On Thursday, 4 October 2018 at 07:31:21 UTC, Ritchie wrote:
Any reason why this works?
https://run.dlang.io/is/TALlyw
Yup.
Alright, so there's a few features in use here - which one are
you asking about?
1. Private constructor.
You can call the private constructor because the unit of
encapsulation in D is the module, not the type. So everything is
visible to everything else inside the same module.
2. @disable this()
This only disables default construction. Since you call a
different constructor, this doesn't affect the compilation.
3. @disable this(this)
This disables copy construction. In the example, no copying
occurs - the variable declaration leads to a move instead. Simply
put, copy construction requires that two copies exist
simultaneously, and in this case the potential second copy is
immediately destroyed, so no copy is necessary.
4. @disable void opAssign()
There's several reasons this doesn't affect compilation here.
First, the signature is wrong - you're disabling an opAssign that
takes no arguments. You're probably wanting to do @disable void
opAssign(X x);.
Now, even that isn't going to cause it to fail to compile in this
case though, since as in #3, what's happening on line 14 is move
construction, not assignment. To force an assignment, you need to
have an object already:
X x = void;
x = X(5); // calls opAssign
So, now we've explained why it compiles. Currently, there's no
way to @disable or hook move construction. There's a DIP in the
works (DIP 1014) that aims to provide a hook into move
construction, but I don't see that it allows for @disabling it
altogether.
Hope this helps!
--
Simen
