std::nullopt

std::nullopt is a constant used to represent an empty or uninitialized state for std::optional. It explicitly signals that an optional contains no value.

Key Properties

• Defined in the <optional> header.
• Used to initialize or assign an std::optional to an empty state.
• Type: std::nullopt_t, which is a distinct type specifically for this purpose.
• Can be used in comparisons to check if an std::optional is empty.
• Helps improve code readability by clearly indicating the absence of a value.

Example

#include <optional>
#include <iostream>
 
std::optional<int> findValue(bool found) {
    if (found)
        return 42;
    else
        return std::nullopt; // explicitly empty
}
 
int main() {
    auto value = findValue(false);
 
    if (value == std::nullopt)
        std::cout << "No value found\n";
    else
        std::cout << "Value: " << *value << "\n";
}