Why can't typed optional arguments have a default of Null?

问题

In ActionScript 3, when you declare an optional argument by giving it a default value, the value null cannot be used on typed arguments.

function Action(Param:int=null){
    // 1184: Incompatible default value of type Null where int is expected.
}
function Action(Param:int=0){
    // No compiler errors
} 

Any workarounds for this, or general purpose values that can apply to all data types?

回答1:

You can change your int to Number and then can set it to NaN which is a special number that means 'not a number' and this can represent your null state for a Number.

To check if something is NaN, you must use the isNaN() function and not val == NaN, or you will not get what you expect.

function Action(param:Number = NaN) : void {
    trace(param);
}

For all other objects, you can set them to null, but 'primitive' numbers are handled differently in Actionscript.

回答2:

int variables cannot be null, that's why you get that error, only reference types like objects can be null

Instead you can use NaN as a special number instead of null. If you want to check if something is NaN you mus use the isNaN function.

回答3:

you could also include a flag in the method signature to avoid having the parameter promoted from int to number:

function whatever(intProvided:Boolean = false, someInt:int = 0):void
{
    if(intProvided)
        doSomeStuff();
}

来源：https://stackoverflow.com/questions/1003100/why-cant-typed-optional-arguments-have-a-default-of-null