Change string by index

Question

I am a beginner in C++ and I am currently working with strings.

My question is why when compiling the code I'm providing below, I can get the string's characters when I use index notation, but cannot get the string itself using cout?

This is the code:

#include <iostream>
#include <string>

using namespace std;

int main()
{
    string original; // original message
    string altered; // message with letter-shift

    original = "abc";
    cout << "Original : " << original << endl; // display the original message

    for(int i = 0; i<original.size(); i++)
        altered[i] = original[i] + 5;

    // display altered message
    cout << altered[0] << " " << altered[1] << " " << altered[2] << endl;
    cout << "altered : " << altered << endl;

    return 0;
}

When I run this, the characters in the string altered are displayed correctly with this line:

cout << altered[0] << " " << altered[1] << " " << altered[2] << endl;

But the string itself is not displayed with this line:

cout << "altered : " << altered << endl;

I would like to know why this happens.

Answer 1

You have not resized your altered string to fit the length of the original string before the loop, thus your code exhibits undefined behavior:

altered[i] = original[i] + 5; // UB -  altered is empty

To fix this, resize altered before the loop:

altered.resize(original.size());

Or use std::string::operator+= or similar to append to altered:

altered += original[i] + 5;

This way, it can be empty before the loop, it will automatically resize itself to contain appended characters.

---

Explanation

The way UB is happening here, is that you're succeeding in writing the data in the static array, which std::string uses for short string optimization (std::string::operator[] does no checks if you're accessing this array past the std::string::size()), but std::string::size() remains 0, as well as std::string::begin() == std::string::end().

That's why you can access the data individually (again, with UB):

cout << altered[0] << " " << altered[1] << " " << altered[2] << endl;

but cout << aligned does not print anything, considering simplified operator<< definition for std::string looks functionally like this:

std::ostream &operator<<(std::ostream &os, std::string const& str)
{
    for(auto it = str.begin(); it != str.end(); ++it) // this loop does not run
        os << *it;

    return os;
}

In one sentence, std::string is not aware of what you did to its underlying array and that you meant the string to grow in length.

---

To conclude, <algoritm> way of doing this transformation:

std::transform(original.begin(), original.end(),
    std::back_inserter(altered), // or altered.begin() if altered was resized to original's length
    [](char c)
    {
        return c + 5;
    }

(required headers: <algorithm>, <iterator>)