Immutable struct fields

[bearophile]

To answer the recent D.learn thread "How would I optimize this parser?", I have tried to find a more efficient way to build the parse tree, so I have used tagged structs, something like this:


enum NodeType { node, text, tag }

struct Node {
    /*immutable*/ NodeType type = NodeType.node;
    Node* parent;
}

struct TextNode {
    /*immutable*/ NodeType type = NodeType.text;
    Node* parent;
    string content;

    public nothrow this(string content) {
        this.content = content;
    }
}

struct TagNode {
    /*immutable*/ NodeType type = NodeType.tag;
    Node* parent;
    string name;
    Node*[] children;

    public nothrow this(string name) {
        this.name = name;
    }

    public nothrow void addChild(Node* newChild) {
        children ~= newChild;
        newChild.parent = cast(Node*)&this;
    }
}


Each struct instance contains a "type" tag that at runtime tells what kind of node it is. This tag never changes in the life of a node, so it's better for it to be immutable, to use the type system to avoid changing it by mistake.

But unfortunately it doesn't work, this is a reduced example:


struct Foo {
    immutable int x = 1;
}
struct Bar {
    immutable int x = 2;
}
static assert(Foo.sizeof == 4);
void main() {
    Foo f;
    assert(f.x == 1);
    assert((cast(Bar)f).x == 1);
}


It seems that immutable fields act like static const fields or enum fields, they have no storage. This seems a little weird to me. Do you know a way to put immutable storage in a struct instance?

Using this doesn't work:

this(int) { type = NodeType.tag; }

And a static this() doesn't seem to work. So in the program I have just used a mutable type field, but it looks silly...

Bye,
bearophile