Just exactly what operands are allowed with ?: operator

[MartinHowe]

Everr time I try to use the ternary conditional operator, I get "Incompatible types for ?: operator". Even something simple as actor origin = (haveMaster ? master : self);. Setting up arguments for function calls is a classic way of using this operator, but if it can't handle integer based values such as actor addresses, how useful is it? Please can anyone tell me just which types it can be used with

[Chris]

That error message suggests the problem is master and self are two different types. The problem isn't "what types it can be used with", but that the types it is used with can't be different. In C/C++, the ?: operator would implicitly cast to a common base if they have one (following the rules about what implicit casts are allowed), and error if it couldn't get the same type for both operands (e.g. if Actor is derived from Object, and you do someBool ? someActor : someObject, the result would be an Object type as someActor can be implicitly cast to its base type; but if Actor was its own type completely separate from Object, it would be a compile error as there would be no common type). Perhaps ZScript is more strict and requires explicit casts when the two operands are different types with a common base.

[MartinHowe]

In C/C++, the ?: operator would implicitly cast to a common base if they have one (following the rules about what implicit casts are allowed).

Thanks Chris. This is a huge fail for ZScript. Both types are descended from Actor; indeed, using the operator that way for actors is one of the main reasons we'd use it at all.

However, it's worse than this: what you describe above is formally called LSP: Liskov Substitution Principle and is fundamental to object orientated programming. The three-operand comparator in ZScript doesn't just break LSP; it throws it under the bus; it would be like having a car with everything perfect, except it doesn't have brakes

Do the devs have any comments before I report as a bug, please?

[Xeotroid]

For what it's worth, the following two pieces of code work perfectly fine:

Code: Select all

bool dontLikeZombies = true;
Actor imp = Actor.Spawn("DoomImp", (0, 0, 0));
Actor zombie = Actor.Spawn("Zombieman", (0, 0, 0));
Actor enemy = dontLikeZombies ? imp : zombie;

Code: Select all

bool dontLikeZombies = true;
Actor.Spawn(dontLikeZombies ? "DoomImp" : "Zombieman", position);

So perhaps the following could work?

Code: Select all

Actor origin = haveMaster ? Actor(master) : Actor(self);

[MartinHowe]

So perhaps the following could work?

Code: Select all

Actor origin = haveMaster ? Actor(master) : Actor(self);

Thanks; I would have thought so; have to look at this again when next working in that part of the code.