etl/variant-variadic.md at feature/timer-refactor

coffee/etl

mirror of https://github.com/ETLCPP/etl.git synced 2026-10-29 02:17:09 +08:00

John Wellbelove 1bd1072303

Reduce the compiled code size for etl::variant (#1623 )

* Replaced pointer table with a recursive visitor call to reduce compiled code size.

* Removed the 32 type limit for visitor and operator visits

Deleted variant_select_do_operator and variant_select_do_visitor headers

* Restored CMakeLists.txt

Applied clang-format

* Updated documentation

Added guards to 'single index' do_operator_at and do_visitor_at

* Unified 'if (Index == index())' order

* Merged new tests from PR #1625

---------

Co-authored-by: John Wellbelove <john.wellbelove@etlcpp.com>

2026-10-12 18:40:19 +01:00

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title
variant

{{< callout type="info">}} Header: variant.h
Similar to: std::variant
For C++11 and above. {{< /callout >}}

template <typename... TTypes>
class variant

A variadic class that can contain one a several specified types in a type safe manner.

Accepts functors, etl::overload and etl::visitor types as visitor callbacks.

Accepts etl::visit since 20.28.0.

Error exception types

class variant_exception

Description
The base for all variant exceptions.
Derived from etl::exception.

class variant_incorrect_type_exception

Description
The error types emitted when a non-supported type is requested from the variant.

Construction

ETL_CONSTEXPR14 variant()

Description
Default constructor.

template <typename T>
ETL_CONSTEXPR14 variant(T&& value)

Description
Constructor from a value.

template <typename T, typename... TArgs>
ETL_CONSTEXPR14 explicit variant(etl::in_place_type_t<T>, TArgs&&... args)

Description
Construct T from arguments.

template <size_t Index, typename... TArgs>
ETL_CONSTEXPR14 explicit variant(etl::in_place_index_t<Index>, TArgs&&... args)

Description
Construct from Index using args.

template <typename T, typename U, typename... TArgs>
ETL_CONSTEXPR14 explicit variant(etl::in_place_type_t<T>, 
                                 std::initializer_list<U> init, 
                                 TArgs&&... args)

Description
Construct from type, initializer_list and arguments.

template <size_t Index, typename U, typename... TArgs>
ETL_CONSTEXPR14 explicit variant(etl::in_place_index_t<Index>, 
                                 std::initializer_list<U> init, 
                                 TArgs&&... args)

Description
Construct from Index, initializer_list and arguments.

ETL_CONSTEXPR14 variant(const variant& other)

Description
Copy constructor.

ETL_CONSTEXPR14 variant(variant&& other)

Description
Move constructor.

Operations

template <typename T, typename... TArgs>
T& emplace(TArgs&&... args)

Description
Set to type T using the construction parameters args.

template <typename T>
variant& operator =(T&& value)

Description

variant& operator =(const variant& other)

Description
Copies other to the current variant.

variant& operator =(variant&& other)

Description
Moves other to the current variant.

constexpr bool valueless_by_exception() const noexcept

Description
Returns true if the type id is etl::variant_npos.

constexpr size_t index() const noexcept

Description
Returns the index of the type contained in the variant.
Returns etl::variant_npos if the variant does not hold a valid type.

void swap(variant& rhs) noexcept

Description
Swaps rhs with the current variant.

Access

Non-member functions

get

template <size_t Index, typename... TTypes>
ETL_CONSTEXPR14 etl::variant_alternative_t<Index, etl::variant<TTypes...>>&
  get(etl::variant<TTypes...>& v)

Description
Get the value based on Index.
If Index matches the current index of the variant, returns a reference to the stored value, otherwise, asserts etl::variant_incorrect_type_exception.

Parameters
A reference to a variant.

template <size_t Index, typename... TTypes>
ETL_CONSTEXPR14 etl::variant_alternative_t<Index, etl::variant<TTypes...>>&&
  get(etl::variant<TTypes...>&& v)

Parameters
An rvalue reference to a variant.

template <size_t Index, typename... TTypes>
ETL_CONSTEXPR14 const etl::variant_alternative_t<Index, const etl::variant<TTypes...>>&
  get(const etl::variant<TTypes...>& v)

Parameters
A const reference to a variant.

template <size_t Index, typename... TTypes>
ETL_CONSTEXPR14 const etl::variant_alternative_t<Index, const etl::variant<TTypes...>>&&
  get(const etl::variant<TTypes...>&& v)

Parameters
A const rvalue reference to a variant.

template <typename T, typename... TTypes>
ETL_CONSTEXPR14 T& get(etl::variant<TTypes...>& v)

Description
Get the value based on T.
If T matches the current type stored in the variant, returns a reference to the stored value, otherwise, asserts etl::variant_incorrect_type_exception.

Parameters
A reference to a variant.

template <typename T, typename... TTypes>
ETL_CONSTEXPR14 T&& get(etl::variant<TTypes...>&& v)

Description
Get the value based on T.
If T matches the current type stored in the variant, returns a reference to the stored value, otherwise, asserts etl::variant_incorrect_type_exception.

Parameters
An rvalue reference to a variant.

template <typename T, typename... TTypes>
ETL_CONSTEXPR14 const T& get(const etl::variant<TTypes...>& v)

Description
Get the value based on T.
If T matches the current type stored in the variant, returns a reference to the stored value, otherwise, asserts etl::variant_incorrect_type_exception.

