std::optional(3)

💖 std::optional — the C++ Standard Library page, but make it a vibe ✨

okay so bestie, this is the manual page for std::optional(3) and it is literally the “maybe there’s a value, maybe there isn’t” queen of C++17. no cap, she’s iconic. 🫶

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🌸 NAME

std::optional — std::optional (yes bestie, that’s what the page says; the name is the name 💅)

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💫 Synopsis

Defined in header <optional>:

template< class T >           (since C++17)
class optional;

The class template std::optional manages an optional contained value, i.e. a value that may or may not be present. 👑

A common use case for optional is the return value of a function that may fail. As opposed to other approaches, such as std::pair<T, bool>, optional handles expensive-to-construct objects well and is more readable, as the intent is expressed explicitly. ✨

Any instance of optional<T> at any given point in time either contains a value or does not contain a value.

If an optional<T> contains a value, the value is guaranteed to be allocated as part of the optional object footprint, i.e. no dynamic memory allocation ever takes place. Thus, an optional object models an object, not a pointer, even though operator*() and operator->() are defined. 💀 (like, she’s giving object, not pointer, periodt.)

When an object of type optional<T> is contextually converted to bool, the conversion returns true if the object contains a value and false if it does not contain a value.

The optional object contains a value in the following conditions:

• The object is initialized with/assigned from a value of type T or another optional that contains a value.

The object does not contain a value in the following conditions:

• The object is default-initialized.
• The object is initialized with/assigned from a value of type std::nullopt_t or an optional object that does not contain a value.
• The member function reset() is called.

There are no optional references, functions, arrays, or cv void; a program is ill-formed if it instantiates an optional with such a type. In addition, a program is ill-formed if it instantiates an optional with the (possibly cv-qualified) tag types std::nullopt_t or std::in_place_t. 🚨

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🧬 Template parameters

• T — the type of the value to manage initialization state for. The type must meet the requirements of Destructible (in particular, array and reference types are not allowed). 🎀

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🧁 Member types

• value_type — T

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🛠️ Member functions

• constructor — constructs the optional object (public member function)
• destructor — destroys the contained value, if there is one (public member function)
• operator= — assigns contents (public member function)

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👀 Observers

• operator-> — accesses the contained value (public member function)
• operator* — accesses the contained value (public member function)
• operator bool — checks whether the object contains a value (public member function)
• has_value — checks whether the object contains a value (public member function)
• value — returns the contained value (public member function)
• value_or — returns the contained value if available, another value otherwise (public member function)

🪄 Monadic operations

• and_then (C++23) — returns the result of the given function on the contained value if it exists, or an empty optional otherwise (public member function)
• transform (C++23) — returns an optional containing the transformed contained value if it exists, or an empty optional otherwise (public member function)
• or_else (C++23) — returns the optional itself if it contains a value, or the result of the given function otherwise (public member function)

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🔧 Modifiers

• swap — exchanges the contents (public member function)
• reset — destroys any contained value (public member function)
• emplace — constructs the contained value in-place (public member function)

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🫱 Non-member functions

• operator== — compares optional objects (C++17) (function template)
• operator!= — compares optional objects (C++17) (function template)
• operator< — compares optional objects (C++17) (function template)
• operator<= — compares optional objects (C++17) (function template)
• operator> — compares optional objects (C++17) (function template)
• operator>= — compares optional objects (C++17) (function template)
• operator<=> — compares optional objects (C++20) (function template)
• make_optional — creates an optional object (C++17) (function template)
• std::swap(std::optional) — specializes the std::swap algorithm (C++17) (function template)

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🧰 Helper classes

• std::hash<std::optional> — hash support for std::optional (C++17) (class template specialization)
• nullopt_t — indicator of optional type with uninitialized state (C++17) (class)
• bad_optional_access — exception indicating checked access to an optional that doesn’t contain a value (C++17) (class)

🧚 Helpers

• nullopt — an object of type nullopt_t (C++17) (constant)
• in_place, in_place_type, in_place_index — in-place construction tag (tag) (C++17)
• in_place_t, in_place_type_t, in_place_index_t — in-place construction tag (tag) (C++17)

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🧞 Deduction guides

(this section is literally just the heading in the original — no extra bullet points listed, so we’re not inventing any 💅)

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📝 Notes

Feature-test macro, value, standard, and feature:

• __cpp_lib_optional 201606L (C++17) — std::optional
• __cpp_lib_optional 202106L (C++20) — Fully constexpr (DR)
• __cpp_lib_optional 202110L (C++23) — Monadic operations

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🌈 Example

Run this code, bestie:

// Run this code

#include <iostream>
#include <optional>
#include <string>

// optional can be used as the return type of a factory that may fail
std::optional<std::string> create(bool b)
{
    if (b)
        return "Godzilla";
    return {};
}

// std::nullopt can be used to create any (empty) std::optional
auto create2(bool b)
{
    return b ? std::optional<std::string>{"Godzilla"} : std::nullopt;
}

int main()
{
    std::cout << "create(false) returned "
              << create(false).value_or("empty") << '\n';

    // optional-returning factory functions are usable as conditions of while and if
    if (auto str = create2(true))
        std::cout << "create2(true) returned " << *str << '\n';
}

Output:

create(false) returned empty
create2(true) returned Godzilla

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👑 See also

• variant — a type-safe discriminated union (C++17) (class template)
• any — objects that hold instances of any CopyConstructible type (C++17) (class)

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okay bye bestie 🫶 this page came from http://cppreference.com, version/date 2024.06.10 — now go make an optional that slays 💅✨