tmp/tmp1so0ktcw/{from.md → to.md}
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| 1 |
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### Non-propagating cache <a id="range.nonprop.cache">[[range.nonprop.cache]]</a>
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Some types in subclause [[range.adaptors]] are specified in terms of an
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exposition-only class template *`non-propagating-{}cache`*.
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`non-propagating-cache<T>` behaves exactly like `optional<T>` with the
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following differences:
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- `non-propagating-cache<T>` constrains its type parameter `T` with
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`is_object_v<T>`.
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- The copy constructor is equivalent to:
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``` cpp
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constexpr non-propagating-cache(const non-propagating-cache&) noexcept {}
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```
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- The move constructor is equivalent to:
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``` cpp
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constexpr non-propagating-cache(non-propagating-cache&& other) noexcept {
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other.reset();
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}
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```
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- The copy assignment operator is equivalent to:
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``` cpp
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constexpr non-propagating-cache& operator=(const non-propagating-cache& other) noexcept {
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if (addressof(other) != this)
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reset();
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return *this;
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}
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```
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- The move assignment operator is equivalent to:
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``` cpp
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constexpr non-propagating-cache& operator=(non-propagating-cache&& other) noexcept {
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reset();
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other.reset();
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return *this;
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}
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```
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- `non-propagating-cache<T>` has an additional member function template
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specified as follows:
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``` cpp
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template<class I>
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constexpr T& emplace-deref(const I& i); // exposition only
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```
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*Mandates:* The declaration `T t(*i);` is well-formed for some
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invented variable `t`.
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\[*Note 1*: If `*i` is a prvalue of type cv `T`, there is no
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requirement that it is movable [[dcl.init.general]]. — *end note*]
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*Effects:* Calls `reset()`. Then direct-non-list-initializes the
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contained value with `*i`.
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*Ensures:* `*this` contains a value.
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*Returns:* A reference to the new contained value.
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*Throws:* Any exception thrown by the initialization of the contained
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value.
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*Remarks:* If an exception is thrown during the initialization of `T`,
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`*this` does not contain a value, and the previous value (if any) has
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been destroyed.
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[*Note 1*: *`non-propagating-cache`* enables an input view to
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temporarily cache values as it is iterated over. — *end note*]
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