On Thursday, 1 January 2015 at 23:06:30 UTC, Peter Alexander
wrote:
Can someone please explain this behaviour? I find it totally
bizarre.
auto f(T)(T x) {
struct S {
T y;
this(int) { }
}
return S(0);
}
void main() {
f(f(0));
}
Error: constructor f376.f!(S).f.S.this field y must be
initialized in constructor, because it is nested struct
Why must y be initialized in the constructor? It isn't const.
Why isn't it default initialized?
Is this explained anywhere in the docs? I can't see anything in
the nested struct section, or in any constructor section.
A simplification of your code that helped me understand what's
going on:
auto f() {
struct S1 {
this(int) { }
}
return S1();
}
struct S2 {
typeof(f()) y; /* Error: field y must be initialized in
constructor, because it is nested struct */
this(int) { }
}
Apparently dmd thinks that the result of f must be a nested
struct. I.e. it needs a context pointer. And I guess hell would
break loose if you'd use a nested struct with a null context
pointer. At least when the context pointer is actually used,
unlike here.
If the struct needed to be nested, the compiler would maybe do
the right thing here: preventing null/garbage dereferencing. As
it is, it should maybe see that S1 doesn't need a context pointer.
You can explicitly mark the struct as not-nested by making it
"static".
