Exit Zen
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``` x.reserve(n); ```
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13
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`reserve(n)` reserves space for at least `n` elements, allocating enough buckets
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14
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so as to not exceed the maximum load factor. + Because the maximum load factor is defined as the number of elements divided by the total number of available buckets, this function is logically equivalent to: + ``` x.rehash(std::ceil(n / x.max_load_factor())) ``` + See the xref:reference/unordered_map.adoc#unordered_map_rehash[reference for more details] on the `rehash` function.
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:idprefix: comparison_
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Comparison with Associative Containers
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Associative Containers
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Unordered Associative Containers
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19
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Parameterized by an ordering relation `Compare`
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Parameterized by a function object `Hash` and an equivalence relation `Pred`
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Keys can be compared using `key_compare` which is accessed by member function `key_comp()`, values can be compared using `value_compare` which is accessed by member function `value_comp()`.
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Keys can be hashed using `hasher` which is accessed by member function `hash_function()`, and checked for equality using `key_equal` which is accessed by member function `key_eq()`. There is no function object for compared or hashing values.
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Constructors have optional extra parameters for the comparison object.
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24
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Constructors have optional extra parameters for the initial minimum number of buckets, a hash function and an equality object.
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Keys `k1`, `k2` are considered equivalent if `!Compare(k1, k2) && !Compare(k2, k1)`.
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Keys `k1`, `k2` are considered equivalent if `Pred(k1, k2)`
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27
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Member function `lower_bound(k)` and `upper_bound(k)`
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28
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No equivalent. Since the elements aren't ordered `lower_bound` and `upper_bound` would be meaningless.
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`equal_range(k)` returns an empty range at the position that `k` would be inserted if `k` isn't present in the container.
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`equal_range(k)` returns a range at the end of the container if `k` isn't present in the container. It can't return a positioned range as `k` could be inserted into multiple place. +
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**Closed-addressing containers:** To find out the bucket that `k` would be inserted into use `bucket(k)`. But remember that an insert can cause the container to rehash - meaning that the element can be inserted into a different bucket.
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