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Writing a custom iterator in modern C++

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Re: Writing a custom iterator in modern C++

#11

Post by gpderetta »

arcticbull wrote: post #2

There's six different kinds of iterator?! Off hand, is it just me or... - Isn't an iterator that outputs data to its recipient container not an iterator, but rather some type of filling API? - Isn't a random access iterator not an iterator but rather a slice view? - Isn't a contiguous iterator a leaky abstraction in that an iterator is just an interface that shouldn't expose how the data backing it is stored? This it…

> Isn't an iterator that outputs data to its recipient container not an iterator, but rather some type of filling API?

Sure, but it makes sense for this api to have the same interface as an iterator so that existing iterators can be used without an adapter.

> Isn't a random access iterator not an iterator but rather a slice view?

It is an iterator that supports additional functions. You can use a random access oterators with functions that take output iterators, forward iterators, multipass iterators etc. Of you had untelated concepts for each use case the system would be more complex.

> Isn't a contiguous iterator a leaky abstraction in that an iterator is just an interface that shouldn't expose how the data backing it is stored?

Seeing thought abstractions and specializing algorithms is what makes STL style generic code efficient.

And it is not inconsistent at all. Stepanov has written in grat details about the theoretical fundations of his design.

Re: Writing a custom iterator in modern C++

#12

Post by quietbritishjim »

arcticbull wrote: post #2

There's six different kinds of iterator?! Off hand, is it just me or... - Isn't an iterator that outputs data to its recipient container not an iterator, but rather some type of filling API? - Isn't a random access iterator not an iterator but rather a slice view? - Isn't a contiguous iterator a leaky abstraction in that an iterator is just an interface that shouldn't expose how the data backing it is stored? This it…

You seem to have mixed up an objection to the API with an objection to the user of the word "iterator" for something a bit more general than the normal/original meaning. That conflation isn't terribly helpful. Words can have more than one meaning. So long as everyone's on the same page, that's OK.

Re: Writing a custom iterator in modern C++

#13

Post by mzsmyk »

creato wrote: post #9
arcticbull wrote: post #2

There's six different kinds of iterator?! Off hand, is it just me or... - Isn't an iterator that outputs data to its recipient container not an iterator, but rather some type of filling API? - Isn't a random access iterator not an iterator but rather a slice view? - Isn't a contiguous iterator a leaky abstraction in that an iterator is just an interface that shouldn't expose how the data backing it is stored? This it…

It's funny how two of the most common bandwagon posts you can see on HN and other programming venues is: * C++ is a terrible bloated language that no one should use. * The power of modern computers is wasted on lazy software engineers writing software with too many layers of abstraction.

You don't need to use a "terribly bloated language" to write efficient software.

Re: Writing a custom iterator in modern C++

#14

Post by millstone »

shakna wrote: post #3
arcticbull wrote: post #2

There's six different kinds of iterator?! Off hand, is it just me or... - Isn't an iterator that outputs data to its recipient container not an iterator, but rather some type of filling API? - Isn't a random access iterator not an iterator but rather a slice view? - Isn't a contiguous iterator a leaky abstraction in that an iterator is just an interface that shouldn't expose how the data backing it is stored? This it…

> Isn't a random access iterator not an iterator? Why would it not be? It still allows you to iterate over an entire collection, however the order is left undefined so that certain performance characteristics can be exposed to the underlying implementation. This is the kind of tool you'd reach for if you don't care about the order of a collection, but you do care about performance, and probably care about the entire…

Random access iterators mean you can efficiently advance or retreat an iterator by an arbitrary amount. So a vector (via pointer arithmetic) but not a linked list.

Re: Writing a custom iterator in modern C++

#15

Post by quietbritishjim »

The classic iterator categories (input, output, ...) discussed in this article have been deprecated in C++20, and replaced with new concepts. I would expect a post from 2020 claiming to be about iterators "in modern C++" to be specifically about that change, but actually it's just about the legacy categories, so this is a pretty worthless article.

Re: Writing a custom iterator in modern C++

#16

Post by cma »

quietbritishjim wrote: post #15

The classic iterator categories (input, output, ...) discussed in this article have been deprecated in C++20, and replaced with new concepts. I would expect a post from 2020 claiming to be about iterators "in modern C++" to be specifically about that change, but actually it's just about the legacy categories, so this is a pretty worthless article.

It is mentioned in the article early on:

"The first thing to do is to assign the iterator some properties. Until C++17 this is done by tagging it with the tag dispatch mechanism, while C++20 uses concepts: in this article I will follow the traditional approach."

Re: Writing a custom iterator in modern C++

#17

Post by ribs »

First sentence: “ An iterator is an object that points to an element inside a container”

This is not the primary purpose of an iterator. Does anyone want to hazard a guess as to what an iterator’s primary purpose is? You? You? Bueller?

Re: Writing a custom iterator in modern C++

#18

Post by millstone »

arcticbull wrote: post #2

There's six different kinds of iterator?! Off hand, is it just me or... - Isn't an iterator that outputs data to its recipient container not an iterator, but rather some type of filling API? - Isn't a random access iterator not an iterator but rather a slice view? - Isn't a contiguous iterator a leaky abstraction in that an iterator is just an interface that shouldn't expose how the data backing it is stored? This it…

Iterators are inconsistent, but this plays to the strengths of C++'s duck-typed generics. The same code can either copy or insert, depending on whether you use std::begin or std::back_inserter; this isn't principled but it's powerful.

The "six types of iterators" overstates it. You don't need to sweat those details unless you are writing boost or something. Just add what you need.

I miss C++ iterators in other languages. For example, in Rust it's hard to talk about "a position in a String" which may be advanced or retreated. Usually you give up, use an integer index, and this is much worse.

Re: Writing a custom iterator in modern C++

#19

Post by pumpkinpal »

emmanueloga_ wrote: post #6

It would be nice to write a DSL to generate some of the C++ boilerplate (say, containers, iterators, etc) required as a baseline before starting to solve the actual problem. A DSL could codify rules like the ones described in this article and generate at least a rough first pass to be hand-tuned later.

Boost STL Interfaces provides this functionality as a library. Not just for iterators but both views and containers of sequences too.

https://www.boost.org/doc/libs/1_75_0/doc/html/stl_interface...

Re: Writing a custom iterator in modern C++

#20

Post by DoofusOfDeath »

quietbritishjim wrote: post #15

The classic iterator categories (input, output, ...) discussed in this article have been deprecated in C++20, and replaced with new concepts. I would expect a post from 2020 claiming to be about iterators "in modern C++" to be specifically about that change, but actually it's just about the legacy categories, so this is a pretty worthless article.

Honest question: what does it mean for something to be "deprecated" in (specifically) the c++ language spec?
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