std::expected::value(3)

Hey bestie 🫶 welcome to the std::expected::value(3) page from the C++ Standard Libary manual — yes, that spelling is in the header, and honestly? Keeping it 💀. This is the whole tea on pulling a value out of a std::expected when the vibe check passes, plus the very specific exception situation when it doesn't 😭.

---

💖 NAME 💖

std::expected::value — std::expected::value 💅

✨ Synopsis ✨

Okay so first up: when T is not (possibly cv-qualified) void, you get these overloads:

T is not (possibly cv-qualified) void
constexpr T& value() &;               (1) (since C++23)
constexpr const T& value() const&;    (2) (since C++23)
constexpr T&& value() &&;             (3) (since C++23)
constexpr const T&& value() const&&;  (4) (since C++23)
T is (possibly cv-qualified) void
constexpr void value() const&;        (5) (since C++23)
constexpr void value() &&;            (6) (since C++23)

If *this contains an expected value, it returns a reference to the contained value. Slay 💅.

It returns nothing if T is (possibly cv-qualified) void — literally nothing, no cap.

Otherwise, it throws an exception of type std::bad_expected_access<std::decay_t<E>> that contains a copy of error(). Not the fun kind of surprise 😭.

Now the “program is ill-formed” checkpoints, because C++ loves a prerequisite:

• 1,2) — if std::is_copy_constructible_v<E> is false, the program is ill-formed.
• 3,4) — if std::is_copy_constructible_v<E> or std::is_constructible_v<E, decltype(std::move(error()))> is false, the program is ill-formed.
• 5) — if std::is_copy_constructible_v<E> is false, the program is ill-formed.
• 6) — if std::is_move_constructible_v<E> is false, the program is ill-formed.

🎀 Parameters 🎀

(none) — bestie, there are literally no parameters. Iconic minimalism ✨.

🦋 Return value 🦋

• 1-4) — the expected value contained in *this.
• 5,6) — (none).

🌸 Exceptions 🌸

• 1,2,5) — throws std::bad_expected_access(std::as_const(error())) if *this contains an unexpected value.
• 3,4,6) — throws std::bad_expected_access(std::move(error())) if *this contains an unexpected value.

These are the “checked access” moments, so yeah — if the unexpected value is sitting there, it is not quietly letting it slide 😤.

💅 Example 💅

This section is incomplete — Reason: no example 😭.

Not me staring at an empty example section. The audacity. Anyway, that’s exactly what the page says, bestie.

🔥 Defect reports 🔥

Heads up: the following behavior-changing defect reports were applied retroactively to previously published C++ standards. History said “not today,” but the committee said “actually, yes” 💀.

• DR: LWG 3940 — Applied to: C++23. Behavior as published: E was not required to be copy/move-constructible for overloads (5,6). Correct behavior: required.

👑 See also 👑

Go peek at these besties:

• value_or — returns the expected value if present, another value otherwise (public member function).
• operator-> — accesses the expected value.
• operator* — (public member function).
• error — returns the unexpected value (public member function).
• bad_expected_access (C++23) — exception indicating checked access to an expected that contains an unexpected value (class template).

🌈 Category 🌈

• * Todo no example

---

Anyway, that’s the page, bestie 💖 it lives at http://cppreference.com, dated 2024.06.10, and it’s std::expected::value(3) — go forth and check your expected values, mwah 🫶🎀✨