The second item in the documentation mentions "any number of default parameters" when describing copy constructor syntax:

  https://dlang.org/spec/struct.html#struct-copy-constructor

1) I can't figure out how to use those extra parameters. For example, I can't find a special function name to call explicitly:

  S.__cctor(a, 42);  // No go

2) Unlike the examples there, I think the parameter should most usefully be defined as 'const' unless there is a special reason:

struct S {

  // const(S) instead of S:
  this(ref const(S) that) {
  }
}

Do you agree?

Thank you,
Ali

On Thursday, 30 December 2021 at 01:04:10 UTC, Ali Çehreli wrote:
> 2) Unlike the examples there, I think the parameter should most usefully be defined as 'const' unless there is a special reason:
>
> struct S {
>
>   // const(S) instead of S:
>   this(ref const(S) that) {
>   }
> }
>
> Do you agree?

When the compiler generates a copy constructor for you, it always qualifies both the source and destination objects with `inout`:

https://dlang.org/spec/struct.html#implicit-copy-constructors

Therefore, when you write your own copy constructors, you should always use `inout` if possible, so that compiler-generated copy constructors will be able to copy instances of your struct that appear as members of other structs.

On 12/29/21 5:14 PM, Paul Backus wrote:

> Therefore, when you write your own copy constructors, you should always use `inout` if possible, so that compiler-generated copy constructors will be able to copy instances of your struct that appear as members of other structs.

Excellent point. I noticed a typo in the documentation:

struct A
{
    this(ref return scope inout A rhs) immutable {}
}

That 'immutable' should be 'inout', right?

Ali

On Thursday, 30 December 2021 at 02:04:30 UTC, Ali Çehreli wrote:
> On 12/29/21 5:14 PM, Paul Backus wrote:
>
>> Therefore, when you write your own copy constructors, you should always use `inout` if possible, so that compiler-generated copy constructors will be able to copy instances of your struct that appear as members of other structs.
>
> Excellent point. I noticed a typo in the documentation:
>
> struct A
> {
>     this(ref return scope inout A rhs) immutable {}
> }
>
> That 'immutable' should be 'inout', right?

I think 'immutable' is correct here, since the usage examples look like this:

    A r1;
    const(A) r2;
    immutable(A) r3;

    // All call the same copy constructor because `inout` acts like a wildcard
    immutable(A) a = r1;
    immutable(A) b = r2;
    immutable(A) c = r3;