Reversing a String in C++

Reversing a std::string is a common operation that can be accomplished efficiently using the standard library. Here's a look at a few methods, starting with the most straightforward and efficient:

1#include <iostream>
2#include <string>
3#include <algorithm>
4
5std::string reverse_string(std::string str) {
6  std::reverse(str.begin(), str.end());  
7  return str;
8}
9
10int main() {
11  std::string text{"Hello, World!"};
12
13  std::cout << "Original: " << text << '\n';
14  std::cout << "Reversed: "
15    << reverse_string(text) << '\n';
16
17  // In-place reversal
18  std::reverse(text.begin(), text.end());  
19  std::cout << "Reversed in-place: "
20    << text << '\n';
21}

1Original: Hello, World!
2Reversed: !dlroW ,olleH
3Reversed in-place: !dlroW ,olleH

The std::reverse() function from the <algorithm> header is the most efficient way to reverse a std::string. It works in-place, modifying the original string, and has a time complexity of $O(n/2)$, where $n$ is the length of the string.

If you need to preserve the original string, you can create a copy first:

1std::string reverse_string(const std::string& str) {
2  std::string reversed{str};
3  std::reverse(reversed.begin(), reversed.end());
4  return reversed;
5}

For educational purposes, here's how you might implement string reversal manually:

1std::string manual_reverse(
2  const std::string& str
3) {
4  std::string reversed;
5  // Preallocate space
6  reversed.reserve(str.length());
7  for (
8    auto it = str.rbegin();
9    it != str.rend();
10    ++it
11  ) {
12    reversed.push_back(*it);
13  }
14  return reversed;
15}

This manual method is less efficient than std::reverse() but demonstrates the concept clearly.

For Unicode strings, simple reversal might not work correctly due to multi-byte characters and combining characters. In such cases, you'd need a more sophisticated approach or a Unicode-aware library.

Remember, when working with std::string, it's often more efficient to modify the string in-place rather than creating a new string for each operation. The in-place reversal using std::reverse() is typically the most efficient method for reversing a string in C++.