wil should add GUID-related APIs

[JeffMill]

WIL has these constants:

    //! For {guid} string form. Includes space for the null terminator.
    constexpr size_t guid_string_buffer_length = 39;
 
    //! For {guid} string form. Not including the null terminator.
    constexpr size_t guid_string_length = 38;

but no complementary APIs to do conversion to/from std::string. Plus things for CoCreateGuid, format to/from strings, etc. I know many teams end up using cppwinrt's helpers to do this instead.

[dunhor]

IMO - if you're trying to heap allocate a GUID string, 9 times out of 10 you're doing it wrong. The length is statically known, so it's almost always better to just have it allocated where you need it as an array - either on the stack or as a member of a struct. Some possible exceptions:

• The string is a member of a struct, but is conditionally set and the extra memory being allocated might be excessive
• You are calling an API that takes ownership of the string (rare) or are implementing an API that gives ownership of a string (less rare). In this case, you'd probably want a LocalAlloc-ed, etc. type, though
• You are, for some reason, storing a dynamically sized list of these strings

I could see something like:

template <typename StringT>
HRESULT to_string_nothrow(const GUID&, StringT&);

template <typename StringT>
StringT to_string(const GUID&);

for these scenarios as quick helpers for the rare times where this is needed.

[jonwis]

And also a create_guid that emits your favorite GUID type, defaulting to GUID, like:

template<typename TGuid = GUID> create_guid()
{
    GUID g;
    THROW_IF_FAILED(::CoCreateGuid(&g));
    return { g };
}

(No nonthrowing variant required since CoCreateGuid already exists.)

[JeffMill]

to implementer: consider that GUIDs have two forms -- with and without surrounding braces, so it would be nice if this implementation also supported this. PowerShell does a good job IMO:

> [Guid]'23f1dd41-a85f-4b5c-8ce0-010bd2f886f7'

Guid
----
23f1dd41-a85f-4b5c-8ce0-010bd2f886f7

> [Guid]'{23f1dd41-a85f-4b5c-8ce0-010bd2f886f7}'

Guid
----
23f1dd41-a85f-4b5c-8ce0-010bd2f886f7

> ([Guid]'{23f1dd41-a85f-4b5c-8ce0-010bd2f886f7}').ToString()
23f1dd41-a85f-4b5c-8ce0-010bd2f886f7

And would be nice to get to underlying struct as well, so one could access the underlying fields of the GUID struct if needed.

[jonwis]

See also the work I did on the static_sid type, which returns a custom type of "the right size." So you could define a new struct guid_string that contains a 39-charcter wchar_t inside, and has an "operator std::wstring_view" (or the new zwstring_view) and has .c_str so you can just get a stack thing that can be used like a string.