Why this code doesn't write two identical lines?

https://dpaste.dzfl.pl/e99aad315a2a

Andrea

On Monday, 2 October 2017 at 08:47:47 UTC, Andrea Fontana wrote:
> Why this code doesn't write two identical lines?
>
> https://dpaste.dzfl.pl/e99aad315a2a
>
> Andrea

A reduced example of where it goes wrong:

class B {}

struct A {
    B b = new B;
}

unittest {
    A a1, a2;
    assert(a1 == a2);
}

In other words, when you initialize the class reference in your struct, it has to be a value that's known at compile-time. So the compiler creates a single instance of B, and every instance of A points to it. So this line:

    A a = A(A(1), 2);

first appends 1 to b.data, then appends 2 to b.data, and it's the same b in both cases.

Not knowing what you're attempting to do, I'm not sure how to fix your problem. But if what I've described above does indeed cover it, initializing b in the constructor is the way to get it to work.

--
  Biotronic

On Monday, 2 October 2017 at 09:08:59 UTC, Biotronic wrote:
> Not knowing what you're attempting to do, I'm not sure how to fix your problem. But if what I've described above does indeed cover it, initializing b in the constructor is the way to get it to work.
>
> --
>   Biotronic

Obviusly real example is quite different and larger.
Anyway: you cant put a default destructor on struct

On Monday, 2 October 2017 at 09:34:29 UTC, Andrea Fontana wrote:
> Anyway: you cant put a default destructor on struct

True. In which case you should either @disable this() (which presents its own set of issues) or hide b behind a @property function, something like:

struct S {
    B _b;

    @property
    B b() {
        if (_b is null) _b = new B();
        return b;
    }
}

This exact same issue also crops up for classes, since typeid(T).initializer is simply blitted over the newly allocated memory. At least for classes we could change the language such that:

class C {
    int[] p = new int[5];
}

is sugar for:

class C {
    int[] p;
    this() {
        p = new int[5];
    }
}

No such solution exists for structs, since they don't have default constructors.

--
  Biotronic