std::optional
Defined in header
<optional>
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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<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 the 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 a value of type
T
- The object is assigned from 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 a value of std::nullopt_t or an
optional
object that does not contain a value. - The object is assigned from a value of std::nullopt_t or from an
optional
that does not contain a value
Contents |
[edit] Template parameters
T | - | the type of the value to manage initialization state for. The type must meet the requirements of Destructible
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[edit] Member types
Member type | Definition |
value_type
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T
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[edit] Member functions
constructs the optional object (public member function) |
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destroys the contained value, if there is one (public member function) |
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assigns contents (public member function) |
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Observers |
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accesses the contained value (public member function) |
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checks whether the object contains a value (public member function) |
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returns the contained value (public member function) |
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returns the contained value if available, another value otherwise (public member function) |
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Modifiers |
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exchanges the contents (public member function) |
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constructs the contained value in-place (public member function) |
[edit] Member objects
Member name | Definition |
val (private)
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pointer to the contained value (which points at a data member of the same object), the name is for exposition only |
[edit] Non-member functions
compares optional objects (function template) |
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(C++17)
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creates an optional object (function template) |
(C++17)
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specializes the std::swap algorithm (function) |
[edit] Helper classes
(C++17)
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specializes the std::hash algorithm (class template specialization) |
(C++17)
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indicator of optional type with uninitialized state (class) |
(C++17)
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disambiguation tag type for in-place construction of optional types (class) |
(C++17)
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exception indicating checked access to an optional that doesn't contain a value (class) |
[edit] Helper objects
(C++17)
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an object of type nullopt_t (constant) |
(C++17)
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an object of type std::in_place_t (constant) |