lib/orelse.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>

template<typename ValT>
std::ostream& operator<< (std::ostream& strm, const std::optional<ValT>& opt)
{
  if (opt) {
    return strm << *opt;   
  }
  else {
    return strm << "<novalue>";  
  }
}

std::optional<int> asOptInt(const std::string& s) // returns int or "no value"
{
  try {
    return std::stoi(s);
  }
  catch (...) {
    return {};
  }
}

int main()
{
  using OptString = std::optional<std::string>;

  for (auto myOpt : {OptString{"42"}, OptString{"data"}, OptString{std::nullopt}}) {

    std::cout << "myOpt: " << myOpt << '\n';

    auto opt = myOpt.or_else([]{
                               std::cout << "there is no string\n";
                               return std::optional<std::string>{"fallback data"};
                             })
                    .and_then(asOptInt)
                    .or_else([]{
                               std::cout << "there is no int\n";
                               return std::optional<int>{42};
                             });

    std::cout << "    => " << opt << "\n\n";
  }
}