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502 lines
11 KiB
Markdown
502 lines
11 KiB
Markdown
---
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title: "variant"
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---
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{{< callout type="info">}}
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Header: `variant.h`
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Similar to: `std::variant`
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{{< /callout >}}
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**For C++11 and above.**
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```cpp
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template <typename... TTypes>
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class variant
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```
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A variadic class that can contain one a several specified types in a type safe manner.
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C++11 and C++14 support up to 32 types for visitor callback callbacks.
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C++17 and above supports any number of visitor callback types.
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Accepts functors, `etl::overload` and `etl::visitor` types as visitor callbacks.
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Accepts `etl::visit` since `20.28.0`.
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## Error exception types
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```cpp
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class variant_exception
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```
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**Description**
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The base for all variant exceptions.
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Derived from `etl::exception`.
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---
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```cpp
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class variant_incorrect_type_exception
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```
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**Description**
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The error types emitted when a non-supported type is requested from the variant.
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## Construction
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```cpp
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ETL_CONSTEXPR14 variant()
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```
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**Description**
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Default constructor.
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---
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```cpp
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template <typename T>
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ETL_CONSTEXPR14 variant(T&& value)
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```
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**Description**
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Constructor from a value.
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---
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```cpp
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template <typename T, typename... TArgs>
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ETL_CONSTEXPR14 explicit variant(etl::in_place_type_t<T>, TArgs&&... args)
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```
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**Description**
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Construct T from arguments.
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---
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```cpp
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template <size_t Index, typename... TArgs>
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ETL_CONSTEXPR14 explicit variant(etl::in_place_index_t<Index>, TArgs&&... args)
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```
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**Description**
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Construct from Index using args.
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---
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```cpp
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template <typename T, typename U, typename... TArgs>
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ETL_CONSTEXPR14 explicit variant(etl::in_place_type_t<T>,
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std::initializer_list<U> init,
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TArgs&&... args)
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```
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**Description**
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Construct from type, `initializer_list` and arguments.
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---
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```cpp
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template <size_t Index, typename U, typename... TArgs>
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ETL_CONSTEXPR14 explicit variant(etl::in_place_index_t<Index>,
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std::initializer_list<U> init,
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TArgs&&... args)
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```
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**Description**
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Construct from `Index`, initializer_list and arguments.
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---
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```cpp
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ETL_CONSTEXPR14 variant(const variant& other)
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```
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**Description**
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Copy constructor.
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---
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```cpp
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ETL_CONSTEXPR14 variant(variant&& other)
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```
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**Description**
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Move constructor.
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## Operations
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```cpp
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template <typename T, typename... TArgs>
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T& emplace(TArgs&&... args)
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```
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**Description**
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Set to type `T` using the construction parameters args.
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---
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```cpp
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template <typename T>
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variant& operator =(T&& value)
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```
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**Description**
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---
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```cpp
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variant& operator =(const variant& other)
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```
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**Description**
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Copies other to the current variant.
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---
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```cpp
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variant& operator =(variant&& other)
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```
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**Description**
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Moves other to the current variant.
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---
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```cpp
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constexpr bool valueless_by_exception() const noexcept
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```
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**Description**
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Returns `true` if the type id is `etl::variant_npos`.
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---
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```cpp
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constexpr size_t index() const noexcept
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```
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**Description**
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Returns the index of the type contained in the variant.
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Returns `etl::variant_npos` if the variant does not hold a valid type.
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---
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```cpp
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void swap(variant& rhs) noexcept
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```
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**Description**
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Swaps rhs with the current variant.
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## Access
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**Non-member functions**
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### get
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```cpp
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template <size_t Index, typename... TTypes>
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ETL_CONSTEXPR14 etl::variant_alternative_t<Index, etl::variant<TTypes...>>&
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get(etl::variant<TTypes...>& v)
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```
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**Description**
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Get the value based on `Index`.
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If `Index` matches the current index of the variant, returns a reference to the stored value, otherwise, asserts `etl::variant_incorrect_type_exception`.
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**Parameters**
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A reference to a variant.
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---
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```cpp
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template <size_t Index, typename... TTypes>
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ETL_CONSTEXPR14 etl::variant_alternative_t<Index, etl::variant<TTypes...>>&&
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get(etl::variant<TTypes...>&& v)
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```
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**Description**
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Get the value based on `Index`.