Parameters
A const reference to a variant.

template <typename T, typename... TTypes>
ETL_CONSTEXPR14 const T&& get(const etl::variant<TTypes...>&& v)

Description
Get the value based on T.
If T matches the current type stored in the variant, returns a reference to the stored value, otherwise, asserts etl::variant_incorrect_type_exception.

Parameters
A const rvalue reference to a variant.

get_if

template <size_t  Index, typename... TTypes>
ETL_CONSTEXPR14 etl::add_pointer_t<etl::variant_alternative_t<Index, etl::variant<TTypes...>>>
  get_if(etl::variant<TTypes...>* pv) noexcept

Description
Get the value based on Index.
If Index matches the current index of the variant, returns a pointer to the stored value, otherwise returns nullptr.

Parameters
A pointer to a variant.

template <size_t Index, typename... TTypes>
ETL_CONSTEXPR14 etl::add_pointer_t<const etl::variant_alternative_t<Index, etl::variant<TTypes...>>>
  get_if(const etl::variant<TTypes...>* pv) noexcept

Description
Get the value based on Index.
If Index matches the current index of the variant, returns a const pointer to the stored value, otherwise returns nullptr.

Parameters
A const pointer to a variant.

template <typename T, typename... TTypes>
ETL_CONSTEXPR14 etl::add_pointer_t<T> get_if(etl::variant<TTypes...>* pv) noexcept

Description
Get the value based on T.
If T matches the current type stored in the variant, returns a pointer to the stored value, otherwise return nullptr.

Parameters
An pointer to a variant.

template <typename T, typename... TTypes>
ETL_CONSTEXPR14 etl::add_pointer_t<const T> get_if(const etl::variant<TTypes...>* pv) noexcept

Description
Get the value based on T.
If T matches the current type stored in the variant, returns a const pointer to the stored value, otherwise return nullptr.

Parameters
An const pointer to a variant.

Other

struct monostate

Description
A default constructible type.
For use as the first state to turn a non-default constructible variant into a default constructible variant.

template <typename... TTypes>
void swap(etl::variant<TTypes...>& lhs, etl::variant<TTypes...>& rhs)

Description
Swaps lhs and rhs.

template <typename... TTypes>
struct variant_size<etl::variant<TTypes...>>

Description
The member constant value contains the number of types.

For C++17 and above.

template <typename... TTypes>
inline constexpr size_t variant_size_v = variant_size<TTypes...>::value;

Description
Has the value equal to the number of types.

Tests

Non-member functions

template <typename T, typename... TTypes>
ETL_CONSTEXPR14 bool holds_alternative(const etl::variant<TTypes...>& v) noexcept

Description
Checks if the variant v holds the alternative T.

template <size_t Index, typename... TTypes>
ETL_CONSTEXPR14 bool holds_alternative(const etl::variant<TTypes...>& v) noexcept

Description
Checks if the variant v holds the alternative Index.

template <typename... TTypes>
ETL_CONSTEXPR14 bool holds_alternative(size_t index, const etl::variant<TTypes...>& v) noexcept

Description
Checks if the variant v holds the alternative index.

Visitation

etl::variant provides member functions that support three methods of visitation.

Before: `20.50.0`

C++11 and C++14:
Supports up to 32 types of functor, etl::visitor, or etl::overload callback types.
C++17 and above:
Supports any number of functor or etl::visitor, or etl::overloadcallback types.

From: `20.50.0`

Supports any number of visitor callback types.
Compiles to smaller object code.
{{< /callout >}}

etl::visitor

void accept_visitor(etl::visitor<TTypes...>& v)

Description
Accepts an etl::visitor derived type.
{{< callout type="warning">}} Deprecated from 20.36.0 {{< /callout >}}

void accept_visitor(etl::visitor<TTypes...>& v) const

Description
Accepts an etl::visitor derived type.
{{< callout type="warning">}} Deprecated from 20.36.0 {{< /callout >}}

void accept(etl::visitor<TTypes...>& v)

Description
Accepts an etl::visitor derived type.
Since: 20.36.0

void accept(etl::visitor<TTypes...>& v) const

Description
Accepts an etl::visitor derived type.
Since: 20.36.0

[See visitor]({{< relref "../../patterns/visitor.md" >}})

Functors

template <typename TVisitor>
void accept_functor(TVisitor& v)

Description
Accepts a functor that overrides operator() for each type.
{{< callout type="warning">}} Deprecated from 20.36.0 {{< /callout >}}

template <typename TVisitor>
void accept_functor(TVisitor& v) const

Description
Accepts a functor that overrides operator() for each type.
{{< callout type="warning">}} Deprecated from 20.36.0 {{< /callout >}}

template <typename TVisitor>
void accept(TVisitor& v)

Description
Accepts a functor that overrides operator() for each type.
Since: 20.36.0

template <typename TVisitor>
void accept(TVisitor& v) const

Description
Accepts a functor that overrides operator() for each type.
Since: 20.36.0

See overload

etl::visit

For STL style etl::visit.
Since: 20.28.0

The ETL implements an STL style visit function.

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Error exception types

Construction

Operations

Access

get

get_if

Other

Tests

Non-member functions

Visitation

Before: 20.50.0

From: 20.50.0

etl::visitor

Functors

etl::visit

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Before: `20.50.0`

From: `20.50.0`