tmp/tmpdlmwqmw6/{from.md → to.md}
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#### Concept <a id="iterator.concept.bidir">[[iterator.concept.bidir]]</a>
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The `bidirectional_iterator` concept adds the ability to move an
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iterator backward as well as forward.
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``` cpp
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template<class I>
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concept bidirectional_iterator =
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forward_iterator<I> &&
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derived_from<ITER_CONCEPT(I), bidirectional_iterator_tag> &&
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requires(I i) {
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{ --i } -> same_as<I&>;
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{ i-- } -> same_as<I>;
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};
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```
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A bidirectional iterator `r` is decrementable if and only if there
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exists some `q` such that `++q == r`. Decrementable iterators `r` shall
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be in the domain of the expressions `--r` and `r--`.
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Let `a` and `b` be equal objects of type `I`. `I` models
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`bidirectional_iterator` only if:
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- If `a` and `b` are decrementable, then all of the following are
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`true`:
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- `addressof(--a) == addressof(a)`
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- `bool(a-- == b)`
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- after evaluating both `a--` and `--b`, `bool(a == b)` is still
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`true`
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- `bool(++(--a) == b)`
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- If `a` and `b` are incrementable, then `bool(--(++a) == b)`.
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