[英]C++ Iterator for C Linked Lists: to use range-based for loops
I am working with legacy C
code and the new code is written in C++
.我正在使用旧的
C
代码,新代码是用C++
编写的。 To use the C++ standard library, I wrote a simple Iterator
for the legacy LinkedList
as shown below after reading through Bjarne Stroustrup's blog post on Adaptation .为了使用 C++ 标准库,我在阅读了 Bjarne Stroustrup 关于Adaptation的博文后,为旧版
LinkedList
编写了一个简单的Iterator
,如下所示。
My question is:我的问题是:
I want to create another Iterator
for another struct
say struct TokenList
.我想为另一个
struct
创建另一个Iterator
,比如struct TokenList
。 I am not sure how to use namespace
and still be able to use the range-based for
loops.我不确定如何使用
namespace
并且仍然能够使用基于范围的for
循环。 Any pointers would be helpful.任何指针都会有所帮助。
Are the adapters for the Iterator
namely: begin
, end
, ++
, *
, !=
correct? Iterator
的适配器,即: begin
, end
, ++
, *
, !=
正确吗? Currently, I'm an interested in reading the contents of the LinkedList
using range-based for
loops.目前,我对使用基于范围的
for
循环读取LinkedList
的内容很感兴趣。
#include <cstdio>
#include <numeric>
#include <algorithm>
struct LinkedList {
double v;
LinkedList *next;
};
struct Iterator {
LinkedList *current;
LinkedList &c;
};
Iterator begin(LinkedList *c) { return Iterator {c, *c}; }
Iterator end(LinkedList *c) { return Iterator {nullptr, *c}; }
Iterator &operator++(Iterator &p) { p.current = p.current->next; return p; }
LinkedList *operator*(Iterator p) { return p.current; }
bool operator!=(Iterator lhs, Iterator rhs) { return (lhs.current != rhs.current); }
int main()
{
LinkedList *node1 = new LinkedList;
LinkedList *node2 = new LinkedList;
LinkedList *node3 = new LinkedList;
node1->v = 1; node1->next = node2;
node2->v = 2; node2->next = node3;
node3->v = 3; node3->next = nullptr;
printf("// C style: iteration\n");
for (auto ptr = node1; ptr; ptr = ptr->next) {
printf("%e\n", ptr->v);
}
auto head = node1;
// make use of begin(), end(), ++, != and *
printf("// Modern C++ style: range based for-loop\n");
for (const auto& it : head) {
printf("%e\n", it->v);
}
std::for_each(begin(head), end(head), [](auto t) { delete t; });
return 0;
}
Iterators are pseudo-pointer types.迭代器是伪指针类型。 That means they themselves are regular.
这意味着他们自己是有规律的。
struct Iterator {
LinkedList *current;
LinkedList &c;
};
Here you mix references and pointers.在这里你混合引用和指针。 This is a serious anti-pattern, as what does assignment do?
这是一个严重的反模式,赋值是做什么的? There is no sensible answer.
没有明智的答案。
I woukd remove the c
member entirely.我会完全删除
c
成员。
Next you need to broadcast an iterator type.接下来你需要广播一个迭代器类型。 Yours looks like a forward iterator.
你的看起来像一个前向迭代器。 All end iterators can be equal.
所有结束迭代器都可以相等。
Iterator begin(LinkedList *c) { return Iterator {c, *c}; }
Iterator end(LinkedList *c) { return Iterator {nullptr, *c}; }
These look ok.这些看起来不错。
Note that the name does not have to be Iterator
.请注意,名称不必是
Iterator
。 begin
/ end
must be defined in the namespace of LinkedList
, but the return type does not have to be. begin
/ end
必须在LinkedList
的命名空间中定义,但返回类型不必是。
Iterator &operator++(Iterator &p) { p.current = p.current->next; return p; }
I usually implement this as a member function, and implement both pre and post increment;我通常将其实现为成员 function,并实现前后递增; post is implemented using pre and copy.
post 是使用 pre 和 copy 实现的。
LinkedList *operator*(Iterator p) { return p.current; }
This is wrong.这是错误的。 It should return
*p.current
as a double&
.它应该返回
*p.current
作为double&
。
bool operator!=(Iterator lhs, Iterator rhs) { return (lhs.current != rhs.current); }
sure.当然。 Also implement
==
as !(lhs!=rhs)
.还将
==
实现为!(lhs!=rhs)
。
Look up the forward iterator concept and forward iterator tag.查找前向迭代器概念和前向迭代器标签。 Include the types needed for
std::iterator_traits
.包括
std::iterator_traits
所需的类型。
For other things to iterate, give the iterator a different name.对于其他要迭代的东西,给迭代器一个不同的名字。 This can be via a different namespace.
这可以通过不同的命名空间。
If the thing that differs is just the type of the value, you can make it a template pretty easy.如果不同的只是值的类型,你可以很容易地把它变成一个模板。 Then you only have to manually write
begin
/ end
.然后你只需要手动编写
begin
/ end
。
If the name of v
also changes, you could use ADL on a GetValue(List*)
function you write as a customization point.如果
v
的名称也发生变化,您可以在作为自定义点编写的GetValue(List*)
function 上使用 ADL。
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