On 13 June 2017 at 08:31, Murray Cumming wrote:
>
> So, I think:
> 1. We would use std::string_view everywhere if all the C functions took
> a length instead of assuming null-termination. That's not going to
> happen.
>
> 2. Overriding all methods to take either a const char* or a std::string
> (ignoring ustring for now), would avoid the extra array copy, but I
> don't think it's worth it other than for methods that typically take
> very large strings.
>
> 3. GTK+ functions that take very large strings tend to take a length,
> to avoid the need for copying. For instance,
> gtk_text_buffer_set_text(). We could take std::string_view there, but
> then our use of std::string_view::to_string() would be wasteful when
> someone passes a std::string to our method.
>
> This is discouraging, so I hope I'm wrong about something.
>
>
Nope, you're correct. string_view is great if you stay in C++ world, but
suboptimal when you need to pass the string to libc functions or C APIs
taking null-terminated strings.
One possible approach (which I have no experience of in practice, only in
theory) is to use string_view objects which explicitly include the
null-terminator in their length:
template<typename C, typename T>
inline std::basic_string_view<C, T>
make_null_terminated_view(const C* s) noexcept
{ return { s, T::length() + 1 }; }
template<typename C, typename T>
inline bool
is_null_terminated_view(std::basic_string_view<C,T> sv) noexcept
{ return sv.length() && !sv.back(); }
And/or create your own cstring_view / zstring_view type which is guaranteed
to be null-terminated:
struct cstring_view : std::string_view {
cstring_view(const char* s)
: std::string_view(s, traits_type::length(s)+1)
{ }
};
Unlike std::string which has a null-terminator after its content that isn't
counted in the length, these string views would count the null character as
part of their content. You'd need a little more care to use this (i.e. when
using the length remember to subtract one where appropriate) but it does
mean you can pass around views to null-terminated strings efficiently
(along with their length, which is the advantage over a raw pointer).
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