I can't find a way to use a pure constructor to create both mutable and immutable instances of the same class, when one of the fields I assign is a string.

This works fine:
	class A
	{
		int value;
		
		this(int value_) pure
		{
			this.value = value_;
		}	
	}
	
	auto a_mutable = new A(1);
	auto a_immutable = new immutable A(2);

But if I change the field to a string, I get a compilation error:
	class B
	{
		string value;
		
		this(string value_) pure
		{
			this.value = value_;
		}	
	}
	
	auto b_mutable = new B("foo");
	auto b_immutable = new immutable B("bar");

giving a compilation error for the last row:

Error: mutable method B.this is not callable using a immutable object

forcing me to use two separate constructors, which works fine:
	class B
	{
		string value;
		
		this(string value_)
		{
			this.value = value_;
		}	

		this(string value_) immutable
		{
			this.value = value_;
		}	
	}

The question is: am I missing something that would make it possible to use a pure constructor in this case, or is it simply not possible?

On Saturday, 29 November 2014 at 09:41:00 UTC, jostly wrote:
> I can't find a way to use a pure constructor to create both mutable and immutable instances of the same class, when one of the fields I assign is a string.

After poking around a bit, I believe it is caused by issue #10012 https://issues.dlang.org/show_bug.cgi?id=10012 :

"This is current dmd implementation limitation. In complex cases dmd cannot detect that the constructor generates unique object."

So my question then becomes, how can I generate an unique object from the string? I tried using .idup on the incoming string, but that didn't help. Or is it simply still a problem of _detecting_ it for the compiler?

On Saturday, 29 November 2014 at 09:41:00 UTC, jostly wrote:
> I can't find a way to use a pure constructor to create both mutable and immutable instances of the same class, when one of the fields I assign is a string.
[...]
> The question is: am I missing something that would make it possible to use a pure constructor in this case, or is it simply not possible?

I ran some tests, as far as I can tell, they should all work:

mixin template test(T)
{
     class C
     {
         T value;
         this(T value_) pure {this.value = value_;}
     }

     static assert(is(typeof({auto c = new C(T.init);})));

     static if(!is(typeof(new immutable C(T.init))))
         pragma(msg, T, ": immutable construction fails");

     static if(!is(typeof({immutable c = new C(T.init);})))
         pragma(msg, T, ": unique construction fails");
}

/* No indirections (and no (d/w)char): all fine */
mixin test!int;
mixin test!(int[3]);

/* With indirections:
immutable construction fails
unique construction works */
mixin test!string;
mixin test!(immutable int[]);
mixin test!(immutable Object);
mixin test!(immutable int*);

/* No indirections, but (d/w)char:
immutable construction works
unique construction fails
Wat. */
mixin test!dchar;
mixin test!wchar;
mixin test!char;
mixin test!(dchar[3]);