C++ does what it wants | Char[] cout and inverted char[]

Question

I'm actually working on a C++ Invert String/Char[] Methods

#include <iostream>

using namespace std;

void inverse(string input)
{
    int length = input.length();
    char* output = new char[length];

    for(int i = 0; i < length; i++)
    {
        output[length - (i + 1)] = input[i];
    }

    cout << output << endl;
    delete output;
}

int main(int argc, char *argv[])
{
    while(true)
    {
        string in;
        cin >> in;
        inverse(in);
    }

    return 0;
}

The problem is, when i enter a string with 3/5/7 and so on letters, then it will invert, then if its correct, but if i enter a string length of 2/4/6 and so on characters there inverted string has random chars on his and and only when i enter these length of number.

I'm so confused because this error only appears with even numbers.

Here's a little example: Here's a little example:

Here is the new code (everything works fine here), i know it has something to do with the /0 on the end of the array but why only the even numbers.

#include <iostream>

using namespace std;

void inverse(string input)
{
    int length = input.length();
    char* output = new char[length + 1];

    for(int i = 0; i < length; i++)
    {
        output[length - (i + 1)] = input[i];
    }

    output[length] = '\0';
    cout << output << endl;
    delete output;
}

int main(int argc, char *argv[])
{
    while(true)
    {
        string in;
        cin >> in;
        inverse(in);
    }

    return 0;
}

Can anybody help me find a solution for this?

Answer 1

Your string is not null-terminated. Try

void inverse(string input)
{
    reverse( input.begin(), input.end() );
    cout << input << endl;
}

For this to work, you need to include the algorithm header which defines the std::reverse() function.

The reason it does not work for even numbers could be the fact, that memory allocations usually reserve a little more memory, so that memory boundaries are properly aligned. Therefore, rounding up to even numbers is not a waste. That's just a guess. However, I would strongly recommend you avoid undefined behavior.

Answer 2

You got garbage at the end of your string because it wasn't ended with \0. Printing such strings is an undefined behavior in c++.

You were just lucky that your code worked for even lengths. On my machine, for example, it works good for lengths of 1,2,3,5,6,7 and 8 and prints garbage for 4.

UP: Actually, try to avoid the UB: the c++ standard does not specify what happens when you have the UB - theoretically, you program may crash, or your computer may blow up, or a demon may appear from your nose. In this particular case I would recommend using std::string as a container for output if you, for some reason, cannot use std::reverse or manually do all the work in-place.