
#include "q3stringliteral.h"
#include <cctype>

void StringLiteral::main() {
    currentEncoding = C;
    inString = false;
    escapeNext = false;
    logicalLength = 0;

    _Enable {
        for (;;) {
            if (!inString) {
                buffer.clear();
                processedString.clear();
                logicalLength = 0;

                // Check for encoding prefix
                if (ch == 'L' || ch == 'U') {
                    currentEncoding = Integer;
                    buffer += ch;
                    suspend();
                } else if (ch == 'u') {
                    currentEncoding = Wide;
                    buffer += ch;
                    suspend();
                    if (ch == '8') {
                        currentEncoding = UTF8;
                        buffer += ch;
                        suspend();
                    }
                }

                if (ch == '"') {
                    inString = true;
                    buffer += ch;
                } else if (ch == EOF) {
                    suspend();
                } else {
                    _Resume Error() _At resumer();
                    suspend();
                }
            } else {
                if (ch == EOF) {
                    _Resume Error() _At resumer();
                    inString = false;
                    suspend();
                } else if (ch == '"' && !escapeNext) {
                    buffer += ch;
                    _Resume Match(currentEncoding, logicalLength, buffer) _At resumer();
                    inString = false;
                    suspend();
                } else {
                    if (escapeNext) {
                        escapeNext = false;
                        switch (ch) {
                            case 'n': case 'r': case 't': case 'v': case 'b': case 'f': case 'a':
                            case '\\': case '\'': case '"': case '?':
                                processedString += '\\';
                                processedString += ch;
                                logicalLength++;
                                break;
                            default:
                                _Resume Error() _At resumer();
                                suspend();
                        }
                    } else if (ch == '\\') {
                        escapeNext = true;
                    } else {
                        processedString += ch;
                        logicalLength++;
                    }
                    buffer += ch;
                }
                suspend();
            }
        }
    }
}

q3stringliteral.h:
#ifndef STRINGLITERAL_H
#define STRINGLITERAL_H

#include <uC++.h>
#include <string>

_Coroutine StringLiteral {
public:
    enum Encoding { C, Wide /* u */, Integer /* U or L */, UTF8 /* u8 */ };

private:
    char ch; // character passed by cocaller

    // Added private members
    std::string buffer;
    std::string processedString;
    unsigned int logicalLength;
    Encoding currentEncoding;
    bool inString;
    bool escapeNext;
    
    void main(); // coroutine main

public:
    _Exception Match { // last character match
    public:
        Encoding prefix; // string encoding
        unsigned int length; // string length
        std::string & str; // string value
        Match( Encoding prefix, unsigned int length, std::string & str )
            : prefix{prefix}, length{length}, str{str} {}
    };

    _Exception Error {}; // last character invalid

    void next( char c ) {
        ch = c; // communication input
        resume(); // activate
    }

    // Added public destructor
    ~StringLiteral() {}
};

#endif // STRINGLITERAL_H

For my current program, when I input:
"a\n"

The expected output is: 
"a\n" : "a\n" yes
  0 2 "aX"

And my current output is: 

The issue is probably that the string literal is not being stored and processed correctly for length or it's escape sequences.

The original question is as follows:
verifies a string of characters corresponds to a C++ string-literal described by the following grammar:
string-literal :
encoding-prefixopt " s-char-sequenceopt "
encoding-prefixopt :
u8 | u | U | L | ǫ (empty string)
s-char-sequenceopt :
s-char | s-char-sequenceopt s-char | ǫ
s-char :
any character except double-quote ("), backslash (\), or new-line | escape-sequence
escape-sequence :
simple-escape-sequence | octal-escape-sequence | hexadecimal-escape-sequence
simple-escape-sequence :
\’ | \" | \? | \\ | \a | \b | \f | \n | \r | \t | \v
octal-escape-sequence :
\ octal-digit | \ octal-digit octal-digit | \ octal-digit octal-digit octal-digit
octal-digit :
0 | 1 | 2 | 3 | 4 | 5 | 6 | 7
hexadecimal-escape-sequence :
\x hexadecimal-sequence
hexadecimal-sequence :
hexadecimal-sequence hexadecimal-digit | hexadecimal-digit
hexadecimal-digit :
0 | 1 | 2 | 3 | 4 | 5 | 6 | 7 | 8 | 9 | a | b | c | d | e | f | A | B | C | D | E | F
Each hexadecimal escape sequence must be the longest sequence of characters that can constitute the escape
sequence. The following are examples of valid and invalid C++ string literals:
valid invalid
"" "
"a\n" "a
"abc" ab"
"xyz\012" "\\\"
"xyz\x000c" "\9"
L"www" l"www"
To simplify parsing, assume a valid string literal starts at the beginning of an input line, i.e., there is no leading
whitespace. No checking is required for this assumption and no test data contains examples of this form. See
the C library routine isxdigit(c), which returns true if character c is a hexadecimal digit. An octal or hexadecimal
escape sequence represents a single character. Therefore, the escape sequence must be converted from multiple
characters to its single character value. There is a cheap and cheerful C pattern to convert octal/hexadecimal
digits from characters to its value. Marks will be deducted for expensive approaches for converting characters
to values, such as stringstream.
After creation, the coroutine is resumed with a series of characters from a string (one character at a time).
Eventually, the coroutine raises one of the following exceptions at its resumer:
• Match means the characters form a valid string and the exception instance contains detailed information
about the string.
• Error means the last character forms an invalid string.
When generating the return string convert any unprintable characters (isprint(c)) into ’X’, otherwise the unprint-
able characters can cause problems with shell tools. After the coroutine raises an exception, at its last resumer,
it must terminate and NOT be resumed again; sending more characters to a terminated coroutine generates an
error.


Can you figure out where the errors are and fix my program