Why no struct default constructors?

[Sean Eskapp]

== Quote from Jonathan M Davis (jmdavisProg at gmx.com)'s article
> On Saturday 12 February 2011 10:48:04 Sean Eskapp wrote:
> > With the removal of scope classes, the only way of which I know to use RAII
> > objects is to use structs. This is fine, but structs aren't allowed default
> > constructors! Why is this?
> It's because they have to have an init property. _All_ types have an init
> property. It's what they're default initialized to. For bool, it's false. For
> integral types, it's 0. For floating point types, it's NAN. Etc. For classes,
> that's null, so they can have default constructors just fine. However, for
> structs, it must be what the struct's member variables are directly initialized
> to. If you could have an arbitrary default constructor, then you'd have the
> problem of it throwing exceptions as well as it trying to run functions which
> could only be run at runtime. The struct's init value must be known at compile
> time, and it must _always_ be the same. So, it can't involve any functions which
> would have to be run at runtime, and it can't involve anything that would result
> in differing values betwen runs of the default constructor.
> There has been some discussion of allowing structs to have limited default
> constructors, but they would still disallow most of the kinds of things that
> people would want to do with a default constructor. init _must_ be known at
> compile time. So, its value is taken from the value that a struct gets when all
> of its member variables have been directly initialized (or default initialized)
> and no constructor has been called.
> It _can_ be a bit annoying at times, but it's either that or allow undefined
> struct values, which would be a big problem and go against D's philosophy on
> default initialization.
> Since you can't have a default constructor, what you do instead is define a
> static opCall() function for the struct and use that. Then you construct a
> struct (with the name S in this case) like so:
> auto s = S();
> You're still stuck with the init state if you do
> S s;
> or if you're dealing with structs in an array and the like (since they're all
> default constructed to the struct's init property). But you can do RAII just
> fine. Personally, I _always_ use this syntax when declaring struct variables:
> auto S = S();
> So, the situation isn't exactly ideal, but we're kind of stuck, given the
> various constraints that we have to work with.
> - Jonathan M Davis

Alright, thanks for clearing that up!