[PATCH, libstdc++, C++20] Implement P0457R2 String Prefix and Suffix Checking.

[Jonathan Wakely]

On 29/11/18 21:35 -0500, Ed Smith-Rowland wrote:
>Greetings,
>
>This patch implements starts_with and ends_with for basic_string and 
>basic_string_view for C++20.

This was on my TODO list, thanks for taking care of it.

>+#if __cplusplus > 201703L
>+      bool
>+      starts_with(basic_string_view<_CharT, _Traits> __x) const noexcept
>+      { return __sv_type(this->data(), this->size()).starts_with(__x); }
>+
>+      bool
>+      starts_with(_CharT __x) const noexcept
>+      { return __sv_type(this->data(), this->size()).starts_with(__x); }
>+
>+      bool
>+      starts_with(const _CharT* __x) const

This can be noexcept. It isn't in the standard, because it has a
narrow contract (there's a precondition that __x is null-terminated).
But since we can't reliably detect whether __x is null-terminated, and
even if we could we wouldn't want to throw an exception, we can just
make it noexcept. Same for the ends_with(const _CharT*) one, and the
equivalent members in the COW basic_string and basic_string_view.

>Index: include/std/string_view
>===================================================================
>--- include/std/string_view	(revision 266645)
>+++ include/std/string_view	(working copy)
>@@ -227,7 +227,6 @@
> 	__sv = __tmp;
>       }
>
>-
>       // [string.view.ops], string operations:
>
>       size_type
>@@ -387,6 +386,36 @@
> 				      traits_type::length(__str));
>       }
>
>+#if __cplusplus > 201703L
>+      constexpr bool
>+      starts_with(basic_string_view __x) const noexcept
>+      { return this->size() >= __x.size()
>+	    && this->compare(0, __x.size(), __x) == 0; }

Please put the opening and closing braces on their own lines, as for
ends_with below:

>+      constexpr bool
>+      ends_with(basic_string_view __x) const noexcept
>+      {
>+	return this->size() >= __x.size()
>+	    && this->compare(this->size() - __x.size(), npos, __x) == 0;
>+      }

OK with those changes, thanks.