Why allow initializers of non-static members that allocate?

[Mike Parker]

On Saturday, 11 June 2022 at 01:14:06 UTC, matheus wrote:

> So, in the case of "int[] arr = new int[](5)", an array of 
> length 5 of type int will be instantiated and its address will 
> be shared among whoever instantiates "S" and be pointed and 
> accessed through arr.
>
> In the second case, "int[2] arr2", 2 consecutive integer spaces 
> in memory will be allocate independently for each 
> "instantiation" of "S", so different address.
>
> I never saw this before (I mean I never wrote the first case), 
> I'm used to the "int[2] arr2;" way of declaring it, but if I 
> had looked this code without knowing this, I'd be certain that 
> s1.arr and s2.arr would have different addresses.

That's because static arrays are allocated as part of the 
instance:

```d
struct Foo {
     int[] dyn;
}

struct Bar {
     int[10] stat;
}

assert(Foo.sizeof == 16);
assert(Bar.sizeof == 40);
```

>
> This is a bit weird (At least for a newbie like me), I really 
> think the compiler should emit an warning about this.
>

At it's core this is just a matter of knowing how two specific 
language features behave (allocation of static vs. dynamic arrays 
coupled with member field initialization). If you aren't aware of 
it and it bites you, then you learn about it and you know it. So 
would you then really want a warning every time you initialize a 
static array field?

People getting bit by `new` in field initialization often enough 
that I think a warning would be helpful. But any such warnings 
need to be enabled by default to be useful, and must have an off 
switch for people who don't need them. So the question in each 
case would be, where's the line between helpful and annoying?

The compiler should be as helpful as it can, but it has to be 
helpful without getting in the way. There's a significant amount 
of learning by trial and error in any programming language. So I 
think there has to be a distinction between things like "easy to 
do by accident even when you know the deal"  and "once you learn 
it, you're unlikely to do it again".