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If `Index` matches the current index of the variant, returns a reference to the stored value, otherwise, asserts `etl::variant_incorrect_type_exception`.
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**Parameters**
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An rvalue reference to a variant.
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---
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```cpp
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template <size_t Index, typename... TTypes>
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ETL_CONSTEXPR14 const etl::variant_alternative_t<Index, const etl::variant<TTypes...>>&
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get(const etl::variant<TTypes...>& v)
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```
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**Description**
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Get the value based on `Index`.
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If `Index` matches the current index of the variant, returns a reference to the stored value, otherwise, asserts `etl::variant_incorrect_type_exception`.
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**Parameters**
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A const reference to a variant.
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---
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```cpp
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template <size_t Index, typename... TTypes>
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ETL_CONSTEXPR14 const etl::variant_alternative_t<Index, const etl::variant<TTypes...>>&&
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get(const etl::variant<TTypes...>&& v)
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```
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**Description**
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Get the value based on `Index`.
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If `Index` matches the current index of the variant, returns a reference to the stored value, otherwise, asserts `etl::variant_incorrect_type_exception`.
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**Parameters**
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A const rvalue reference to a variant.
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---
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```cpp
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template <typename T, typename... TTypes>
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ETL_CONSTEXPR14 T& get(etl::variant<TTypes...>& v)
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```
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**Description**
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Get the value based on `T`.
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If `T` matches the current type stored in the variant, returns a reference to the stored value, otherwise, asserts `etl::variant_incorrect_type_exception`.
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**Parameters**
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A reference to a variant.
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---
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```cpp
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template <typename T, typename... TTypes>
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ETL_CONSTEXPR14 T&& get(etl::variant<TTypes...>&& v)
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```
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**Description**
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Get the value based on `T`.
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If `T` matches the current type stored in the variant, returns a reference to the stored value, otherwise, asserts `etl::variant_incorrect_type_exception`.
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**Parameters**
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An rvalue reference to a variant.
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---
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```cpp
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template <typename T, typename... TTypes>
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ETL_CONSTEXPR14 const T& get(const etl::variant<TTypes...>& v)
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```
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**Description**
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Get the value based on `T`.
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If `T` matches the current type stored in the variant, returns a reference to the stored value, otherwise, asserts `etl::variant_incorrect_type_exception`.
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**Parameters**
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A const reference to a variant.
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---
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```cpp
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template <typename T, typename... TTypes>
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ETL_CONSTEXPR14 const T&& get(const etl::variant<TTypes...>&& v)
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```
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**Description**
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Get the value based on `T`.
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If `T` matches the current type stored in the variant, returns a reference to the stored value, otherwise, asserts `etl::variant_incorrect_type_exception`.
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**Parameters**
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A const rvalue reference to a variant.
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### get_if
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```cpp
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template <size_t Index, typename... TTypes>
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ETL_CONSTEXPR14 etl::add_pointer_t<etl::variant_alternative_t<Index, etl::variant<TTypes...>>>
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get_if(etl::variant<TTypes...>* pv) noexcept
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```
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**Description**
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Get the value based on `Index`.
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If `Index` matches the current index of the variant, returns a pointer to the stored value, otherwise returns `nullptr`.
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**Parameters**
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A pointer to a variant.
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---
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```cpp
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template <size_t Index, typename... TTypes>
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ETL_CONSTEXPR14 etl::add_pointer_t<const etl::variant_alternative_t<Index, etl::variant<TTypes...>>>
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get_if(const etl::variant<TTypes...>* pv) noexcept
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```
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**Description**
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Get the value based on `Index`.
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If `Index` matches the current index of the variant, returns a const pointer to the stored value, otherwise returns `nullptr`.
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**Parameters**
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A const pointer to a variant.
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---
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```cpp
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template <typename T, typename... TTypes>
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ETL_CONSTEXPR14 etl::add_pointer_t<T> get_if(etl::variant<TTypes...>* pv) noexcept
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```
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**Description**
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Get the value based on `T`.
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If `T` matches the current type stored in the variant, returns a pointer to the stored value, otherwise return `nullptr`.
