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Why does ranges not support std::back_inserter? #867

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yaoxinliu opened this issue Jul 20, 2018 · 2 comments

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@yaoxinliu
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commented Jul 20, 2018

        auto src = std::vector<int>{ 1, 2, 3 };
	std::vector<int> dst;

	// ok
	std::copy(src.begin(), src.end(),
		std::back_inserter(dst));

	// ok
	ranges::copy(src, ranges::back_inserter(dst));

	// not ok
	ranges::copy(src, std::back_inserter(dst));
@ericniebler

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commented Jul 20, 2018

Because it doesn't satisfy the Iterator concept as the range library defines it (it's not default constructible). That will be fixed in C++20. At least, that's the plan.

@correaa

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commented Jun 13, 2019

I bumped into the same problem: https://stackoverflow.com/questions/56571907/why-is-back-inserter-iterator-not-weaklyincrementable-in-rangev3

I think by following existing rules too strickly the library is consistently overconstraining the algorithms. I think there was no need to define Iterator as default constructible in the first place. If that is not possible to change, unfortunately we will have to define eventually NonNecessarelyDefaultConstructibleIterator relaxing that condition.

I once did a crude survey of which STL algorithms in effect need default-constructibility and it turns out that in libc++, at the time, it were std::minmax_element and std::rotate, and in both cases there was no actual need for that. I think partially-formed and/or default-constructibility are not bad per se, but requiring it is the problem.

(Another example is requiring Sentinels to be left and right comparable with iterators when one can adopt the convention that Sentinels to be right and left comparable.)

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