类
<iterator>
std::forward_iterator_tag
struct forward_iterator_tag {};Forward iterator category
Empty class to identify the category of an iterator as a forward iterator
Forward iterators
Forward iterators are iterators that can be used to access the sequence of elements in a range in the direction that goes from its beginning towards its end.
Performing operations on a forward iterator that is dereferenceable never makes its iterator value non-dereferenceable. This enables algorithms that use this category of iterators to use multiple copies of an iterator to pass more than once by the same iterator values.
All bidirectional and random-access iterators are also valid forward iterators.
There is not a single type of forward iterator: Each container may define its own specific iterator type able to iterate through it and access its elements. But all forward iterators support -at least- the following operations
| 属性 | 有效的表达式 |
|---|
| 可以默认构造、拷贝构造、拷贝赋值和析构 | X a;
X b(a);
b = a; |
可以使用相等/不等运算符进行等价比较
（当两个迭代器值指向相同的底层序列时才有意义）。 | a == b
a != b |
| 可以作为右值解引用（如果处于可解引用状态）。 | *a
a->m |
对于可变迭代器（非 const 迭代器）
可以作为左值解引用（如果处于可解引用状态）。 | *a = t |
可以递增（如果处于可解引用状态）。
结果是也可解引用或越界迭代器。
Two iterators that compare equal, keep comparing equal when both are increased.
| ++a
a++
*a++ |
| 属性 | 有效的表达式 |
|---|
| 可以默认构造、拷贝构造、拷贝赋值和析构 | X a;
X b(a);
b = a; |
可以使用相等/不等运算符进行等价比较
（当两个迭代器值指向相同的底层序列时才有意义）。 | a == b
a != b |
| 可以作为右值解引用（如果处于可解引用状态）。 | *a
a->m |
对于可变迭代器（非 const 迭代器）
可以作为左值解引用（如果处于可解引用状态）。 | *a = t |
可以递增（如果处于可解引用状态）。
结果是也可解引用或越界迭代器。
Two iterators that compare equal, keep comparing equal when both are increased.
| ++a
a++
*a++ |
| 左值是可交换的。 | swap(a,b) |
Where X is a forward iterator type, a and b are objects of this iterator type, and t is an object of the type pointed by the iterator type (or some other type that can be assigned to the lvalue returned by dereferencing an object of type X).
常量迭代器 是不满足输出迭代器要求的迭代器；解引用它们会得到对常量元素的引用（例如 const T&）。
These properties are the same as those of bidirectional iterators, except that forward iterators only support being incremented (but not decreased).