std::experimental::optional - cppreference.com (original) (raw)
std::experimental::optional
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Merged into ISO C++ The functionality described on this page was merged into the mainline ISO C++ standard as of 3/2016, see std::optional (since C++17) |
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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 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.
Contents
- 1 Template parameters
- 2 Member types
- 3 Member functions
- 4 Member objects
- 5 Non-member functions
- 6 Helper classes
- 7 Helper objects
[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 |
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| value_type | T |
[edit] Member functions
| (constructor) | constructs the optional object (public member function) [edit] |
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| (destructor) | destroys the contained value, if there is one (public member function) [edit] |
| operator= | assigns contents (public member function) [edit] |
| Observers | |
| operator->operator* | accesses the contained value (public member function) [edit] |
| operator bool | checks whether the object contains a value (public member function) [edit] |
| value | returns the contained value (public member function) [edit] |
| value_or | returns the contained value if available, another value otherwise (public member function) [edit] |
| Modifiers | |
| swap | exchanges the contents (public member function) [edit] |
| emplace | constructs the contained value in-place (public member function) [edit] |
[edit] Member objects
| Member name | Definition |
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| val (private) | pointer to the contained value (which points at a data member of the same object), the name is for exposition only |