optional

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

template <typename T>
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.

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.

Constructor

optional()

Description
Default constructor.

optional(const T& value)
optional(T&& value)

Description
Construct from value. Constructs a T.

optional(const optional<T>& other)
optional(optional<T>&& other)

Description
Construct copy. Constructs a T.

optional(nullopt)

Description
Construct empty value (same as default constructor).

template <typename... TArgs>
ETL_CONSTEXPR14
optional_impl(etl::in_place_t, TArgs&&... args)

Description
Constructor from variadic args. 20.43.0

template <typename U, typename... TArgs>
ETL_CONSTEXPR14
optional_impl(etl::in_place_t, std::initializer_list<U> ilist, TArgs&&... args)

Description
Constructor from variadic args. 20.43.0

~optional()

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

Operations

optional& operator =(etl::nullopt)

Description
Clears the optional.

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

Description
Copy constructor.

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

Description
Assigns a value to the optional.

Note

Unlike std::optional, assigning from { } will not clear the optional item to ’empty’. It will be assigned a default constructed ‘T’.

etl::optional<int> opt;
opt = { }; // opt will contain 0.

void swap(optional& other)

Description
Swaps with the other optional

void reset()

Description
Resets the optional back to default.

emplace

Description
Construct in-place from arguments.

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)

Observers

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

Description
Returns the contained value.

template <typename T>
T value_or(T value)

Description
Returns the contained value if *this is valid, otherwise returns value.

const T* operator->() const
T* operator->()

Description
Returns a pointer to the contained value.

const T& operator *() const
T* operator *()

Description
Returns a reference to the contained value.

Tests

explicit operator bool() const
bool has_value() const

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

Non-member functions

operator ==

Description
true if the contents of the lists are equal, otherwise false.

operator !=

Description
true if the contents of the lists are not equal, otherwise false.

operator <

Description
true if the contents of the lhs are lexicographically less than the contents of the rhs, otherwise false.

operator <=

Description
true if the contents of the lhs are lexicographically less than or equal to the contents of the rhs, otherwise false.

operator >

Description
true if the contents of the lhs are lexicographically greater than the contents of the rhs, otherwise false.

operator >=

Description
true if the contents of the lhs are lexicographically greater than or equal to the contents of the rhs, otherwise false.