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std::optional

From cppreference.com
< cpp‎ | utility
 
 
Utilities library
Language support
Type support (basic types, RTTI)
Library feature-test macros (C++20)
Program utilities
Variadic functions
Coroutine support (C++20)
Contract support (C++26)
Three-way comparison
(C++20)
(C++20)
(C++20)(C++20)(C++20)  
(C++20)(C++20)(C++20)

General utilities
Relational operators (deprecated in C++20)
Integer comparison functions
(C++20)(C++20)(C++20)  
(C++20)
Swap and type operations
(C++20)
(C++14)
(C++11)
(C++11)
(C++23)
(C++11)
(C++17)
Common vocabulary types
(C++11)
optional
(C++17)
(C++17)
(C++17)
(C++11)
(C++17)
(C++23)


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

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 at any given point in time either contains a value or does not contain a value.

If an optional contains a value, the value is guaranteed to be nested within the optional object. 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.

The optional object is a view that contains either one element if it contains a value, or otherwise zero elements if it does not contain a value. The lifetime of the contained element is bound to the object.

(since C++26)

There are no optional references, functions, arrays, or (possibly cv-qualified) 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 .

Contents

[edit] 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).

[edit] Nested types

Type Definition
value_type T
iterator (since C++26) implementation-defined LegacyRandomAccessIterator, ConstexprIterator, and contiguous_iterator whose value_type and reference are std::remove_cv_t <T> and T&, respectively.
const_iterator (since C++26) implementation-defined LegacyRandomAccessIterator, ConstexprIterator, and contiguous_iterator whose value_type and reference are std::remove_cv_t <T> and const T&, respectively.

All requirements on the iterator types of a Container apply to the iterator type of optional as well.

[edit] Data members

T* val a pointer to the contained object (if exists)
(exposition-only member object*)

[edit] Member functions

constructs the optional object
(public member function) [edit]
destroys the contained value, if there is one
(public member function) [edit]
assigns contents
(public member function) [edit]
Iterators
(C++26)
returns an iterator to the beginning
(public member function) [edit]
(C++26)
returns an iterator to the end
(public member function) [edit]
Observers
accesses the contained value
(public member function) [edit]
checks whether the object contains a value
(public member function) [edit]
returns the contained value
(public member function) [edit]
returns the contained value if available, another value otherwise
(public member function) [edit]
Monadic operations
(C++23)
returns the result of the given function on the contained value if it exists, or an empty optional otherwise
(public member function) [edit]
(C++23)
returns an optional containing the transformed contained value if it exists, or an empty optional otherwise
(public member function) [edit]
(C++23)
returns the optional itself if it contains a value, or the result of the given function otherwise
(public member function) [edit]
Modifiers
exchanges the contents
(public member function) [edit]
destroys any contained value
(public member function) [edit]
constructs the contained value in-place
(public member function) [edit]

[edit] Non-member functions

(C++17)(C++17)(C++17)(C++17)(C++17)(C++17)(C++20)
compares optional objects
(function template) [edit]
creates an optional object
(function template) [edit]
specializes the std::swap algorithm
(function template) [edit]

[edit] Helper classes

hash support for std::optional
(class template specialization) [edit]
(C++17)
indicator of an std::optional that does not contain a value
(class) [edit]
exception indicating checked access to an optional that doesn't contain a value
(class) [edit]

[edit] Helpers

(C++17)
an object of type nullopt_t
(constant) [edit]
in-place construction tag
(tag)[edit]

[edit] Helper specializations

template< class T >
constexpr bool ranges::enable_view <std::optional<T>> = true;
(since C++26)

This specialization of ranges::enable_view makes optional satisfy view.

template< class T >
constexpr auto format_kind<std::optional<T>> = range_format::disabled;
(since C++26)

This specialization of format_kind disables the range formatting support of optional.

[edit] Deduction guides

[edit] Notes

Feature-test macro Value Std Feature
__cpp_lib_optional 201606L (C++17) std::optional
202106L (C++23)
(DR20)		 Fully constexpr
202110L (C++23) Monadic operations
__cpp_lib_optional_range_support 202406L (C++26) Range support for std::optional

[edit] Example

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

[edit] Defect reports

The following behavior-changing defect reports were applied retroactively to previously published C++ standards.

DR Applied to Behavior as published Correct behavior
LWG 4141 C++17 the requirement of storage
allocation was confusing 		the contained object must be
nested within the optional object

[edit] See also

(C++17)
a type-safe discriminated union
(class template) [edit]
(C++17)
objects that hold instances of any CopyConstructible type
(class) [edit]
(C++23)
a wrapper that contains either an expected or error value
(class template) [edit]
a view that contains a single element of a specified value
(class template) (customization point object)[edit]
an empty view with no elements
(class template) (variable template)[edit]

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