Non constexpr operator== #34
Description
First of all, great work.
This is probably the most thorough and streamlined backport of string_view publicly available, and I thank you for making it such.
I'm using it to replace Google's Abseil implementation, which still has some rough edges, and certainly drags in a lot of baggage for someone who just needs string_view.
One of my biggest motivations for using string_view is to facilitate string keyed associative containers with compile-time lookup, which opens the door to many useful things with zero runtime overhead. However, this requires constexpr comparison to be available...
This string_view delegates operator== to char_traits::compare() which is part of std, and as such is required NOT to be marked constexpr in pre-C++17 compilation. This is despite the compiler being perfectly able to perform constexpr comparisons.
In my case then, there is little value in backporting string_view if it doesn't backport its benefits too.
My workaround solution for this was deriving my own char_traits to implement a constexpr compare(), and then deriving my own string_view, as shown below. Perhaps you can think of a better way to backport constexpr comparison.
Thx
namespace my_own {
namespace _detail {
struct char_traits : public std::char_traits<char> {
static constexpr int compare( const char_type* s1, const char_type* s2, std::size_t count ) {
... compiler specific ...
}
};
using plain_string_view = nonstd::basic_string_view<char>;
}
class string_view : public nonstd::basic_string_view<char, _detail::char_traits> {
using Base = nonstd::basic_string_view<char, _detail::char_traits>;
public:
using Base::Base;
// need to provide own implicit conversions with std::string, since it uses the default char_traits
operator string() const noexcept { return string(data(), size()); }
string_view(string const & s) noexcept : Base(s.data(), s.size()) {}
// provide implicit conversions with a string view that uses standard char_traits
constexpr operator _detail::plain_string_view() const {return {data(), size()};}
constexpr string_view(_detail::plain_string_view p) : Base(p.data(), p.size()) {}
};
inline std::ostream & operator<<(std::ostream & os, string_view sv) { return os << _detail::plain_string_view(sv); }
} // namespace