Genie Vala Generics and Nullable Types - gtk3

Question

Simple question: In the following generic class, how should the generic type and contained types be defined so that they are nullable? The following will not compile.
class Pair? of G1, G2: Object
_first:G1?
_second:G2?
construct()
_first = null
_second = null
def insert( first:G1, second:G2 )
_first = first
_second = second
def insert_first( value:G1 )
_first = value
def insert_second( value:G2 )
_second = value
def second():G2
return _second
Usage:
var pair = new Pair() of string, string
pair = null

Answer 1

Due to the way Vala Generics work, generic parameters are always nullable.
As long as you don't switch on --enable-experimental-non-null class variables are nullable as well, so your code simplifies to:
[indent=4]
class Pair of G1, G2: Object
_first:G1
_second:G2
construct()
_first = null
_second = null
def insert( first:G1, second:G2 )
_first = first
_second = second
def insert_first( value:G1 )
_first = value
def insert_second( value:G2 )
_second = value
def second():G2
return _second
init
var pair = new Pair of string, string
pair = null
When --enable-experimental-non-null is on, you have to be explicit in the type of the variable. I don't know how to write this in Genie, I tried this, but the compiler does not like it:
init
pair: Pair? of string, string = new Pair of string, string
pair = null
In Vala it's no problem:
class Pair<G1,G2>: Object {
private G1 first;
private G2 second;
public Pair () {
first = null;
second = null;
}
// ...
}
int main () {
Pair<string, string>? pair = new Pair<string, string> ();
pair = null;
return 0;
}

Answer 2

I can't wrap my head around the concept of a type parameter that has null as the type. I don't think that is a useful concept. So the definition of your class would be:
[indent = 4]
class Pair of G1, G2: Object
_first:G1?
_second:G2?
def insert( first:G1, second:G2 )
_first = first
_second = second
def insert_first( value:G1 )
_first = value
def insert_second( value:G2 )
_second = value
def second():G2
return _second
If you must re-assign the variable that has the object instance to null then it would be:
[indent = 4]
init
var pair = new Pair of string,string()
pair = null
The Vala compiler will, however, dereference pair when it goes out of scope. So I'm not sure why you would need to assign null.
The use of nulls would ideally only be used when interfacing with a C library in my view. Accessing a null can lead to a crash (segmentation fault) if it is not checked for properly. For example:
init
a:int? = 1
a = null
var b = a + 1
The Vala compiler does have an experimental non-null mode that does some checking for unsafe code. If you compile the following with the Vala switch --enable-experimental-non-null:
[indent = 4]
init
var pair = new Pair of string,string()
pair = null
you will get the error:
error: Assignment: Cannot convert fromnull' to Pair<string,string>'
If you understand the consequences then you can tell the compiler this is OK with:
[indent = 4]
init
pair:Pair? = new Pair of string,string()
pair = null