103 lines
3.7 KiB
Markdown
103 lines
3.7 KiB
Markdown
[forward.iterators]
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# 24 Iterators library [[iterators]](./#iterators)
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## 24.3 Iterator requirements [[iterator.requirements]](iterator.requirements#forward.iterators)
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### 24.3.5 C++17 iterator requirements [[iterator.cpp17]](iterator.cpp17#forward.iterators)
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#### 24.3.5.5 Forward iterators [forward.iterators]
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[1](#1)
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[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/iterators.tex#L2230)
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A class or pointer typeX meets the *Cpp17ForwardIterator* requirements if
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- [(1.1)](#1.1)
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X meets the *Cpp17InputIterator* requirements ([[input.iterators]](input.iterators "24.3.5.3 Input iterators")),
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- [(1.2)](#1.2)
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X meets the *Cpp17DefaultConstructible* requirements ([[utility.arg.requirements]](utility.arg.requirements "16.4.4.2 Template argument requirements")),
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- [(1.3)](#1.3)
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if X is a mutable iterator, reference is a reference to T;
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if X is a constant iterator, reference is a reference to const T,
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- [(1.4)](#1.4)
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the expressions in Table [81](#tab:forwarditerator "Table 81: Cpp17ForwardIterator requirements (in addition to Cpp17InputIterator)") are valid and have the indicated semantics, and
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- [(1.5)](#1.5)
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objects of type X offer the multi-pass guarantee, described below[.](#1.sentence-1)
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[2](#2)
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[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/iterators.tex#L2249)
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The domain of == for forward iterators is that of iterators over the same
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underlying sequence[.](#2.sentence-1)
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However, value-initialized iterators may be compared and
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shall compare equal to other value-initialized iterators of the same type[.](#2.sentence-2)
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[*Note [1](#note-1)*:
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Value-initialized iterators behave as if they refer past the end of
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the same empty sequence[.](#2.sentence-3)
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â *end note*]
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[3](#3)
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[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/iterators.tex#L2258)
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Two dereferenceable iterators a and b of type X offer the[*multi-pass guarantee*](#def:multi-pass_guarantee "24.3.5.5 Forward iterators [forward.iterators]") if
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- [(3.1)](#3.1)
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a == b implies ++a == ++b and
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- [(3.2)](#3.2)
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X is a pointer type or the expression(void)++X(a), *a is equivalent to the expression *a[.](#3.sentence-1)
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[4](#4)
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[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/iterators.tex#L2267)
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[*Note [2](#note-2)*:
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The requirement thata == b implies++a == ++b (which is not true for input and output iterators)
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and the removal of the restrictions on the number of the assignments through
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a mutable iterator
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(which applies to output iterators)
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allows the use of multi-pass one-directional algorithms with forward iterators[.](#4.sentence-1)
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â *end note*]
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Table [81](#tab:forwarditerator) — *Cpp17ForwardIterator* requirements (in addition to *Cpp17InputIterator*) [[tab:forwarditerator]](./tab:forwarditerator)
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| [ð](#tab:forwarditerator-row-1)<br>**Expression** | **Return type** | **Operational** | **Assertion/note** |
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| --- | --- | --- | --- |
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| [ð](#tab:forwarditerator-row-2) | | **semantics** | **pre-/post-condition** |
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| [ð](#tab:forwarditerator-row-3)<br>r++ | convertible to const X& | { X tmp = r; ++r; return tmp; } | |
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| [ð](#tab:forwarditerator-row-4)<br>*r++ | reference | | |
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[5](#5)
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[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/iterators.tex#L2297)
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If a and b are equal, then either a and b are both dereferenceable
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or else neither is dereferenceable[.](#5.sentence-1)
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[6](#6)
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[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/iterators.tex#L2302)
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If a and b are both dereferenceable, then a == b if and only if*a and *b are bound to the same object[.](#6.sentence-1)
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