• Freddy (22/22) Jun 24 2015 I was surprised when this happened, is it a bug or a feature?
• Adam D. Ruppe (4/5) Jun 24 2015 Feature, overwriting const or immutable data would violate the
• Jonathan M Davis (15/20) Jun 24 2015 This is why you should almost never have structs with const or

"Freddy" <Hexagonalstar64 gmail.com> writes:

I was surprised when this happened, is it a bug or a feature?

---
struct A
{
     const int var;
}

int test()
{
     auto b = A(5);
     auto c = A(7);
     pragma(msg, typeof(b)); //A
     pragma(msg, typeof(c)); //A
     auto d = b; //compiles
     b = c; //doesn't compile
}
---
$ rdmd -main -unittest test
A
A
test.d(13): Error: cannot modify struct b A with immutable members
Failed: ["dmd", "-main", "-unittest", "-v", "-o-", "test.d", 
"-I."]

"Adam D. Ruppe" <destructionator gmail.com> writes:

On Wednesday, 24 June 2015 at 19:41:54 UTC, Freddy wrote:
 I was surprised when this happened, is it a bug or a feature?

Feature, overwriting const or immutable data would violate the 
promise that it never changes. Saving a struct over another 
struct is just another way of overwriting the data.

"Jonathan M Davis" <jmdavisProg gmx.com> writes:

On Wednesday, 24 June 2015 at 19:45:12 UTC, Adam D. Ruppe wrote:
 On Wednesday, 24 June 2015 at 19:41:54 UTC, Freddy wrote:
 I was surprised when this happened, is it a bug or a feature?

 Feature, overwriting const or immutable data would violate the 
 promise that it never changes. Saving a struct over another 
 struct is just another way of overwriting the data.

This is why you should almost never have structs with const or 
immutable members. It's overly limiting. Once one member is const 
or immutable, the whole thing might as well be, and if that's 
what you want, just declare the variable const or immutable and 
don't have individual members be const or immutable. If read-only 
is what you're looking for, then wrap it in an  property function 
which returns the value but don't provide a corresponding setter 
property, and then the only way to overwrite it is to assign a 
value to the entire struct.

Classes are different, since they're reference types, but with 
structs, I'm not sure I've ever seen a case where I thought that 
it was a good idea to have any of its members be const or 
immutable.

- Jonathan M Davis