Learning D

Objects with const and immutable

In the context of class and struct instances, it's easier to understand the relationship const and immutable have with data. Here's a simple example:

struct ModMe {
    int x;
}
void main() {
    immutable ModMe mm;
    immutable(ModMe)* pmm;
    mm.x = 1;   // Error
    pmm.x = 2;  // Error
}

The declaration of mm creates an instance of type immutable(ModMe). Not only can no assignments be made to mm, mm.x cannot be modified. pmm is a pointer to immutable data, so the pointer can be reassigned, but pmm.x cannot be modified. Now look at this:

struct ModMeHolder {
    ModMe mm;
}
void main() {
    immutable ModMeHolder mmh;
    mmh.mm.x = 1;
}

As immutable is transitive, applying it to mmh causes mm to also be immutable. If, in turn, it had any class ...