Why is there a question mark at the end of the string? - c++

Question

Please help me to figure out why there is a question mark at my string output. I have been working on this simple reverse a string practice. My code can do it correctly. Then I try to store the reversed string into an array and then convert this array to a string. So here is something wired happened. There is always a question mark at the end of that string. Please teach me the reason of it, and how to get rid of this question mark. Here is my code. Thank you so much
#include <string>
using namespace std;
int main()
{
cout<<"Please enter a string."<<endl;
string str;
cin >> str;
int i=0;
int length = str.length();
char arr[length];
//cout<<length;
while (length != 0) {
arr[i] = str.at(length-1);
length--;
cout<<arr[i];
++i;
}
cout<<endl;
string stri(arr);
cout<<endl<<stri<<endl;
cout<<stri[4];
return 0;
}

Answer 1

A string in C (or many other languages) needs to be terminated by a '\0'. After all, you don't know how large the memory your char* points to is. So you need char[length + 1]. Also, variable-length arrays aren't part of C++. You need to use new[]/delete[] or malloc()/free():
char * arr = new char[length + 1]; // enough space for string + '\0'
char[length] = '\0'; // terminate the string with '\0'
while (length != 0) {
// ... // reverse
}
cout << endl;
string stri(arr); // create the new string
delete[] arr; // don't forget to deallocate the memory
However, if you're dropping down to manual memory allocation, you're usually missing something from the standard library. And indeed, you could simply use the right constructor (it's (4), simplified below):
template <class InputIt>
string::string(InputIt first, InputIt last);
And luckily, std::string provides input iterators that traverse the string backwards via std::string::rbegin() and std::string::rend(). Now your code gets a lot easier:
#include <iostream>
#include <string>
int main()
{
std::cout << "Please enter a string." << std::endl;
std::string str;
std::cin >> str;
std::cout << std::endl;
// Create the new reversed string directly:
std::string reversed_string(str.rbegin(), str.rend());
std::cout << reversed_string << std::endl;
return 0;
}

Answer 2

char arr[length];
should be
char arr[length + 1];
EDIT: or rather (as Jonathan Potter points out, since length is not a compile time constant, your code likely only compiles because this is permitted by your specific compiler, e.g. GNU C++):
char *arr = new char[length + 1];
(and delete [] arr; at some point)
to store the terminating '\0':
arr[length] = '\0';