I do not understand copy constructors

[Paul Backus]

On Thursday, 12 August 2021 at 12:10:49 UTC, Learner wrote:
>
> That worked fine, but the codebase is @safe:
>
> ```d
> cast from `int[]` to `inout(int[])` not allowed in safe code
> ```
>
> So copy constructors force me to introduce trusted methods, 
> while that was not necessary with postblits?

A postblit would simply ignore the type qualifier--which can lead 
to undefined behavior. (Scroll down to the paragraph that begins 
"An unqualified postblit..." under ["Struct Postblits"][1] in the 
spec.) The copy constructor merely forces you to be honest about 
the safety of your code.

In your case, I would recommend encapsulating the unsafe cast in 
a function like the following:

```d
T[] dupWithQualifiers(T[] array)
{
     auto copy = array.dup;
     return (() @trusted => cast(T[]) copy)();
}
```

You can then use this function in place of `dup` in your copy 
constructor.

[1]: https://dlang.org/spec/struct.html#struct-postblit