2.15. Metamethods¶
Metamethods are a mechanism that allows the customization of certain aspects of the
language semantics. Those methods are normal functions placed in a table
parent(delegate) or 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 tables, the VM will check if the table has a method in his parent called ‘_cmp’; if so it will call it to determine the relation between the tables.:
local comparable={
_cmp = function (other)
{
if(name<other.name)return -1;
if(name>other.name)return 1;
return 0;
}
}
local a={ name="Alberto" }.setdelegate(comparable);
local b={ name="Wouter" }.setdelegate(comparable);
if(a>b)
print("a>b")
else
print("b<=a");
for classes the previous code become:
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;
}
local a = Comparable("Alberto");
local b = Comparable("Wouter");
if(a>b)
print("a>b")
else
print("b<=a");
2.15.1. _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’.
2.15.2. _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’.
2.15.3. _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.
2.15.4. _delslot¶
_delslot(key)
invoked when a script deletes a slot from a table. if the method is invoked squirrel will not try to delete the slot himself
2.15.11. _typeof¶
_typeof()
invoked by the typeof operator on tables, userdata, and class instances.
Returns the type of this
as string
2.15.12. _cmp¶
_cmp(other)
invoked to emulate the < > <= >=
and <=>
operators
returns an integer as follow:
returns | relationship |
---|---|
> 0 | if this > other |
0 | if this == other |
< 0 | if this < other |
2.15.14. _cloned¶
_cloned(original)
invoked when a table or class instance is cloned(in the cloned table)
2.15.15. _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.
2.15.16. _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.
2.15.17. _inherited¶
_inherited(attributes)
invoked when a class object inherits from the class implementing _inherited
.
The this
contains the new class.
Return value is ignored.
2.15.18. _newmember¶
_newmember(index,value,attributes,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.