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digitalmars.D.learn - Struct bug?

reply Andrea Fontana <nospam example.com> writes:
Why this code doesn't write two identical lines?

https://dpaste.dzfl.pl/e99aad315a2a

Andrea
Oct 02 2017
parent reply Biotronic <simen.kjaras gmail.com> writes:
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
Oct 02 2017
parent reply Andrea Fontana <nospam example.com> writes:
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
Oct 02 2017
parent Biotronic <simen.kjaras gmail.com> writes:
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
Oct 02 2017