A C++ template library for embedded applications
MIT licensed
Designed and
maintained by
John Wellbelove
Support the development
of the ETL

optional


A class where the value is optional.
If a value is not assigned then an object of the contained type is not constructed.

etl::optional<T>

If _DEBUG is defined then accesses to invalid data is detected either by throwing (ETL_THROW_EXCEPTIONS defined) or
calling the error handler.
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Template deduction guides
C++17 and above

template <typename T>
etl::optional(T)
  -> etl::optional<T>;

Example

std::string text = "Hello";
etl::optional opt{ text };

Defines opt as containing a std::string, containing the supplied text.

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Constructor


optional()

Default constructor.
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optional(const T& value)
optional(T&& value)

Construct from value. Constructs a T.
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optional(const optional<T>& other)
optional(optional<T>&& other)

Construct copy.  Constructs a T.
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optional(nullopt)

Construct empty value (same as default constructor).
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~optional()

Destructor. Destructs a T if a value has been assigned.

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Operations


optional& operator =(etl::nullopt)

optional& operator =(const optional<T>& value)
optional& operator =(optional<T>&& value)

optional& operator =(const T& value)
optional& operator =(T&& value)

Note:
Unlike std::optional, assigning from { } will not clear the optional item to 'empty'.
It will be assigned a default constructed 'value'.

etl::optional<int> opt;
opt = { }; // opt will contain 0.
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void swap(optional& other)

Swaps with the other optional

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void reset()

Resets the optional back to default.
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C++03

template <typename T1>
void emplace(const T1& value1)

template <typename T1, typename T2>
void emplace(const T1& value1, const T2& value2)

template <typename T1, typename T2, typename T3>
void emplace(const T1& value1, const T2& value2, const T3& value3)

template <typename T1, typename T2, typename T3, typename T4>
void emplace(const T1& value1, const T2& value2, const T3& value3, const T4& value4)

C++11

template <typename ... Args>
void emplace(Args && ... args)

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Observers


const T& value() const
T& value();

Returns the contained value.
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template <typename T >
T value_or(T value) const

template <typename T>
T value_or(T value)

Returns the contained value if *this is valid, otherwise returns value.
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const T* operator->() const
T* operator->();

Returns a pointer to the contained value.
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const T& operator *() const
T* operator *();

Returns a reference to the contained value.

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Tests


explicit operator bool() const
bool has_value() const

Checks whether *this is in a valid state, i.e. whether the contained value is initialised.
Returns true if valid, otherwise false.

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Non-member functions


==  true if the contents are equal, otherwise false.
!=  true if the contents are not equal, otherwise false.
<   true if the contents of the lhs are less than the contents of the rhs,  otherwise false.
<=  true if the contents of the lhs are less than or equal to the contents of the rhs, otherwise false.
>   true if the contents of the lhs are greater than the contents of the rhs,  otherwise false.
>=  true if the contents of the lhs are greater than or equal to the contents of the rhs, otherwise false.
optional.h