core/rbfor1.cpp

The following code example is taken from the book
C++23 - The Complete Guide by Nicolai M. Josuttis, Leanpub, 2026
The code is licensed under a Creative Commons Attribution 4.0 International License.

---

// raw code

#include <iostream>
#include <string>
#include <optional>

std::optional<std::string> getOptString(int num)
{
  if (num <= 0) {
    return std::nullopt;  // return no value
  }
  // return a test string with num characters:
  return std::string(num, '+');
}

int main()
{
  // OK:
  std::cout << getOptString(20).value() << '\n';

  // RUNTIME ERROR (undefined behavior) before C++23:
  for (const auto& elem : getOptString(20).value()) {
    std::cout << elem << ' ';
  }
  std::cout << '\n';
}