Program crashes when using string instead of char array - c++

Question

I was trying to write a program to enter texts like passwords (display "*" instead of the character which in input by the user).
The problem is, when I use char arrays to store the password, the program works fine, but when I use a string class variable for the same purpose, my program crashes while displaying the string.
Here is the code:
// *********THIS CODE WORKS FINE***********
#include <iostream>
#include<string>
#include<conio.h>
int main()
{
using namespace std;
int i=0;
cout<<"Enter a password,press ENTER to finish writing"<<endl;
char passw[20];
passw[i]=getch();
while(passw[i]!=13)
{
i++;
cout<<"*";
passw[i]=getch();
}
passw[i+1]='\0';
cout<<"\nPassword is "<<passw;
return 0;
}
Now when I replace char passw[20] with string passw:
//**********THIS CODE CRASHES!!*********
#include <iostream>
#include<string>
#include<conio.h>
int main()
{
using namespace std;
int i=0;
cout<<"Enter a password,press ENTER to finish writing"<<endl;
string passw;
passw=getch();
while(passw[i]!=13)
{
i++;
cout<<"*";
passw[i]=getch();
}
cout<<"\nPassword is "<<passw;
return 0;
}
Can anybody explain why this is happening?
Just started with strings and there's just too much for me to know about strings :)

Answer 1

In string you can use string concatenation.
you can save your char and add it to your string variable.
char ch;
string pass;
ch=getch();
pass=pass+ch;

Answer 2

You're accessing a location (passw[i]) that is not yet existing in your std::string and you're not storing it correctly to a string. You should use the method passwd.push_back(char) to store it.
Check for reference: http://en.cppreference.com/w/cpp/string/basic_string/push_back

Answer 3

A string works differently compared to a char array. You probably know that char passw[20] in your first example creates an array with space for up to 20 chars. These can then be accessed using operator[] without a problem. A string, unlike a basic array, is a dynamic data structure, meaning it grows and shrinks over time as your program runs. Its content can also be accessed using operator[], however, this will not append to the string, it can only be used to change what is already there.
It's important to note that in c++, not all operations will perform what is known as bounds checking. In your second example, you access the i-th character of your string and write a character to that location. However, what would happen if you try to write, for instance, to the 10-th character of a string that only has space for 5? When doing so via operator[], the program will write to whatever location the 10-th character would be at, even though that part of memory is actually not part of the string. Anything could be there and will be overwritten by that operation. As you can probably imagine, this is not desirable at all and can lead to all sorts of problems later down the line. If you're lucky, your program will crash immediately on the write. It did not in your case, however, that's not a good thing; writing out of bounds can cause a heap corruption (the heap being a certain part of memory) which generally leads to a crash at some point down the line.
It's fine to use operator[] to change a strings characters if and only if you know whatever index you're accessing is within the strings size. Otherwise, you need to append to the string using an appropriate member function. These will enlarge the string if necessary.
You always need to pay attention when working with dynamic memory and what parts of it you're accessing (in your first example, for instance, you would also write out of bounds if your password is too long).