Copy constructors for lazy initialization

[Lionello Lunesu]

Nice. This could also be used to implement unique_ptr(T), with move
semantics.

L.

On 29-5-2010 9:26, Andrei Alexandrescu wrote:
> Walter has had a great idea last night: allow classes to define
> 
> this(ref S src);
> 
> where S is the name of the struct being defined, as an alternative for
> 
> this(this);
> 
> The result would be a function similar with a C++ copy constructor.
> 
> Such an abstraction allows us to perform lazy initialization in a way
> that allows the kind of problems associated with non-shared hash tables:
> 
> void foo(int[int] x)
> {
>    x[5] = 5;
> }
> 
> void main()
> {
>    int[int] x;
>    foo(x);
>    assert(x[5] == 5); // fails
> }
> 
> If you change the first line of main() with
> 
> int[int] x = [ 42:42 ];
> 
> the assert passes.
> 
> The idea of the copy constructor is to lazy initialize the source and
> the target if the source has null state. That would take care of this
> problem and the similar problems for shared state.
> 
> There is still a possibility to call a method against an object with
> null state. I think that's acceptable, particularly because lazy
> initialization saves some state allocation.
> 
> What do you think?
> 
> 
> Andrei