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.

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".

On Friday, 2 January 2015 at 00:08:02 UTC, anonymous wrote:
> 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.

Ah, I see. So the problem is that the nested struct doesn't really have a sensible default value, meaning you must initialize it explicitly in the constructor.

Thanks for the clarification.