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optional.md

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execute true

What It Does

std::optional<T> is a class template containing either a value of type T or no value. It provides a type-safe representation of an optional value without pointer semantics, sentinel values, or output parameters. Member functions has_value() and value() provide value checking and access, with value() throwing std::bad_optional_access if no value is present.

Why It Matters

Nullable pointers shift null-check responsibility to each use site without compile-time enforcement. Sentinel values overload the value domain of a type and require convention-based interpretation. std::optional encodes value presence or absence in the type itself, enabling static analysis and compile-time verification of access patterns.

Example

#include <optional>
#include <string>
#include <map>
#include <print>

std::optional<std::string> find_user(int id) {
    static auto users = std::map<int, std::string>{{1, "Alice"}, {2, "Bob"}};

    auto it = users.find(id);
    if (it != users.end()) {
        return it->second;
    }

    return std::nullopt;  // no value
}

int main() {
    if (auto user = find_user(1)) {
        std::println("Found: {}", *user);
    }

    // Or use value_or for a default
    std::println("{}", find_user(99).value_or("Unknown"));

    auto maybe_number = std::optional<int>();
    std::println("Has value: {}", maybe_number.has_value());  // false
    maybe_number = 42;
    std::println("Value: {}", *maybe_number);  // 42
}