Why cant I use std::copy with std::string to another std::string?

Question

The following code prints an empty string and i am unable to understand why it is so?

#include <string>
#include <algorithm>
#include <iostream>

int main()
{
    std::string s="hello";
    std::string r;
    std::copy(s.rbegin(),s.rend(), r.begin());
    std::cout<<r;
    return 0;
}

No i am not looking to copy to a vector of chars. I am looking at copying the string to another string. I am aware of other string copying mechanisms but i expected this to work as well. — Argha Sen, Aug 16 '20 at 13:42
`std::copy` doesn't allocate memory. You have the resize the container. You can `std::string r(s.length(), '\0');` — Thomas Sablik, Aug 16 '20 at 13:45
Because `r` is empty, and the only thing that can be copied into an empty string is another empty string. Is there any specific reason why `r=s;` will not work for you? — Sam Varshavchik, Aug 16 '20 at 13:46
There's also a constructor for `std::string` that uses iterators. You can do `std::string r(s.rbegin(), s.rend());` to get this effect, bypassing `std::copy`. — JohnFilleau, Aug 16 '20 at 13:49
@sam varshachik i was doing it more as a learning experience — Argha Sen, Aug 16 '20 at 13:51

score 6 · Accepted Answer · answered Aug 16 '20 at 13:46

The problem is r is an empty std::string, it contains no chars. std::copy is trying to copy-assign chars since r.begin(), which leads to UB.

You can make r containing 5 elements in advance.

std::string r(5, '\0');

Or

std::string r;
r.resize(5);

Or use std::back_inserter.

std::string r;
std::copy(s.rbegin(),s.rend(), std::back_inserter(r));

Why cant I use std::copy with std::string to another std::string?

1 Answers1