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; Is possible to change many aspect of a table/class instance behavior by just defining a metamethod. Class objects(not instances) supports 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");
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 defferentieate between a runtime error and a 'index not found'.
function _set(idx,val) //returns val
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 defferentieate between a runtime error and a 'index not found'.
function _get(idx) //return the fetched values
invoked when a script tries to add a new slot in a table.
function _newslot(key,value) //returns val
if the slot already exists in the target table the method will not be invoked also if the “new slot” operator is used.
invoked when a script deletes a slot from a table.
if the method is invoked squirrel will not try to delete the slot himself
function _delslot(key)
invoked by the typeof operator on tables ,userdata and class instances
function _typeof() //returns the type of this as string
invoked to emulate the < > <= >= operators
function _cmp(other)
returns an integer:
| >0 | if this > other |
| 0 | if this == other |
| <0 | if this < other |
invoked when a table, userdata or class instance is called
function _call(original_this,params…)
invoked when a table or class instance is cloned(in the cloned table)
function _cloned(original)
invoked when a userdata or class instance is iterated by a foreach loop
function _nexti(previdx)
if previdx==null it means that it is the first iteration. The function has to return the index of the ‘next’ value.
invoked when during string conacatenation or when the print function prints a table, instance or userdata. The method is also invoked by the sq_tostring() api
function _tostring()
must return a string representation of the object.
invoked when a class object inherits from the class implementing _inherited the this contains the new class.
function _inherited(attributes)
return value is ignored.