[英]What is the difference between STL containers iterator and C pointer iterator
i am writing a reverse iterator as a part of my own custom vector class. 我正在编写一个反向迭代器,作为我自己的自定义矢量类的一部分。 Up till now what i have written is as following. 到目前为止,我所写的内容如下。
class MyVector
{
public:
typedef T value_type;
typedef value_type* pointer;
typedef const value_type* const_pointer;
typedef value_type& reference;
typedef const value_type& const_reference;
typedef pointer iterator;
typedef const_pointer const_iterator;
typedef size_t size_type;
class reverse_iterator
{
private:
iterator iter;
public:
inline reverse_iterator(iterator a=0) : iter(a) {}
inline reverse_iterator(const reverse_iterator& rev_iter) : iter(rev_iter.iter) {}
inline reverse_iterator& operator++()
{
--iter;
return *this;
}
// and remaining other operator functions
};
inline iterator begin (void) { return ((iterator)data_array); }
inline iterator end (void) { return ((iterator)data_array+number_of_elements); }
inline reverse_iterator rbegin(void) { return end()-1;}
inline reverse_iterator rend(void) { return begin()-1;}
//functions for myvector class
}; //end of Myvector class
in above class iterater is just C style pointer and reverse_iterator is class. 在上面的类中,iterater只是C风格的指针,而reverse_iterator是类。 So when i do 所以当我做
main()
{
myVector<int> i;
myVector<int>::reverse_iterator rit= i.begin();
}
my reverse_iterator is initialised with begin() function and code is compiling and running. 我的reverse_iterator用begin()函数初始化,代码正在编译并运行。 But this does not happen in case of STL's iterator and reverse_iterator. 但这在STL的迭代器和reverse_iterator情况下不会发生。 They prevent such initialisation. 它们阻止了这种初始化。 eg reverse_iterator not allow himself to be initialised with begin() function, it must be initialised with rbegin() . 例如:reverse_iterator不允许使用begin()函数初始化自己,而必须使用rbegin()初始化。
So what should i do to avoid such kind of initialization ?. 那么我该怎么做才能避免这种初始化呢? Should i write a different iterator like which is in STL. 我应该写一个像STL中一样的不同迭代器吗? And i think it is not possible to write it.. Please give me some solution... 而且我认为这是不可能的..请给我一些解决方案...
Iterators and reverse iterators have a few differences. 迭代器和反向迭代器有一些区别。
The most obvious: incrementing each iterator moves them in opposite directions. 最明显的是:增加每个迭代器的方向是相反的。
Less obvious: the return of begin()
is not the end of a reverse iterator. 不太明显: begin()
的返回不是反向迭代器的结尾。 begin()-1
would be. begin()-1
将会是。 Remember that end()
represents "one-past-the-end", as C++ ranges are half open, ie [begin, end). 记住end()
表示“一个过去”,因为C ++范围是半开的,即[begin,end)。 If you just swap begin()
and end()
in your reverse iterator then your reverse iterator will have the range (begin, end]. 如果仅在反向迭代器中交换begin()
和end()
,则反向迭代器将具有范围(开始,结束)。
In other words, there's a reason why STL containers have both begin()
and end()
and rbegin()
and rend()
, even if the iterators and reverse iterators themselves might be compatible in some ways. 换句话说,即使迭代器和反向迭代器本身在某些方面兼容,STL容器也具有begin()
和end()
以及rbegin()
和rend()
的原因是有原因的。
To completely prevent conversion from iterator
(pointer) to reverse_iterator
, make the constructor of reverse_iterator
private, and make MyVector
a friend of reverse_iterator
: 为了完全防止从iterator
(指针)到reverse_iterator
转换,请将reverse_iterator
的构造reverse_iterator
MyVector
私有,并使MyVector
成为reverse_iterator
的朋友:
class MyVector
{
// ...
class reverse_iterator
{
private:
friend class MyVector;
reverse_iterator(iterator a=0) : iter(a) {}
// ...
};
// ...
};
If you want that conversion available, but not automatic, then just make the corresponding constructor explicit
. 如果您希望该转换可用,但不是自动转换,则只需使相应的构造函数explicit
。
class MyVector
{
// ...
class reverse_iterator
{
public:
explicit reverse_iterator(iterator a=0) : iter(a) {}
// ...
};
// ...
};
In an unrelated note, I also notice that your implementation of rbegin()
probably invokes undefined behaviour (I can't tell for sure because it depends on the code initializing data_array
): You are not allowed to decrement the pointer to the beginning of an array. 在不相关的说明中,我还注意到,您对rbegin()
可能会调用未定义的行为(我不能确定,因为这取决于初始化data_array
的代码):不允许将指针递减到数组。
By the way, the keyword inline
is not necessary in your case; 顺便说一句,关键字inline
在您的情况下不是必需的。 if you write the member function body inside the class definition, it is automatically inline. 如果您在类定义内编写成员函数主体,则会自动内联。 Of course, the inline
doesn't hurt either. 当然, inline
也不会有任何伤害。
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