Convert C program to C++ [closed] - c++

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Closed 10 years ago.
Help! I'm trying to figure out this code our professor gave us -
#include <stdio.h>
#include <string.h>
void encrypt(int offset, char *str) {
int i,l;
l=strlen(str);
printf("\nUnencrypted str = \n%s\n", str);
for(i=0;i<l;i++)
if (str[i]!=32)
str[i] = str[i]+ offset;
printf("\nEncrypted str = \n%s \nlength = %d\n", str, l);
}
void decrypt(int offset, char *str) {
// add your code here
}
void main(void) {
char str[1024];
printf ("Please enter a line of text, max %d characters\n", sizeof(str));
if (fgets(str, sizeof(str), stdin) != NULL)
{
encrypt(5, str); // What is the value of str after calling "encrypt"?
// add your method call here:
}
}
We are suppose to do the following:
Convert the C code to C++.
Add codes to the "decrypt" method to decipher the encrypted text.
Change the code to use pointer operations instead of array operations to encrypt and decrypt messages.
In the main method, call the "decrypt" method to decipher the encrypted text (str).
This is as far as I managed to go, but I'm pretty much stuck now. Especially since I have no background in the C language. Any help would be appreciated.
#include <iostream>
#include <string.h>
void encrypt(int offset, char *str)
{
std::cout << "\nUnencrypted str = \n" << str;
char *pointer = str;
while(*pointer)
{
if (*pointer !=32)
*pointer = *pointer + offset;
++pointer;
}
std::cout <<"\nEncrypted str =\n" << str << "\n\nlength = ";
}
void decrypt(int offset, char *str) {
// add your code here
}
void main(void) {
char str[1024];
std::cout << "Please enter a line of text max " << sizeof(str) << " characters\n";
if (fgets(str, sizeof(str), stdin) != NULL)
{
encrypt(5, str); // What is the value of str after calling "encrypt"?
// add your method call here:
}
}

Answer 1

The code you posted should work in C++ as well as C. There shouldn't be a need to "convert" anything, unless there are specific requirements that you haven't told us about.
Your array-to-pointer conversion looks correct, although I would argue that the code is more readable in the array form.
For your decrypt method, you will want to write code that does the inverse of what the encrypt method does. The best way to approach this is to run some sample text through encrypt and examine what the output looks like. The function transforms the input a single character at a time, so you should be able to map input to output on a byte-by-byte basis. With this information, you can detect the pattern and construct a function that makes the transformation in the other direction.

Answer 2

Most well formed C code is compilable as C++, rename the file .cpp, compile the code using C++ compilation and fix what breaks. The declaration of main() as returning void at least should break (it is at best questionable C code, and explicitly incorrect C++).
It all really depends on what your professor expects from you; the requirement to "convert it to C++" is too vague. What features of C++ are you expected to use?. While simple recompilation as C++ technically makes it C++ code (even if it is also valid C code), I somehow doubt that was the intention of the exercise.
The point is that while a superficial conversion by recompilation is possible, C++ offers opportunities for coding it differently. For example:
The header file names <stdio.h> and <string.h> are deprecated in C++, you might use <cstdio> and <cstring> instead. That said in this code <string.h> is redundant; none of the code is dependent on it.
If you use the non-deprecated headers, all the standard library is then in the std:: namespace, so all standard library symbols require scope resolution by prefixing them std:: or (less favourably) by using a using namespace std' directive.
The code uses the C standard library, which is also part of the C++ standard library, but C++ has alternatives that are in many ways superior. <cstdio> for example is largely replaced by <iostream> and its derivatives such as <stringstream> and <fstream>, and string handling and in fact a string data type is provided by <string>. The use <iostream> and <string> to implement this code could drastically simplify it.
If you were to use the std::string class, you might then use iterators to traverse the string content.
C++ supports OOP. In this case you might create a class that contains both encrypt and decrypt methods for example. Although the argument for doing so in this case is possibly weak other than perhaps to exemplify your understanding of the concept.
So you see the scope for "conversion" is very broad, from next to no work to a complete redesign. On the design note, one thing I would do is separate the encrypt/decrypt methods from the output operation. These methods would be reusable if they did not insist on outputting their results to the console. They would do better to return the data to the caller where the caller could do what it needed with it. Of course that too may be beyond the scope of this exercise is that is how the assignment were presented to you.