When should I use std::optional in C++?

You should consider using std::optional in C++ when you have a value that may or may not be present, such as:

Function return values

If a function might not always return a meaningful value, std::optional is a good way to represent that. For example:

1#include <optional>
2#include <string>
3
4std::optional<std::string> GetMiddleName(
5  const std::string& fullName) {
6  // Parse full name
7  // if middle name found, return it
8  // else return empty optional
9  return std::nullopt;
10}

Class members

If a class has a field that is not always applicable or known at construction time, std::optional is a safe way to model that:

1#include <optional>
2#include <string>
3
4class Character {
5  std::string name;
6
7  // age may not be known
8  std::optional<int> age;
9};

Optional function arguments

While default arguments are often used for this, std::optional can make it more explicit:

1#include <optional>
2
3void SetVolume(std::optional<double> volume) {
4  if (volume) {
5    // set volume to *volume
6  } else {
7    // set to default volume
8  }
9}

In general, std::optional is useful when you need to distinguish between a value not being present vs the value being present but potentially 0, empty string, null pointer etc. It makes your code's intent clearer.