• Peter Alexander (18/18) Jan 01 2015 Can someone please explain this behaviour? I find it totally
• anonymous (25/43) Jan 01 2015 A simplification of your code that helped me understand what's
• Peter Alexander (5/10) Jan 01 2015 Ah, I see. So the problem is that the nested struct doesn't

"Peter Alexander" <peter.alexander.au gmail.com> writes:

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.

"anonymous" <anonymous example.com> writes:

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".

"Peter Alexander" <peter.alexander.au gmail.com> writes:

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.