C++ string returning as "

So I am trying to write a function that reverses a string in C++. I’ve figured out most of the code, and when I cout the string, its showing what I need. But when I test the code, the result is that I got: " from the function. Here’s my code:

#include <iostream>
#include <string>
using namespace std; 
string result;
string reverseString (string str)
{
  for(int i = str.length(); i >= -1; i--) {
    result+= str[i];
  }
  cout << result;
  return result;
}

>Solution :

In this for loop in the first its iteration

  for(int i = str.length(); i >= -1; i--) {
    result+= str[i];
  }

the terminating zero character '\0' is written in the first position of the object result because the expression str[i] is equivalent in this case to the expression str[str.length()].

So the result string can be outputted as an empty string.

Also you are trying to access the source string using the negative index -1 that results in undefined behavior.

Instead of this for loop you could just write

result.assign( str.rbegin(), str.rend() );

If you want to do the task using the for loop then the loop can look for example the following way

  result.clear();
  result.reserve( str.length() ); 

  for ( auto i = str.length(); i != 0; ) {
    result += str[--i];
  }

Pay attention to that it is a bad idea to use the global variable result within the function. The function could look like

std::string reverseString( const std::string &str )
{
    return { str.rbegin(), str.rend() };
}