Accessing the value in a std::optional

There are several ways to access the value stored in a std::optional, each with its own use case:

value()

This returns a reference to the contained value. If the optional is empty, it throws a std::bad_optional_access exception. Use this if you are certain the optional contains a value, and you want an exception if it doesn't.

1#include <iostream>
2#include <optional>
3
4int main() {
5  std::optional<int> maybe_int = 10;
6  std::cout << maybe_int.value();   // prints 10
7
8  maybe_int = std::nullopt;
9  // throws std::bad_optional_access
10  std::cout << maybe_int.value();
11}

operator* and operator->

These also return a reference to the contained value, but they do not check whether the optional contains a value. Use these if you are certain the optional contains a value, and you want undefined behavior if it doesn't.

1#include <iostream>
2#include <optional>
3
4int main() {
5  std::optional<int> maybe_int = 10;
6  std::cout << *maybe_int;  // prints 10
7
8  maybe_int = std::nullopt;
9
10  // undefined behavior
11  std::cout << *maybe_int; 
12}

value_or()

This returns the contained value if the optional has one, otherwise it returns the argument. Use this if you want to provide a default value in case the optional is empty.

1#include <iostream>
2#include <optional>
3
4int main() {
5  std::optional<int> maybe_int = 10;
6
7  // prints 10
8  std::cout << maybe_int.value_or(0) << ", ";
9
10  maybe_int = std::nullopt;
11
12  // prints 0
13  std::cout << maybe_int.value_or(0);
14}

110, 0

In general, use value() when you expect the optional to have a value and you want an exception if it doesn't. Use operator* or operator-> if you are absolutely certain the optional has a value. Use value_or() when you want to provide a default in case the optional is empty.