Delegate

So far, we've seen many things, interfaces, structs extending other structs, operators, dynamic arrays. You might be asking yourself, is it possible he would add another way to confuse me like heck? Do not despair! Delegate is the answer.

delegate is a strange feature, a delegate is just a member of the struct that does stuff for it. The struct just delegates the work to another object, in this case, work would mean 'methods'. It would appear as pointless or just an abbreviation, however it can be very useful and a interesting alternative to extends. The whole delegate idea is in use in some other languages, just notice that Jass is not very dynamic and vJass does inherit a lot of its flaws. For the better or the worse, delegation is a completely compile-time deal in the case of vJass.

A delegate does the struct's job, that is a very simple way to put it, a more complicated way would be, that during compile, if jasshelper cannot find a certain requested member of method, it will begin to look up for that member in the struct's delegates, if it finds this member in one of the delegates it will then compile it as a call to the delegate's member instead.

//Array structs are hard to explain, but should be simple to understand with an example

struct A
    private real x
    private real y

    public method performAction takes nothing returns nothing
        call DestroyEffect( AddSpecialEffect("path\\model.mdl", this.x, this.y) )
    endmethod

endstruct

struct B
    delegate A deleg

    static method create takes nothing returns B
     local B b = B.allocate()
        set B.deleg = A.create()
    endmethod

endstruct

function testsomething takes nothing returns nothing
    local B myB = B.create()

    call myB.performAction()

    //Since performAction() is not a member of struct B, jasshelper will check out the
    //delegator, it does have a method called performAction, so it will just try to call
    //it, the result would be the same as:

    call myB.deleg.performAction()

endfunction

Some considerations:

You can have multiple delegates, however that should probably be the exception rather than the rule.
jasshelper gives priority to the struct's members before its delegates' this means that if both the struct and a delegate have the same member, jasshelper will always consider the struct's over the delegate's.
Between delegates in the same struct, the priorities are the same as the declaration order.
You can do a lot of quacky things like delegating to an array member, you will even be able to use .size() and [] on the struct in that case.
You can also do non-sense as making a integer member a delegate, this will not cause a syntax error but does not really do much by itself, considering that integers have no members.
Right now, you cannot make a delegate's method fulfill a interface's rules, for example if struct B was extending a certain interface that required a method called performAction, jasshelper would not recognize the delegate's method and will cause a syntax error, this might change in the future.
You would usually have to initialize the delegate if you do not want bugs in your code.
You can have a private delegate, it would only be accessible by outside code in cases where a member is necessary.
If you try to call/use a delegate's private member, it will probably appear as a syntax error about not being able to find it in the struct rather than telling you that it is a private member of the delegate.

Last modified: 16 October 2024