3.16. Metamethods
Metamethods are a mechanism that allows the customization of certain aspects of the
language semantics. Those methods are normal functions placed in a class declaration;
It is possible to change many aspects of a table/class instance behavior by just defining
a metamethod. Class objects (not instances) support only 2 metamethods _newmember, _inherited
.
For example when we use relational operators other than ‘==’ on 2 instances, the VM will check if the class has a method in his parent called ‘_cmp’; if so it will call it to determine the relation between the objects.:
class Comparable {
constructor(n)
{
name = n;
}
function _cmp(other)
{
if(name<other.name) return -1;
if(name>other.name) return 1;
return 0;
}
name = null;
}
let a = Comparable("Alberto");
let b = Comparable("Wouter");
if(a>b)
print("a>b")
else
print("b<=a");
3.16.1. _set
_set(idx,val)
invoked when the index idx is not present in the object or in its delegate chain.
_set
must ‘throw null’ to notify that a key wasn’t found but the there were not runtime errors (clean failure).
This allows the program to differentiate between a runtime error and a ‘index not found’.
3.16.2. _get
_get(idx)
invoked when the index idx is not present in the object or in its delegate chain. _get must ‘throw null’ to notify that a key wasn’t found but the there were not runtime errors (clean failure). This allows the program to differentiate between a runtime error and a ‘index not found’.
3.16.3. _newslot
_newslot(key,value)
invoked when a script tries to add a new slot in a table.
if the slot already exists in the target table the method will not be invoked also if the “new slot” operator is used.
3.16.4. _delslot
_delslot(key)
invoked when a script deletes a slot from a table. if the method is invoked quirrel will not try to delete the slot himself
3.16.11. _typeof
_typeof()
invoked by the typeof operator on tables, userdata, and class instances.
Returns the type of this
as string
3.16.12. _cmp
_cmp(other)
invoked to emulate the < > <= >=
and <=>
operators
returns an integer as follow:
returns |
relationship |
---|---|
> 0 |
if |
0 |
if |
< 0 |
if |
3.16.14. _cloned
_cloned(original)
invoked when a table or class instance is cloned(in the cloned table)
3.16.15. _nexti
_nexti(previdx)
invoked when a userdata or class instance is iterated by a foreach loop.
If previdx==null it means that it is the first iteration. The function has to return the index of the ‘next’ value.
3.16.16. _tostring
_tostring()
Invoked when during string concatenation or when the print
function prints a table, instance, or userdata.
The method is also invoked by the sq_tostring() API.
Must return a string representation of the object.
3.16.17. _inherited
_inherited()
invoked when a class object inherits from the class implementing _inherited
.
The this
contains the new class.
Return value is ignored.
3.16.18. _newmember
_newmember(index,value,isstatic)
invoked for each member declared in a class body (at declaration time).
If the function is implemented, members will not be added to the class.