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std::optional(3) C++ Standard Libary std::optional(3) NAME std::optional - std::optional 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 in- tent is expressed explicitly. Any instance of optional<T> at any given point in time either con- tains 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. 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; a program is ill-formed if it in- stantiates an optional with a reference type. Alternatively, an optional of a std::reference_wrapper of type T may be used to hold a reference. In addition, a program is ill-formed if it instantiates an optional with the (pos- sibly cv-qualified) tag types std::nullopt_t or std::in_place_t. Template parameters the type of the value to manage initialization state for. The type must meet the T - requirements of Destructible (in particular, array and reference types are not allowed). Member types Member type Definition value_type T 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) Observers operator-> accesses the contained value operator* (public member function) operator bool checks whether the object contains a value has_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 returns the result of the given function on the con- tained value if it (C++23) exists, or an empty optional otherwise (public member function) transform returns an optional containing the transformed con- tained value if it (C++23) exists, or an empty optional otherwise (public member function) or_else returns the optional itself if it contains a value, or the result of (C++23) the given function otherwise (public member function) 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) Non-member functions operator== operator!= operator< operator<= operator> operator>= operator<=> compares optional objects (C++17) (function template) (C++17) (C++17) (C++17) (C++17) (C++17) (C++20) make_optional creates an optional object (C++17) (function template) std::swap(std::optional) specializes the std::swap algorithm (C++17) (function template) Helper classes std::hash<std::optional> specializes the std::hash algorithm (C++17) (class template specialization) nullopt_t indicator of optional type with uninitial- ized state (C++17) (class) bad_optional_access exception indicating checked access to an optional that (C++17) doesn't contain a value (class) Helpers nullopt an object of type nullopt_t (C++17) (constant) in_place in_place_type in_place_index in-place construction tag in_place_t (class template) in_place_type_t in_place_index_t (C++17) Deduction guides Notes Feature-test macro: __cpp_lib_optional Example // Run this code #include <string> #include <functional> #include <iostream> #include <optional> // 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; } // std::reference_wrapper may be used to return a reference auto create_ref(bool b) { static std::string value = "Godzilla"; return b ? std::optional<std::reference_wrap- per<std::string>>{value} : 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'; } if (auto str = create_ref(true)) { // using get() to access the reference_wrapper's value std::cout << "create_ref(true) returned " << str->get() << '\n'; str->get() = "Mothra"; std::cout << "modifying it changed it to " << str->get() << '\n'; } } Output: create(false) returned empty create2(true) returned Godzilla create_ref(true) returned Godzilla modifying it changed it to Mothra See also variant a type-safe discriminated union (C++17) (class template) any Objects that hold instances of any CopyConstructible type. (C++17) (class) http://cppreference.com 2022.07.31 std::optional(3)
NAME | Synopsis | Template parameters | Member types | Member functions | Observers | Modifiers | Non-member functions | Helper classes | Notes | Example | Output: | See also
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