Converting std::vector to char*

Question

I am a c++ student working on a project to receive and encode a message with a simple cipher. To do so, the program accepts each word as a string, converts the string into a vector of characters, and then modifies each character before storing it in a file. In the line that returns the encoded message to the main function, I get this error message: < cannot convert std::vector<char, std::allocator<char> >' to char*' for argument 1' to char* cipher(char*, int)' . Despite this error, the program will run, however, it will simply stop after a word has been entered and end the program. This is the code I am using:

// Cipher.cpp
// This program accepts a message and encodes it 
// Written by Will Grindle

#include<iostream.h>
#include<string.h>
#include<iomanip.h>
#include<math.h>
#include<fstream.h>
#include<vector.h>

using namespace std;

string get_message(); // function prototypes
char* cipher(char broken[], int length);

int main()
{
   int index, length; // declare index for vectors

   string message; // declare string

   ofstream outfile; // declare filestream

   outfile.open("MESSAGE.DAT", ios::in); // create file and open it

   message = get_message(); // use function to retrieve data

   length = message.length(); // find length of message

   // declare vector for breaking the strings down
   vector <char> broken(message.begin(), message.end()); 

   vector <char> encoded(50); // declare vector for encoded message

   encoded[index] = cipher(broken, length); // initialize encoded to new         message

   for(index = 0; index <= length - 1; index++)// loop for displaying values
   {
      cout << encoded[index] << endl;
   }

   if(outfile) // if there is no error with the file
   {
      for(index = 0; index <= length - 1; index++) // loop for writing encoded
      {                                            // message to file
         outfile << encoded[index] << endl;
         cout << "Data written to file." << endl;
      }
   }
   else
   {
      cout << "Error opening file." << endl; 
   }         

   outfile.close(); // close file

   system("pause");
   return 0;
}

string get_message()
{
   string entry; // declare string

   cout << "Please enter the word to be entered." << endl; // request entry
   cin >> entry; // user enters word

   system ("pause");

   return entry;
}

char* cipher(char broken[], int length)
{
   char index; // declare index

   if( broken[index] < 123 && broken[index] > 96 ) // if characters are lowercase, 
   {                                               // make them uppercase
      broken = broken - 32;
   }

   for(index = 0; index <= length - 1; index ++)
   {
      broken[index] = broken[index] * (2/3); 
      broken[index] = broken[index] - 12;
      broken[index] = broken[index] ^ 2;
   }

   cout << "Message encoded." << endl;

   system ("pause");

   return(broken);
} 

I am open to any suggestions on how to make this work. Thank you!

Answer 1

In C++, a vector isn't actually an array of chars, so you can't pass it into a function expecting a char array. You have a few options here.

First, I'd recommend updating your code so that the cipher function takes in as a parameter either a vector or a std::string. These objects are safer than raw arrays because they know their sizes and can do reallocations if you need them to grow.

Second, you could change the call to cipher to pass in a pointer to the vector's underlying array, like this:

cipher(broken.data(), broken.size());

I think this is a less elegant and more error-prone solution, but you're welcome to use it if you'd like.

Answer 2

Your code looks more like C...

Maybe start learning the basic data types and data structures first.

Then make sure you understand the usage of the vector template

This seems good as well: Differences between C and C++ It also points out some things about style and what you could do, but shouldn't do.

And of course this Programmers.StackExChange Post: What are the fundamental differences between C and C++?

Try to use a modern C++ IDE such as CodeBlocks, Eclipse, Netbeans, ...; This will help you to prevent some faults at the beginning.