Re: [boost] [hash2][review] Early review (due to holidays)

On 12/7/24 00:51, Claudio DeSouza via Boost wrote:

On Fri, Dec 6, 2024 at 9:47 PM Andrey Semashev via Boost < boost@lists.boost.org> wrote:

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On 12/7/24 00:42, Vinnie Falco wrote:

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On Fri, Dec 6, 2024 at 1:38 PM Andrey Semashev via Boost mailto:boost@lists.boost.org> wrote:

Unless this is a new addition, I don't see such a member in std::span.

https://en.cppreference.com/w/cpp/container/span/subspan

Ah, yes, I missed this, thanks.

Anyway, it doesn't work as the offset is a runtime value in this case.

So to go back to my point: If you really want to avoid checks, and you do know the size beforehand, you can entirely avoid it by harnessing this

type

of compile-time information and leveraging the compiler in your favour. It all depends how dedicated you are to make sure you get that. Granted, with dynamic extent the number of checks will depend on several factors, including implementation, and hardening settings the user is building with.

It is win after win with span.

To convert all the checks that are done in runtime with the code I presented earlier you would have to convert that loop to a series of statements such as this: x[ 0 ] = detail::read32le( block.subspan<0, 4>() ); x[ 1 ] = detail::read32le( block.subspan<4, 4>() ); x[ 3 ] = detail::read32le( block.subspan<8, 4>() ); x[ 4 ] = detail::read32le( block.subspan<12, 4>() ); // ... x[ 15 ] = detail::read32le( block.subspan<60, 4>() ); Or perform some kind of tricks with variadic templates and fold expressions. At least, until `template for` (P1306) is accepted. So, I suppose, it is doable in this limited case. But not something I would willingly do, given that I can achieve the same result with a plain loop and pointers. There's your example of span getting in the way.