• Inkrementator (39/39) Oct 22 2023 Consider this almost minimal example:
• Inkrementator (6/10) Oct 22 2023 Sorry, obviously it should be:
• Paul Backus (7/14) Oct 22 2023 Nested structs contain a context pointer that needs to be
• Inkrementator (5/12) Oct 23 2023 In my case, it makes sense to not initialize the object, so this

Inkrementator <anon anon.org> writes:

Consider this almost minimal example:
```
import std.algorithm;
import std.range;
import std.stdio;

struct S(Nested){
	Nested member; // = I.init; // Uncommenting this wouldn't help
	int g;
	this(Nested member){
		this.member = member;
	}
	
	this(int g){
		this.g = g;
		version(fix)
			member = Nested.init;
	}
	
}

// IFTI to handle type of map
auto makeS(Nested)(Nested member) => S!(Nested)(member);

import std.sumtype;
// This just works and needs no fix
auto makeSum(I)(I i) => SumType!(string, I)(i);

void main(){
	auto input = iota(5).map!(b => b + 1).map!(b => b - 1);

	auto s = makeS(input);
	auto s2 = makeSum(input);
	writeln(s);
	writeln(s2);
}
```

Running the code with `rdmd -version=fix app.d` works, but 
running `rdmd -version=fix app.d` produces:
`Error: field `member` must be initialized in constructor, 
because it is nested struct`
Why?
I didn't find anything about this in [the 
spec.](https://dlang.org/spec/struct.html#nested)

Inkrementator <anon anon.org> writes:

On Sunday, 22 October 2023 at 21:02:32 UTC, Inkrementator wrote:
 Running the code with `rdmd -version=fix app.d` works, but 
 running `rdmd -version=fix app.d` produces:
 `Error: field `member` must be initialized in constructor, 
 because it is nested struct`

Sorry, obviously it should be:
Running the code with `rdmd -version=fix app.d` works, but 
running just `rdmd app.d` produces:
`Error: field `member` must be initialized in constructor, 
because it is nested struct`

Paul Backus <snarwin gmail.com> writes:

On Sunday, 22 October 2023 at 21:02:32 UTC, Inkrementator wrote:
 Running the code with `rdmd -version=fix app.d` works, but 
 running `rdmd -version=fix app.d` produces:
 `Error: field `member` must be initialized in constructor, 
 because it is nested struct`
 Why?
 I didn't find anything about this in [the 
 spec.](https://dlang.org/spec/struct.html#nested)

Nested structs contain a context pointer that needs to be 
initialized at runtime. If you don't initialize them, the pointer 
gets set to `null`, and the struct will not be able to access its 
context.

Using `.init` doesn't fix this because `.init` is determined at 
compile time, and also has `null` in place of the context pointer.

Inkrementator <anon anon.org> writes:

On Sunday, 22 October 2023 at 23:49:40 UTC, Paul Backus wrote:
 Nested structs contain a context pointer that needs to be 
 initialized at runtime. If you don't initialize them, the 
 pointer gets set to `null`, and the struct will not be able to 
 access its context.

I see, thanks for the explanation.

 Using `.init` doesn't fix this because `.init` is determined at 
 compile time, and also has `null` in place of the context 
 pointer.

In my case, it makes sense to not initialize the object, so this 
isn't an issue. Void-initialization would probably be better to 
document my intent, but the compiler won't let me do that.