tmp/tmpgrduy0s9/{from.md → to.md}
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| 1 |
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#### Overview <a id="flat.multimap.overview">[[flat.multimap.overview]]</a>
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A `flat_multimap` is a container adaptor that provides an associative
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container interface that supports equivalent keys (i.e., possibly
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containing multiple copies of the same key value) and provides for fast
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retrieval of values of another type `T` based on the keys.
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`flat_multimap` supports iterators that meet the *Cpp17InputIterator*
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requirements and model the `random_access_iterator` concept
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[[iterator.concept.random.access]].
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A `flat_multimap` meets all of the requirements for a container
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[[container.reqmts]] and for a reversible container
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[[container.rev.reqmts]], plus the optional container requirements
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[[container.opt.reqmts]]. `flat_multimap` meets the requirements of an
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associative container [[associative.reqmts]], except that:
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- it does not meet the requirements related to node handles
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[[container.node]],
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- it does not meet the requirements related to iterator invalidation,
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and
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- the time complexity of the operations that insert or erase a single
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element from the map is linear, including the ones that take an
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insertion position iterator.
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[*Note 1*: A `flat_multimap` does not meet the additional requirements
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of an allocator-aware container
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[[container.alloc.reqmts]]. — *end note*]
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A `flat_multimap` also provides most operations described in
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[[associative.reqmts]] for equal keys. This means that a `flat_multimap`
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supports the `a_eq` operations in [[associative.reqmts]] but not the
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`a_uniq` operations. For a `flat_multimap<Key, T>` the `key_type` is
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`Key` and the `value_type` is `pair<Key, T>`.
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Except as otherwise noted, operations on `flat_multimap` are equivalent
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to those of `flat_map`, except that `flat_multimap` operations do not
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remove or replace elements with equal keys.
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[*Example 1*: `flat_multimap` constructors and emplace do not erase
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non-unique elements after sorting them. — *end example*]
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A `flat_multimap` maintains the following invariants:
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- it contains the same number of keys and values;
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- the keys are sorted with respect to the comparison object; and
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- the value at offset `off` within the value container is the value
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associated with the key at offset `off` within the key container.
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If any member function in [[flat.multimap.defn]] exits via an exception,
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the invariants are restored.
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[*Note 2*: This can result in the `flat_multimap` being
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emptied. — *end note*]
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Any type `C` that meets the sequence container requirements
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[[sequence.reqmts]] can be used to instantiate `flat_multimap`, as long
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as `C::iterator` meets the *Cpp17RandomAccessIterator* requirements and
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invocations of member functions `C::size` and `C::max_size` do not exit
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via an exception. In particular, `vector` [[vector]] and `deque`
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[[deque]] can be used.
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[*Note 3*: `vector<bool>` is not a sequence container. — *end note*]
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The program is ill-formed if `Key` is not the same type as
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`KeyContainer::value_type` or `T` is not the same type as
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`MappedContainer::value_type`.
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The effect of calling a constructor that takes both `key_container_type`
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and `mapped_container_type` arguments with containers of different sizes
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is undefined.
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The effect of calling a constructor or member function that takes a
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`sorted_equivalent_t` argument with a container, containers, or range
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that are not sorted with respect to `key_comp()` is undefined.
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