lazy construction of an immutable object

[Puming via Digitalmars-d-learn]

On Tuesday, 15 July 2014 at 17:09:04 UTC, Meta wrote:
> On Tuesday, 15 July 2014 at 15:48:10 UTC, Puming wrote:
>> wow, that's interesting :-) Is it the idiomatic approach to 
>> initiate immutable objects lazily? Or do people use data class 
>> with immutable fields and generate a companion builder class 
>> at compile time?
>
> There's no real idiomatic approach, I think, because this is 
> somewhat unexplored territory. I'd say constructing the object 
> inside a pure method so it can be implicitly cast to immutable 
> is more typesafe, but if you're storing the immutable reference 
> to the object in a class or struct, it can only be initialized 
> in that class's/struct's constructor. So the following isn't 
> possible:
>
> struct Test
> {
> 	immutable(int*) n;
> 	
> 	private int* getPtr() pure
> 	{
> 		int* n = new int;
> 		*n = 42;
> 		
> 		return n;
> 	}
> 	
> 	public void initialize() pure
> 	{
> 		n = getPtr();
> 	}
> }
>
> void main()
> {
> 	auto t = new Test();
> 	test.initialize();
> }

So this means that the pure method approach should also be 
transitive: the containing class should also be mutable (having 
the desired immutable object as a mutable field) and later freeze 
itself.

If there is a complex hierarchy of objects, there are three 
strategies:

1. define all classes and fields mutable, and use immutable value 
in the business logic code. This would work, but lose the 
compilers help with checking immutability in all conditions. For 
example, you coud modify the mutable reference if it is leaked.
2. define all classes and fields immutable, and only use 
constructor to eagerly create objects and there fields. This is 
the most strict, but will incur some inconvenience in 
asynchronous environments, where not all parts of the data is 
available at start.
3. define all classes and use template magic to generate 
companion builders just like protobuffer does. But that would be 
complicated and I don't know how to do it. Do you have any 
suggestion for this approach?