C++ Iterators
===
[ISO C++ N4861 Post-Prague 2020 C++ working draft](https://github.com/cplusplus/draft/releases)
平常大家在撰寫C++的時候，會使用迭代器(Iterator)操作各式各樣在標準函式庫的資料結構，例如: 遍歷vector
```Clike=
std::vector<int> example_vec;
vector<int>::iterator example_vec_begin = example_vec.begin();
vector<int>::iterator example_vec_end = example_vec.end();
while(example_vec_begin!=example_vec_end)
{
*example_vec_begin = 1;
cout << *example_vec_begin;
example_vec_begin++;
}
```
但對於Iterator其實有分為幾個種類。每種操作的方式都不太一樣。這也是這一篇我主要想要探討和釐清的觀念。(因為我也不懂，所以找資料學習)。
---
在ISO C++是這麼定義的:
> 迭代器(Iterator)是指針的一般化，它允許C++以統一的方式使用不同的資料結構(容器)。
也就是Iterator就類似C Language的pointer，但是他操作不同的資料結構有統一的方法。
因此可以這樣操作:
```Clike=
cout << *example_vec_begin;
```
* ISO C++ 27.2.1 說明了iterator的輸入和輸出:
> An input iterator _i_ supports the expression _*i_, resulting in a value of some object type T, called the value typeof the iterator.
> An output iterator _i_ has a non-empty set of types that are writable to the iterator; for each such type T, the expression _*i_ = _o_ is valid where _o_ is a value of type T. An iterator _i_ for which the expression(_*i_)
---
* ISO C++ 27.2.1定義了幾種C++會使用到的iterator:
> 
* 分別為:
1. input iterator, [ISO C++ 23.3.4.9 Concept input_iterator]
2. output iterator, [ISO C++ 23.3.4.10 Concept Output_iterator]
3. forward iterator, [ISO C++ 23.3.4.11 Concept Forward_iterator]
4. bidirectional iterator [ISO C++ 23.3.4.12 Concept Bidirectional_iterator]
5. random access iterator [ISO C++ 23.3.4.13 Concept Random_access_iterator]
6. contiguous iterator [ISO C++ 23.3.4.14 Concept contiguous_iterator]
---
* input iterator:
> The input_iterator concept defines requirements for a type whose referenced values can be read and which can be both pre- and post-incremented.
input_iterator概念定義了對一種類型的要求，該類型的參考值可以讀取，並且可以在讀取前和讀取後遞增。
> ISO C++ 23.3.5.2 Input iterators 定義了一些合法的用法:
```
a != b
*a
a->m
++r
(void)r++
*r++
```
* output_iterator:
> The output_iteratorconcept defines requirements for a type that can be used to write values, and which can be both pre- and post-incremented.
output_iterator概念定義了對可用於寫入值的類型的要求，並且該類型可以先增加後再增加。
> ISO C++ 23.3.5.3 Output iterators 定義了一些合法的用法:
```
*r = o
++r
r++
*r++ = o
```
* forward iterator:
> The forward_iterator concept adds copyability, equality comparison, and the multi-pass guarantee.
forward_iterator概念增加了可複製性，相等比較和多次通過保證。
> ISO C++ 23.3.5.4 forward_iterator 定義了一些合法的用法:
```
r++
*r++
```
特別的是(input iterator和output_iterator不支援):
> _a_ == _b_ implies ++ _a_ == _b_ ++ and _X_ is a pointer type or the expression (void)++_X_(_a_), _*a_ is equivalent to the expression _*a_.
在forward_iterator
* bidirectional_iterator:
> The bidirectional_iterator concept adds the ability to move an iterator backward as well as forward.
bidirectional_iterator概念增加了向後和向前移動迭代器的功能。
> ISO C++ 23.3.5.5 bidirectional_iterator 定義了一些合法的用法:
```
--r
r--
*r--
```
* random_access_iterator:
> The random_access_iterator concept adds support for constant-time advancement with+=,+,-=, and-, as well as the computation of distance in constant time with-. Random access iterators also support array notation via subscripting.
random_access_iterator概念增加了對使用+ =，+，-=和-進行連續時間前進的支持，以及使用-進行連續時間的距離計算。Random access iterators還通過下標([])支持陣列符號。
```
r += n
a + n
n + a
r -= n
a - n
b - a
a[n]
a < b
a > b
a >= b
a <= b
```
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