What exactly is `span_p<>`?

[mbeutel]

The Core Guidelines specify span_p<> like this:

• span_p<T> // {p, predicate} [p:q) where q is the first element for which predicate(*p) is true

But why would this be a dedicated type rather than simply a span<> algorithm?

template <typename T, typename PredT>
span<T> take_until(span<T> s, PredT&& pred)
{
    ...
}

Perhaps I'm misunderstanding the intention of span_p<>? Could somebody elaborate?

[vmgolovin]

I think the concept arose from views into zero-terminated strings. std::string_view has no information about is the referenced part of string is zero-terminated, which can be useful.

[cubbimew]

I'd say C++20 ranges include this functionality as views::take_while. For example,

    int a[] = {1,2,3,0,-1,-2,-3};
    auto up_to_zero = views::take_while(a, [](int n){return n != 0;});
    for (int n: up_to_zero) std::cout << n; // prints 123

live demo: https://wandbox.org/permlink/OX4smWQLC8uOh6PJ

views into zero-terminated strings.

That's zstring from F.25

[BjarneStroustrup]

On 10/1/2020 9:47 PM, Sergey Zubkov wrote:

I'd say C++20 ranges include this functionality as views::take_while https://en.cppreference.com/w/cpp/ranges/take_while_view. For example,

|int a[] = {1,2,3,0,-1,-2,-3}; auto up_to_zero = views::take_while(a, [](int n){return n != 0;}); for (int n: up_to_zero) std::cout << n; // prints 123|

|Yes. I wouldn't call that pretty, though and I worry about |

|int a[] = {1,2,3,9,-1,-2,-3}; auto up_to_zero = views::take_while(a, [](int n){return n != 0;}); for (int n: up_to_zero) std::cout << n; // prints what? |

||

||

live demo: https://wandbox.org/permlink/OX4smWQLC8uOh6PJ https://wandbox.org/permlink/OX4smWQLC8uOh6PJ

views into zero-terminated strings.

That's |zstring| from F.25

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[shaneasd]

is span_p(a, [](int n){ return n!= 0; }) prettier than take_while(a, [](int n){ return n != 0; }) or am I misunderstanding the syntax?

Does http://isocpp.github.io/CppCoreGuidelines/CppCoreGuidelines#p13-use-support-libraries-as-appropriate apply here? I would have assumed stl > gsl > libraries > my code. Though technically what P.13 says is stl = gsl > libraries > my code. I always assumed that was because the gsl was assumed to be designed with P.13 in mind.

[MikeGitb]

I don't think span_p actually exists does it?

[cubbimew]

int a[] = {1,2,3,9,-1,-2,-3}

in that case the take_while is just a view into the original range, in this case, finite range with 7 elements in it

I don't think span_p actually exists does it?

No in any existing GSL, because the Guidelines are not specific enough about it.