[英]How can I make an iterator over fields of container's items?
So, say, I have a class that has std::list<std::pair<int, std::string>>
inside of it; 所以,比方说,我有一个在其中有
std::list<std::pair<int, std::string>>
的类; how can I implement an iterator for this class such that it iterates over strings contained in this list? 如何为此类实现迭代器,以便迭代此列表中包含的字符串?
Or, for example, in my class I have a vector of struct
s with fields a
, b
and c
; 或者,例如,在我的班级中,我有一个带有字段
a
, b
和c
的struct
的向量; can I create an iterator (maybe inherit vector's iterator? don't know), which, when dereferenced, will return a std::pair
, corresponding to (b, c)
? 我可以创建一个迭代器(也许继承向量的迭代器吗?不知道),当取消引用时,它将返回一个
std::pair
,对应于(b, c)
?
By iterator I mean something like std::vector
's iterators: something, that I can get via whatever.begin()
and iterate over, as mentioned, strings inside the list. 通过迭代器,我的意思是像
std::vector
的迭代器:我可以通过whatever.begin()
得到的whatever.begin()
,如上所述,迭代遍历列表中的字符串。
UPD Okay, here's more info on what I want. UPD好的,这里有关于我想要的更多信息。 In my
HashMap
class I have items
: a list of structs
: each with a key, a value and a pointer to it's place in the table. 在我的
HashMap
类中,我有一些items
:一个structs
列表:每个structs
都有一个键,一个值和一个指向它在表中的位置的指针。 What I need is an iterator; 我需要的是一个迭代器; but not the one that I can get by doing
items.begin()
, since this iterator, when dereferenced, will return my struct. 但不是我可以通过执行
items.begin()
获得的items.begin()
,因为这个迭代器在取消引用时会返回我的结构。 I need an iterator, such that I can return it when user calls HashMap.begin()
, and it should dereference into a std::pair
, corresponding to (key, value). 我需要一个迭代器,这样我可以在用户调用
HashMap.begin()
时返回它,它应该取消引用到std::pair
,对应于(key,value)。
This should, hopefully, make my question clearer. 希望这应该让我的问题更清楚。
UPD2 Here is my struct
, if that helps: UPD2这是我的
struct
,如果有帮助:
template<class KeyType, class ValueType>
struct node {
KeyType key;
ValueType value;
node** place;
node(KeyType key_ = KeyType(), ValueType value_ = ValueType()): key(key_), value(value_) {};
};
One elegant way is to use transforming iterators or a transformed range: 一种优雅的方法是使用转换迭代器或转换范围:
#include <iostream>
#include <list>
#include <boost/range/adaptor/transformed.hpp>
#include <boost/iterator/transform_iterator.hpp>
int main() {
std::list<std::pair<int, std::string>> l;
auto extractor = [](auto&& elem) { return elem.second; };
// Using a transformed range.
for(auto&& v: l | boost::adaptors::transformed(extractor))
std::cout << v << '\n';
// Using transform iterators.
for(auto i = boost::make_transform_iterator(l.begin(), extractor), j = boost::make_transform_iterator(l.end(), extractor); i != j; ++i)
std::cout << *i << '\n';
}
To use a transform iterator for you containers you can do something like: 要为容器使用转换迭代器,您可以执行以下操作:
struct MyContianer
{
std::list<std::pair<int, std::string>> container;
static auto constexpr first_extractor = [](auto&& elem) { return elem.second; };
using iterator_first = decltype(boost::make_transform_iterator(container.begin(), first_extractor));
iterator_first begin_first() { return {container.begin(), first_extractor}; }
iterator_first end_first() { return {container.end(), first_extractor}; }
static auto constexpr second_extractor = [](auto&& elem) { return elem.second; };
using iterator_second = decltype(boost::make_transform_iterator(container.begin(), second_extractor));
iterator_second begin_second() { return {container.begin(), second_extractor}; }
iterator_second end_second() { return {container.end(), second_extractor}; }
};
decltype(MyContianer::first_extractor) constexpr MyContianer::first_extractor;
decltype(MyContianer::second_extractor) constexpr MyContianer::second_extractor;
int main() {
MyContianer c;
c.begin_first();
c.begin_second();
}
声明:本站的技术帖子网页,遵循CC BY-SA 4.0协议,如果您需要转载,请注明本站网址或者原文地址。任何问题请咨询:yoyou2525@163.com.