如何使C ++中的每个循环函数使用自定义类 [英] How to make the for each loop function in C++ work with a custom class

问题描述

我刚刚接触C / C ++编程。我在C ++中做了一个自定义类，在C＃中作为一个列表。我用这种方式：

  List< int> ls; 
 int whatever = 123; 
 ls.Add（1）; 
 ls.Add（235445）; 
 ls.Add（whatever）; 
  

它具有动态分配。我有一个 T *向量变量，我把我的数组，当列表即将完全填充，我调整为这样的大小： 8 *（pow（++ realloc_count，4）* 4）。它的工作很好，不知道这个计算是否是最正确的实现这一点。但是无论如何。

我喜欢重新发明一个更低级别的语言，当我可以，这就是为什么我不使用 T> 类。我这样做只是为了好玩。

现在我想循环遍历我的列表的项目。我不喜欢使用好的旧 for（int i = 0; i 循环，所以我想：如果我可以使用一个漂亮漂亮的 foreach-like 循环，使用如下语法：

  foreach（int item in list）{Work（item）;} 
  

code> 在视觉工作室，等待Intellisense，它建议我这：

  for each（object var in collection_to_loop）
 {
 
} 
  

NICE！有一个foreach循环的集合在C + +我想，但我的类OBVIOUSLY将不工作，我的列表是非常基本的。经过一些研究，我注意到，这将只适用于STL库...

我可以用UGLIEST KLUDGE我可以THINK，我想摆脱它：

  #define foreach（type，var，list）\ 
 int _i_ = 0; \ 
 ## type var; \ 
 for = 0，var = list [_i_]; _i_   

我使用它像这样：

  foreach（int，i，ls）{
 doWork（i）; 
} 
  

请不要杀我。

解决方案

那么，有没有办法写一个甜美美丽的foreach样循环？ div>

首先， C ++ 中的 for-each 循环的语法不同于 C＃（它也被称为以循环为基础的范围），形式如下：

  for（< type> < name> ：< collection>）{...} 
  

c> std :: vector< int> vec ，它会是这样：

  for（int i：vec）{... } 
  

覆盖下，这有效地使用 begin c $ c>和 end（）成员函数，它返回迭代器。因此，要允许您的自定义类使用 for-each 循环，您需要提供 begin（） end（）函数。这些通常是重载的，返回一个迭代器或者一个 const_iterator 。实现迭代器可能很棘手，虽然有一个向量类的类不是太难。

  template< typename T& 
 struct List 
 {
 T * store; 
 std :: size_t size; 
 typedef T * iterator; 
 typedef const T * const_iterator; 
 
 .... 
 
迭代器begin（）{return& store [0]; } 
 const_iterator begin（）const {return& store [0]; } 
 iterator end（）{return& store [size]; } 
 const_iterator end（）const {return& store [size]; } 
 
 ... 
}; 
  

使用这些实现，您可以使用上述基于范围的循环。

I'm new to C/C++ programming. I made a custom class in C++ that works as a List in C#. I use it this way:

List<int> ls;
int whatever = 123;
ls.Add(1);
ls.Add(235445);
ls.Add(whatever);

It has dynamic allocation. I have a T* vector variable where I put my array, and when the list is about to be completely filled, I resize it to this size: 8 * (pow(++realloc_count,4) * 4). It's working good, not sure if this calculation is the most correct to achieve this. But whatever.

I like to reinvent the wheel in a lower-level language when I can, that's why I'm not using the vector<T> class. I'm doing this just for fun.

Now I want to loop through the items of my list. I don't like to use the good old for(int i=0;i<n; i==) loop, so I thought: "What if I could use a nice and beautiful foreach-like loop, using a syntax like this:

foreach (int item in list) { Work(item); }

And then I typed for in the visual studio, awaited for Intellisense, and it suggested me this:

for each (object var in collection_to_loop)
{

}        

"NICE! There is a foreach-loop for collections in C++" I thought. But my class OBVIOUSLY won't work with that. My List is very basic. And after some research I noticed that this will only work with the STL Libraries...

I can do that with the UGLIEST KLUDGE I COULD THINK, and I want to get rid of it:

#define foreach(type, var, list)\
int _i_ = 0;\
##type var;\
for (_i_ = 0, var=list[_i_]; _i_<list.Length();_i_++,var=list[_i_]) 

And I use it like this:

foreach(int,i,ls){
    doWork(i);
}

Please don't kill me.

So, is there a way to write a sweet and beautiful foreach-like loop?

解决方案

Firstly, the syntax of a for-each loop in C++ is different from C# (it's also called a range based for loop. It has the form:

for(<type> <name> : <collection>) { ... }

So for example, with an std::vector<int> vec, it would be something like:

for(int i : vec) { ... }

Under the covers, this effectively uses the begin() and end() member functions, which return iterators. Hence, to allow your custom class to utilize a for-each loop, you need to provide a begin() and an end() function. These are generally overloaded, returning either an iterator or a const_iterator. Implementing iterators can be tricky, although with a vector-like class it's not too hard.

template <typename T>
struct List
{
    T* store;
    std::size_t size;
    typedef T* iterator;
    typedef const T* const_iterator;

    ....

    iterator begin() { return &store[0]; }
    const_iterator begin() const { return &store[0]; }
    iterator end() { return &store[size]; }
    const_iterator end() const { return &store[size]; }

    ...
 };

With these implemented, you can utilize a range based loop as above.

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