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**Parameters**
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An pointer to a variant.
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---
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```cpp
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template <typename T, typename... TTypes>
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ETL_CONSTEXPR14 etl::add_pointer_t<const T> get_if(const etl::variant<TTypes...>* pv) noexcept
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```
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**Description**
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Get the value based on `T`.
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If `T` matches the current type stored in the variant, returns a const pointer to the stored value, otherwise return `nullptr`.
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**Parameters**
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An const pointer to a variant.
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## Other
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```cpp
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struct monostate
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```
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**Description**
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A default constructible type.
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For use as the first state to turn a non-default constructible variant into a default constructible variant.
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---
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```cpp
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template <typename... TTypes>
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void swap(etl::variant<TTypes...>& lhs, etl::variant<TTypes...>& rhs)
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```
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**Description**
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Swaps lhs and rhs.
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---
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```cpp
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template <typename... TTypes>
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struct variant_size<etl::variant<TTypes...>>
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```
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**Description**
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The member constant value contains the number of types.
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For C++17 and above.
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```cpp
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template <typename... TTypes>
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inline constexpr size_t variant_size_v = variant_size<TTypes...>::value;
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```
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**Description**
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Has the value equal to the number of types.
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## Tests
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### Non-member functions
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```cpp
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template <typename T, typename... TTypes>
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ETL_CONSTEXPR14 bool holds_alternative(const etl::variant<TTypes...>& v) noexcept
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```
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**Description**
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Checks if the variant v holds the alternative T.
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---
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```cpp
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template <size_t Index, typename... TTypes>
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ETL_CONSTEXPR14 bool holds_alternative(const etl::variant<TTypes...>& v) noexcept
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```
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**Description**
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Checks if the variant v holds the alternative Index.
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---
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```cpp
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template <typename... TTypes>
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ETL_CONSTEXPR14 bool holds_alternative(size_t index, const etl::variant<TTypes...>& v) noexcept
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```
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**Description**
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Checks if the variant v holds the alternative index.
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## Visitation
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For `etl::visitor`
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For C++11 and C++14 the maximum number of visitor types is 32.
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Code size may be minimised if one of the following macros are defined.
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```cpp
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ETL_VARIANT_CPP11_MAX_8_TYPES
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ETL_VARIANT_CPP11_MAX_16_TYPES
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ETL_VARIANT_CPP11_MAX_24_TYPES
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```
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**Description**
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C++17 and above support any number of visitor callback types.
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`etl::variant` provides member functions that support two methods of visitation.
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---
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```cpp
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void accept_visitor(etl::visitor<TTypes...>& v)
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```
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**Description**
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Accepts an `etl::visitor` derived type.
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Deprecated from `20.36.0`
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```cpp
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void accept_visitor(etl::visitor<TTypes...>& v) const
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```
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**Description**
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Accepts `an etl::visitor` derived type.
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Deprecated
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Since: `20.36.0`
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---
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```cpp
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void accept(etl::visitor<TTypes...>& v)
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```
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**Description**
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Accepts an `etl::visitor` derived type.
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Since: `20.36.0`
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---
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```cpp
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void accept(etl::visitor<TTypes...>& v) const
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```
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**Description**
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Accepts an `etl::visitor` derived type.
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Since: `20.36.0`
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See `visitor`
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---
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```cpp
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template <typename TVisitor>
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void accept_functor(TVisitor& v)
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```
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**Description**
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Accepts a functor that overrides `operator()` for each type.
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Deprecated since `20.36.0`
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---
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```cpp
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template <typename TVisitor>
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void accept_functor(TVisitor& v) const
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```
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**Description**
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Accepts a functor that overrides `operator()` for each type.
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Deprecated since `20.36.0`
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---
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```cpp
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template <typename TVisitor>
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void accept(TVisitor& v)
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```
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**Description**
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Accepts a functor that overrides `operator()` for each type.
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Since: `20.36.0`
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---
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```cpp
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template <typename TVisitor>
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void accept(TVisitor& v) const
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```
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**Description**
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Accepts a functor that overrides `operator()` for each type.
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Since: `20.36.0`
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See overload
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## Visitation
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For STL style `etl::visit`.
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Since: `20.28.0`
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The ETL implements an STL style visit function.
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