如何让我自己的惰性迭代器？ [英] How could I make my own lazy iterator?

问题描述

我正在做一个C ++ 11类，产生大量的数据。该数据目前来自数据库，它不能完全适合内存。我想为用户提供一个迭代器，它的行为就像普通的STL迭代器，但这将是懒惰的。更准确地说，我可以做这样的事情：

I'm making a C++11 class that produces a huge amount of data. That data currently comes from a database and it cannot fit entirely in memory. I would like to provide the user with an iterator that behaves like regular STL iterators, but that would be lazy. More precisely, I would be able to do something like that :

for (auto& item : big_bunch_of_data) {
    do_stuff_with(item);
}

仅在每次迭代时从数据库检索项目。如果我是对的，这个新的语法是糖

With item being retrieved from the database only at each iteration. If I'm right, this new syntax is sugar for

for (stuff::iterator it = big_bunch_of_data.begin();it != big_bunch_of_data.end();it++) {
    do_stuff_with(*it);
}

这是否意味着提供 begin ， end 和 operator ++ ，我可以有期望的行为？而且，这些方法应该做什么？

Does it mean that by providing begin, end and operator++, I could have the desired behavior ? And, what are these methods supposed to do ? I mean, can I make them lazy without breaking stuff ?

推荐答案

几乎;如果找不到 begin 或 end 对容器类的方法;这是基于范围的for循环在数组上的工作，没有 begin 和 end 成员。它还将通过ADL寻找自由函数 begin 和 end ，最终还会查找 std :: begin 和 std :: end ，因此有很多机会来改进基于范围的for循环支持现有容器。第6.5.4节涵盖了细节。

Almost; the compiler will look in a few other places to get the begin and end iterators if it can't find begin or end methods on the container class; this is how range-based for loops work on arrays, that don't have begin and end members. It will also look for free functions begin and end by ADL, and eventually std::begin and std::end, so there's plenty of opportunity to retrofit range-based for loop support to existing containers. Section 6.5.4 covers the details.

对于你的其他问题，迭代器绝对可以是惰性的！一个好的例子是 std :: istream_iterator ，它从控制台读取输入时为延迟。

For your other question, iterators absolutely can be lazy! A good example is std::istream_iterator which has to be lazy as it reads input from the console.

在中为循环使用迭代器的要求是它应该满足输入迭代器类别，第24.2.3节;该类别所需的操作为！= ，一元 * ，前后增量 ++ 。

The requirement to use an iterator in a for loop is that it should satisfy the input iterator category, which is described in section 24.2.3; the required operations for that category are !=, unary *, and pre- and post-increment ++.

为了让语言知道你创建了一个输入迭代器，你应该继承 std :: iterator< std :: input_iterator_tag，T，void，T *，T&> 其中 T （第24.4.3节）。

To let the language know that you've created an input iterator, you should inherit from std::iterator<std::input_iterator_tag, T, void, T *, T &> where T is the type your iterator deals in (section 24.4.3).

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