tmp/tmp23k0ckt2/{from.md → to.md}
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| 1 |
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### Movable wrapper <a id="range.move.wrap">[[range.move.wrap]]</a>
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Many types in this subclause are specified in terms of an
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exposition-only class template *`movable-box`*. `movable-box<T>` behaves
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exactly like `optional<T>` with the following differences:
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- `movable-box<T>` constrains its type parameter `T` with
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`move_constructible<T> && is_object_v<T>`.
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- The default constructor of `movable-box<T>` is equivalent to:
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``` cpp
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constexpr movable-box() noexcept(is_nothrow_default_constructible_v<T>)
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requires default_initializable<T>
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: movable-box{in_place} {}
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```
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- If `copyable<T>` is not modeled, the copy assignment operator is
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equivalent to:
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``` cpp
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constexpr movable-box& operator=(const movable-box& that)
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noexcept(is_nothrow_copy_constructible_v<T>)
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requires copy_constructible<T> {
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if (this != addressof(that)) {
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if (that) emplace(*that);
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else reset();
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}
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return *this;
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}
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```
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- If `movable<T>` is not modeled, the move assignment operator is
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equivalent to:
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``` cpp
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constexpr movable-box& operator=(movable-box&& that)
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noexcept(is_nothrow_move_constructible_v<T>) {
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if (this != addressof(that)) {
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if (that) emplace(std::move(*that));
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else reset();
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}
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return *this;
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}
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```
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*Recommended practice:*
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- If `copy_constructible<T>` is `true`, `movable-box<T>` should store
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only a `T` if either `T` models `copyable`, or
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`is_nothrow_move_constructible_v<T> && is_nothrow_copy_constructible_v<T>`
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is `true`.
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- Otherwise, `movable-box<T>` should store only a `T` if either `T`
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models `movable` or `is_nothrow_move_constructible_v<T>` is `true`.
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