Struct problems

[Don Clugston]

On 17/09/12 20:47, Maxim Fomin wrote:
> I consider current struct creation one of the confusing parts of the
> language (may be the most), due to set of incompatible creation
> semantics masked by same syntax, complicated by couple of semi-bugs
> (7210, 1310, 4053) and naive default arguments embedding into the
> language(3438).
>
> Current creation rules look as follows (http://dpaste.dzfl.pl/a4344ad0):
> 1) if a struct object declaration has no initializer, its members are
> default initialized to either explicit initializer in struct declaration
> or type default initializer (note: actually, there are no implicit
> struct constructors). If it has void initializer, it contains garbage
> 2) otherwise, if it has initialization form S() than:
> a) if opCall member is present, it is called (must have no arguments)
> b) otherwise, initialization proceed according to 1)
> 3) otherwise, if it has initialization form S(T1 arg1, ...), than:
> a) if opCall member is present, it is called (its parameters must be
> consistent with arguments)
> b) otherwise, if at least one ctor exists, it is called (and again, its
> parameters must be consistent with arguments)
> c) otherwise, this initialization form is called struct literal and
> struct members are initialized to arguments in accordance with their
> order in struct declaration

There is also 4) struct static initializers, which have their own 
complicated set of rules.

>
> This means that if you have S(), or S(x, y, x) - it is impossible to
> know without looking into struct definition what is going on: a function
> call (constructor or opCall) or just initialization. If naive default
> argument treatment is considered, I may add that there is no sense of
> setting default argument to one-argument struct constructor, since
> S()-like expression would call something else (3438). By the way, .init
> property may be hijacked (good news is that it doesn't affect default
> initializer).
>
> Basically, the question is, is it considered to be good, well designed
> feature or not. And if not, would it be changed at some time? What was
> original design of structures in D?

Struct constructors and struct literals are late additions. opCall was, 
I think, a workaround for the lack of struct constructors.

There's a comment in the source code:
  ***************************************
   * This works by transforming a struct initializer into
   * a struct literal. In the future, the two should be the
   * same thing.
   */
  Expression *StructInitializer::toExpression()

That's the only plan I've heard of.