c++ - Substitution Cipher, Strings and Functions -

where stuck @ encryptstring function. encryptfile function take .txt file , convert .txt c string equivalent, pass encrpytstring function. when pass in, substitution function work cipher string , encrypt individual characters, spit out new string called encrypted_string. however, cannot function accept argument passed it. guidance?

this program is:

#include <iostream> #include <fstream> #include <string> #include <cstdlib>  using namespace std;  char substitution_cipher(string cipher_key, char char_to_encrypt); char reverse_substitution_cipher(string cipher_key, char char_to_encrypt);  string encryptstring(string &cipher_key, string string_to_be_encrypted); string decryptstring(string &cipher_key, string string_to_be_decrypted);  void rotatecipherkey(string &cipher_key); void displayfile(string filename);   void encryptfile(string cipher_key, string filename_from, string filename_to); void decryptfile(string cipher_key, string filename_from, string filename_to);  int main() {     string cipher_key = "qwertyuiopasdfghjklzxcvbnm";      encryptfile(cipher_key, "test.txt", "test-encrypted.txt");     decryptfile(cipher_key, "test-encrypted.txt", "test-ed.txt");      displayfile("test.txt");     displayfile("test-encrypted.txt");     displayfile("test-ed.txt");      system("pause");     return 0; }   //  rotate cipher key. example: abcdef becames bcdefa  void rotatecipherkey(string &cipher_key) {     rotate(cipher_key.begin(), cipher_key.begin() + 1, cipher_key.end()); }  // perform substitution cipher on single character  // using specified cipher key char substitutioncipher(string cipher_key, char char_to_encrypt) {     (int iii = 0; iii < cipher_key.length(); iii++)     {         rotatecipherkey(cipher_key);         char_to_encrypt = cipher_key[iii];     }     return char_to_encrypt; }  // perform "reverse" substitution cipher on single character  // using specified cipher key char reversesubstitutioncipher(string cipher_key, char char_to_decrypt) {     (int iii = 0; iii < cipher_key.length(); iii++)     {         rotatecipherkey(cipher_key);         char_to_decrypt = cipher_key[iii];     }     return char_to_decrypt; }  // encrypt string , return  // use substitutioncipher() function encrypt // individual characters //  // note: call rotatecipherkey() after each time encrypt  // character. string encryptstring(string &cipher_key, string string_to_be_encrypted) {     char *y = string_to_be_encrypted.c_str();     {         //substitutioncipher(cipher_key, string_to_be_encrypted);     }     cout << " " << string_to_be_encrypted;     string encrypted_string = string_to_be_encrypted;     return encrypted_string; }  // decrypt string , return  // use reversesubstitutioncipher() function decrypt // individual characters // // note: call rotatecipherkey() after each time encrypt  // character. string decryptstring(string &cipher_key, string string_to_be_decrypted) {     string decrypted_string = string_to_be_decrypted;      return decrypted_string; }  // display file specified filname parameter  void displayfile(string filename) {     string str;     ifstream infile;     infile.open(filename);     infile >> str;     while (infile)     {         cout << " " << str;         infile >> str;     }     cout << endl; }   // encrypt specified file using specified cipher key ,  // write output different file // function complete void encryptfile(string cipher_key, string filename_from, string filename_to) {     string input;     ifstream infile;     ofstream outfile;      infile.open(filename_from.c_str());     outfile.open(filename_to.c_str());      if (!infile)     {         cout << "can not open input file " + filename_from << endl;         exit(0);     }      if (!outfile)     {         cout << "can not open output file " + filename_to << endl;         exit(0);     }       while (getline(infile, input))     {         outfile << encryptstring(cipher_key, input) << endl;     }     infile.close();     outfile.close(); }  // decrypt specified file using specified cipher key ,  // write output different file // function complete void decryptfile(string cipher_key, string filename_from, string filename_to) {     string input;     ifstream infile;     ofstream outfile;      infile.open(filename_from.c_str());     outfile.open(filename_to.c_str());      if (!infile)     {         cout << "can not open input file " + filename_from << endl;         exit(0);     }      if (!outfile)     {         cout << "can not open output file " + filename_to << endl;         exit(0);     }       while (getline(infile, input))     {         outfile << decryptstring(cipher_key, input) << endl;     }     infile.close();     outfile.close(); } 

edit//

string encryptstring(string &cipher_key, string string_to_be_encrypted) {     char new_char;     (int iii = 0; iii < string_to_be_encrypted.length(); iii++)     {         new_char = substitutioncipher(cipher_key, string_to_be_encrypted[iii]);         rotatecipherkey(cipher_key);     }     string encrypted_string = string_to_be_encrypted;     cout << " " << encrypted_string;     return encrypted_string; } 

ok here new code modifications.

the problem have c_str() returns pointer const, const char*, can not assign char* in line

char* y=string_to_be_encrypted.c_str(); // cannot assign `const char*` `char*` 

use directly string perform work. there no other way of getting pointer non-const char data of std::string.