1.15. Metamethods

Metamethods are a mechanism that allows the customization of certain aspects of the language semantics. Those methods are normal functions placed in a class declaration; It is possible to change many aspects of a table/class instance behavior by just defining a metamethod.

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");

1.15.1. _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’.

1.15.2. _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’.

1.15.3. _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.

1.15.4. _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

1.15.5. _add

_add(other)

the + operator

returns this + other

1.15.6. _sub

_sub(other)

the - operator (like _add)

1.15.7. _mul

_mul(other)

the * operator (like _add)

1.15.8. _div

_div(other)

the / operator (like _add)

1.15.9. _modulo

_modulo(other)

the % operator (like _add)

1.15.10. _unm

_unm()

the unary minus operator

1.15.11. _typeof

_typeof()

invoked by the typeof operator on tables, userdata, and class instances.

Returns the type of this as string

1.15.12. _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`

1.15.13. _call

_call(other)

invoked when a table, userdata, or class instance is called

1.15.14. _cloned

_cloned(original)

invoked when a table or class instance is cloned(in the cloned table)

1.15.15. _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.

1.15.16. _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.

1.15.17. _lock

_lock()

Metamethods of class object only. Called when a class is locked (manually by an explicit call to cls.lock(), when the class is being inherited or when its instance is first created). this references the class object itself. At this stage the class still can be modified.