http://d.puremagic.com/issues/show_bug.cgi?id=4571
Summary: Non-null class references/pointers
Product: D
Version: D2
Platform: All
OS/Version: All
Status: NEW
Severity: enhancement
Priority: P2
Component: DMD
AssignedTo: nob...@puremagic.com
ReportedBy: bearophile_h...@eml.cc
--- Comment #0 from bearophile_h...@eml.cc 2010-08-02 16:27:19 PDT ---
This is a starting point for an enhancement request about a type modifier to
specify that a class reference or pointer can't be null.
This is not an enhancement request about "nullable values" (values wrapped in a
struct that contains a boolean that is true if the value is "null").
This enhancement request assumes that on default both class references and
pointers are nullable (because today defaulting to non-null class references is
probably an impossible change in D).
A possible D syntax to denote a non-null reference or pointer is to use a @
suffix (better looking alternatives are possible):
class T {}
T nullable_reference;
T@ nonnullable_reference = new T@();
struct S {}
S nullable_pointer;
S@ nonnullable_pointer = new S@();
A possible alternative is to use the - (or +) suffix:
class T {}
T nullable_reference;
T- nonnullable_reference = new T-();
struct S {}
S nullable_pointer;
S- nonnullable_pointer = new S-();
A possible problem with non-null class references can be seen with this D
program that uses the trailing @ syntax:
class Foo {}
class A {
Foo@ name;
this(Foo@ s) {
this.name = s;
this.m();
}
void m() { /*...*/ }
}
class B : A {
Foo@ path;
this(Foo@ p, Foo@ s) {
super(s);
this.path = p;
}
override void m() {
// here this.path is null despite it's a non-null
assert(this.path !is null);
}
}
void main() {
new B(new Foo, new Foo);
}
I have adapted that example from this paper, it discusses about partially
uninitialized objects too:
http://research.microsoft.com/pubs/67461/non-null.pdf
A comment about that program from the paper:
>The problem with the code is that during the base call to A's constructor, the
>virtual method B.m may be invoked. At this time, field path of the object
>under construction has not yet been initialized. Thus, accesses of this.path
>in method B.m may yield a possibly-null value, even though the field has been
>declared as being non-null.<
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