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digitalmars.D.learn - Can you constrain a type based on properties of what it is being

reply aliak <something something.com> writes:
To further explain what I mean:

struct A if (!is(this == immutable) && !is(this == shared))
{
}

shared a = A() // error
auto a = A() // ok
immutable a = A() // error
const a = A() // ok

Fake syntax above of course.

I was thinking about this because I read a few posts about const, 
and approaching it via allowing an object to define a head 
mutable version of itself (see post in general forum by Simon for 
details [1])

And there're few constructs like RefCounted which don't make 
sense with an immutable qualifier.

And then there're situations where a mutable version does not 
make sense (I think this would be a much bigger application), ie: 
handles, resource identifiers, application configuration details, 
etc.

[1] 
https://forum.dlang.org/thread/cpxfgdmklgusodqouqdr forum.dlang.org
Feb 02
next sibling parent aliak <something something.com> writes:
On Friday, 2 February 2018 at 14:19:37 UTC, aliak wrote:

 ...  (see post in general forum by Simon for details [1])
*Simen Gah! Sorry!
Feb 02
prev sibling parent Steven Schveighoffer <schveiguy yahoo.com> writes:
On 2/2/18 9:19 AM, aliak wrote:
 To further explain what I mean:
 
 struct A if (!is(this == immutable) && !is(this == shared))
 {
 }
 
 shared a = A() // error
 auto a = A() // ok
 immutable a = A() // error
 const a = A() // ok
In this case, no. A struct is bit-copyable, so you must be able to move it bit-by-bit to another copy. In the case of attribute addition or removal, the compiler allows anything as long as there are no reference types contained in the struct. Your example has no members, which means no references, so it's a value type. If you put a pointer inside your struct, you will get the behavior you are looking for I think. However, you may have to disable the constructor for your exact requirements above. -Steve
Feb 02