C++23 std::views::split size() usage

Question

C++23 std::views::split size() usage

David Lowndes 1,485 MVP

With the changes to split in C++23 I was naively expecting this code to compile:

static void T3( )
{
	std::string s = "1.2.3.4";
	auto splits{ s | std::views::split( '.' ) };
    auto Num = splits.size();
}

The error from msvc is:
error C7500: 'size': no function satisfied its constraints

gcc also doesn't like it - example on Compiler Explorer.

Given the example shown, size() would permit ensuring that there are 4 elements.

Presumably this is because std::views::split only has to support contiguous_range and not sized_range?
Is there a common practical reason for this?
Is there an alternative (built-in means) I've not spotted?

Minxin Yu 8,106 Microsoft Vendor

It seems to be due to sized_range.

   _NODISCARD constexpr auto size()
       requires forward_range<_Derived> && sized_sentinel_for<sentinel_t<_Derived>, iterator_t<_Derived>>

David Lowndes 1,485 Reputation points MVP

2023-11-09T09:37:25.9+00:00

That was what I concluded. Is there a reason for this constraint, and is there a way to work around it?
I should have mentioned that I initially tried to use the size() method because IntelliSense showed it as being available.
Viorel 106.4K Reputation points

2023-11-09T11:01:44.02+00:00
Try this too:

auto Num = std::ranges::distance( splits );
David Lowndes 1,485 Reputation points MVP

2023-11-09T13:29:07.6733333+00:00

That works, but doubles the time taken since distance() loops through the range to compute the value.
Viorel 106.4K Reputation points

2023-11-09T15:03:35.67+00:00

Probably there is no other time that was doubled; the string was not traversed yet.
David Lowndes 1,485 Reputation points MVP

2023-11-09T15:50:01.9133333+00:00

Sorry, I wasn't clear. I was referring to my real trial code (that itself loops round the elements) rather than just the snippet above. It's actually much more performant to just count the iterations in my loop rather than to use distance(), but if there were a fast size() method, I could skip doing the looping entirely if the count didn't match expectations.

This is purely a self learning exercise to get more familiar with newer C++.

Minxin Yu 8,106 Reputation points Microsoft Vendor

2023-11-09T06:54:24.4233333+00:00

It seems to be due to sized_range.

_NODISCARD constexpr auto size() requires forward_range<_Derived> && sized_sentinel_for<sentinel_t<_Derived>, iterator_t<_Derived>>
David Lowndes 1,485 Reputation points MVP

2023-11-09T09:37:25.9+00:00

That was what I concluded. Is there a reason for this constraint, and is there a way to work around it?
I should have mentioned that I initially tried to use the size() method because IntelliSense showed it as being available.
Viorel 106.4K Reputation points

2023-11-09T11:01:44.02+00:00

Try this too:

auto Num = std::ranges::distance( splits );
David Lowndes 1,485 Reputation points MVP

2023-11-09T13:29:07.6733333+00:00

That works, but doubles the time taken since distance() loops through the range to compute the value.
Viorel 106.4K Reputation points

2023-11-09T15:03:35.67+00:00

Probably there is no other time that was doubled; the string was not traversed yet.
David Lowndes 1,485 Reputation points MVP

2023-11-09T15:50:01.9133333+00:00

Sorry, I wasn't clear. I was referring to my real trial code (that itself loops round the elements) rather than just the snippet above. It's actually much more performant to just count the iterations in my loop rather than to use distance(), but if there were a fast size() method, I could skip doing the looping entirely if the count didn't match expectations.

This is purely a self learning exercise to get more familiar with newer C++.

C++23 std::views::split size() usage

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