On Tuesday, 31 January 2017 at 23:52:31 UTC, Ali Çehreli wrote:
On 01/31/2017 03:15 PM, bitwise wrote:
[...]
Thanks for the response, but this doesn't really solve the
problem.
> If the object is defined at module scope as shared static
> immutable
It is indeed possible to initialize immutable objects by pure
functions as done inside shared static this() below:
I didn't mean that I wanted my object shared-static-immutable,
but only that a solution would have to account for that
possibility.
Yes, the situation is different from C++ but it's always
possible to call a function (which constructor is one) to make
the object.
I'm saying that a caller should not have to explicitly initialize
an object before use, but that a programmer should not have to
add boilerplate to deal with zombie objects all over the place
either.
A container for example:
struct Container(T) {
void pushBack(T); // ok: mutable method, can lazily
initialize payload.
// not ok: container may be immutable
Range!(const T) opSlice() const;
Iterator!(const T) find(T) const;
bool empty() const;
size_t count() const;
}
Container!int c; // = Container!int() -> can't do this.
if(c.empty) // can't initialize here either..
c.pushBack(1);
Such innocent looking code will fail without boilerplate inserted
everywhere.
-I can't lazily initialize the container in "empty()".
-I can't pre-emptively initialize it in a default constructor.
This problem causes the propagation of null checks all over the
place.
Objects returned from the container will have to have a "zombie"
state as well, and check validity at each use.
I wouldn't classify this as "a difference", but as a hole.
Although I don't remember where, I recently saw a discussion
about how "mutable" may possibly be implemented. IIRC, there was
no solution stated in that discussion.
The only solution that comes to mind would be to somehow relax
the constraints of const, and make it possible to prevent a
struct from being declared immutable, so that lazy initialization
could be done.
Recent discussions seem to indicate structs having default ctors
is not an option.
