Is it Undefined Behavior to Directly Remove Elements from the Underlying Range of filter_view?

Are you a C++ enthusiast struggling to understand the intricacies of ranges and views? Do you find yourself wondering if it’s safe to remove elements directly from the underlying range of a filter_view? Look no further! In this article, we’ll delve into the world of C++ ranges and explore the answer to this crucial question.

What is filter_view?

Before we dive into the main topic, let’s take a step back and understand what filter_view is. filter_view is a C++20 range adapter that allows you to filter elements from an underlying range based on a predicate. In simpler terms, it’s a view that shows you only the elements that satisfy a certain condition.

auto numbers = std::vector{1, 2, 3, 4, 5, 6, 7, 8, 9, 10};
auto even_numbers = numbers | std::views::filter([](int i){ return i % 2 == 0; });

In this example, even_numbers is a filter_view that shows only the even numbers from the original vector.

The Underlying Range of filter_view

Now that we know what filter_view is, let’s talk about the underlying range. The underlying range is the original range that the filter_view is operating on. In the previous example, the underlying range is the vector numbers.

Here’s the important part: the underlying range is not modified when you create a filter_view. The filter_view is a separate entity that only shows you a filtered view of the original range. This means that you can have multiple filter_views operating on the same underlying range, each showing you different elements based on their respective predicates.

The Question: Is it Undefined Behavior to Directly Remove Elements from the Underlying Range of filter_view?

So, what happens if you try to remove elements directly from the underlying range of a filter_view? Will the world come to an end? Will your program crash and burn? Let’s find out.

According to the C++20 standard, it is indeed undefined behavior to directly remove elements from the underlying range of a filter_view while it’s still in use.

auto numbers = std::vector{1, 2, 3, 4, 5, 6, 7, 8, 9, 10};
auto even_numbers = numbers | std::views::filter([](int i){ return i % 2 == 0; });

// This is undefined behavior!
numbers.erase(std::remove(numbers.begin(), numbers.end(), 4), numbers.end());

Why is this undefined behavior, you ask? It’s because the filter_view is still referring to the original range, and by modifying the underlying range, you’re invalidating the filter_view’s iterators. This can lead to all sorts of nasty consequences, including crashes, incorrect results, or even demons flying out of your nose (just kidding about that last one… or am I?).

The Solution: Use the filter_view’s Iterator

So, how do you remove elements from the underlying range of a filter_view safely? The answer is to use the filter_view’s iterator!

auto numbers = std::vector{1, 2, 3, 4, 5, 6, 7, 8, 9, 10};
auto even_numbers = numbers | std::views::filter([](int i){ return i % 2 == 0; });

// Get the iterator from the filter_view
auto it = even_numbers.begin();

// Remove the element at the iterator
numbers.erase(std::remove(it.base(), numbers.end(), *it), numbers.end());

By using the filter_view’s iterator, you’re ensured that the underlying range is modified correctly, and the filter_view will still work as expected.

Best Practices

To avoid undefined behavior and keep your code sane, follow these best practices:

• Never modify the underlying range of a filter_view while it’s still in use.
• Always use the filter_view’s iterator to remove elements from the underlying range.
• Make sure to update the filter_view after modifying the underlying range.

Conclusion

In conclusion, it is indeed undefined behavior to directly remove elements from the underlying range of a filter_view. However, by using the filter_view’s iterator and following best practices, you can safely modify the underlying range and keep your code correct and efficient.

Remember, C++ is a powerful language that requires careful handling. By understanding the intricacies of ranges and views, you can unlock its full potential and write code that’s both elegant and correct.

Best Practices	Description
Don’t modify the underlying range	Never modify the underlying range of a filter_view while it’s still in use.
Use the filter_view’s iterator	Always use the filter_view’s iterator to remove elements from the underlying range.
Update the filter_view	Make sure to update the filter_view after modifying the underlying range.

Stay tuned for more C++ awesomeness, and remember to always keep your code undefined-behavior-free!

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