在C ++ 11 foreach循环中获取索引 [英] Get index in C++11 foreach loop
问题描述
是否有一种便捷的方法来获取C ++ 11 foreach循环中当前容器条目的索引,例如python中的 enumerate
:
Is there a convenient way to get the index of the current container entry in a C++11 foreach loop, like enumerate
in python:
for idx, obj in enumerate(container):
pass
我可以想象有一个迭代器也可以返回索引或类似内容。
I could imagine an iterator that can also return the index or similar.
我当然可以有一个计数器,但是迭代器通常不保证在容器上进行迭代的顺序。
Of course I could have a counter, but often iterators don't give guarantees of the order they iterate over a container.
推荐答案
该功能的良好实现可以在这里找到:
A good implementation of the feature you are requested can be found here:
https://github.com / ignatz / pythonic
其背后的想法是,您可以使用自定义迭代器来构建包装结构来进行计数。下面是一个非常简单的示例性实现,以说明该想法:
The idea behind is, that you build a wrapper struct with a custom iterator that does the counting. Below is a very minimal exemplary implementation to illustrate the idea:
#include <iostream>
#include <vector>
#include <tuple>
// Wrapper class
template <typename T>
class enumerate_impl
{
public:
// The return value of the operator* of the iterator, this
// is what you will get inside of the for loop
struct item
{
size_t index;
typename T::value_type & item;
};
typedef item value_type;
// Custom iterator with minimal interface
struct iterator
{
iterator(typename T::iterator _it, size_t counter=0) :
it(_it), counter(counter)
{}
iterator operator++()
{
return iterator(++it, ++counter);
}
bool operator!=(iterator other)
{
return it != other.it;
}
typename T::iterator::value_type item()
{
return *it;
}
value_type operator*()
{
return value_type{counter, *it};
}
size_t index()
{
return counter;
}
private:
typename T::iterator it;
size_t counter;
};
enumerate_impl(T & t) : container(t) {}
iterator begin()
{
return iterator(container.begin());
}
iterator end()
{
return iterator(container.end());
}
private:
T & container;
};
// A templated free function allows you to create the wrapper class
// conveniently
template <typename T>
enumerate_impl<T> enumerate(T & t)
{
return enumerate_impl<T>(t);
}
int main()
{
std::vector<int> data = {523, 1, 3};
for (auto x : enumerate(data))
{
std::cout << x.index << ": " << x.item << std::endl;
}
}
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