Checking if a std::optional has a value

There are several ways to check if a std::optional contains a value:

has_value()

This is an explicit way to check. It returns true if the optional contains a value, and false if it is empty.

1#include <iostream>
2#include <optional>
3
4int main() {
5  std::optional<int> maybe_int = 10;
6  if (maybe_int.has_value()) {
7    std::cout << "has value" << std::endl;
8  } else {
9    std::cout << "no value" << std::endl;
10  }
11}

1has value

operator bool()

std::optional has a conversion operator to bool, so it can be used directly in boolean contexts like if statements. It returns true if the optional contains a value, and false if it is empty.

1#include <optional>
2#include <iostream>
3
4std::optional<int> maybe_int = 10;
5if (maybe_int) {
6    std::cout << "has value" << std::endl;
7} else {
8    std::cout << "no value" << std::endl;
9}

1has value

Comparing to std::nullopt

You can directly compare an optional to std::nullopt to check if it is empty.

1#include <iostream>
2#include <optional>
3
4int main() {
5  std::optional<int> maybe_int = 10;
6  if (maybe_int != std::nullopt) {
7    std::cout << "has value" << std::endl;
8  } else {
9    std::cout << "no value" << std::endl;
10  }
11}

1has value

In general, using has_value() is the most explicit and self-explanatory way. Using the bool conversion operator or comparing to std::nullopt can lead to more concise code, but it might be less obvious what the intent is.

Remember, trying to access the value of an empty optional leads to undefined behavior (if using operator* or operator->) or an exception (if using value()). Always check that the optional has a value before trying to access it.