cpp/experimental/optional: Difference between revisions
engaged/disengaged is not in the TS. It's "contains a value" now. |
D41D8CD98F (talk | contribs) the expression `val->T::~T()` is used in several places, needs to say what `val` is |
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{{dsc inc | cpp/experimental/optional/dsc swap}} | {{dsc inc | cpp/experimental/optional/dsc swap}} | ||
{{dsc inc | cpp/experimental/optional/dsc emplace}} | {{dsc inc | cpp/experimental/optional/dsc emplace}} | ||
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Revision as of 06:28, 16 May 2015
| Defined in header <experimental/optional>
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template< class T >
class optional;
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(library fundamentals TS) | |
The class template std::experimental::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
optionalthat 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::experimental::nullopt_t or an
optionalobject that does not contain a value. - The object is assigned from a value of std::experimental::nullopt_t or from an
optionalthat does not contain a value
Template parameters
| T | - | the type of the value to manage initialization state for. The type must meet the requirements of Template:concept |
Member types
| Member type | Definition |
value_type
|
T
|
Member functions
| constructs the optional object (public member function) | |
| destroys the contained value, if there is one (public member function) | |
| assigns contents (public member function) | |
Observers | |
| accesses the contained value (public member function) | |
| checks whether the object contains a value (public member function) | |
| returns the contained value (public member function) | |
| returns the contained value if available, another value otherwise (public member function) | |
Modifiers | |
| exchanges the contents (public member function) | |
| constructs the contained value in-place (public member function) | |
Member objects
| Member name | Definition |
val (private)
|
pointer to the contained value, the name is for exposition only |
Non-member functions
compares optional objects (function template) | |
creates an optional object (function template) | |
| specializes the std::swap algorithm (function) |
Helper classes
| specializes the std::hash algorithm (class template specialization) | |
(library fundamentals TS) |
indicator of optional type with uninitialized state (class) |
Helper objects
(library fundamentals TS) |
an object of type nullopt_t (function) |