Language Guide: Parameters

Added by dholton dholton

Parameters are the objects you can pass to Functions, Closures, or Callable Types which handle Events.

A parameter is declared with a name, followed by "as", followed by the type: paramname as type. If there is no "as type", then the type is assumed to be type object.

Examples

Method/Function example

def mymethod(x as int, y as long):
        ...

Closure

c = def(z):
         print z
 
obj = "a string"
c(obj)

Callable Type + Event

import System
 
class Sandwich:
    event Eating as EatingEvent
    callable EatingEvent(sammich as object, type as string)
 
    def Eat():
        Eating(self, "Turkey sammich.")
 
turkeyAndSwiss = Sandwich()
turkeyAndSwiss.Eating += def(obj, sammich):
    print "You're eating a $sammich! It must be good."
turkeyAndSwiss.Eat()

Variable Number of Parameters

Boo allows you to call or declare methods that accept a variable (unknown) number of parameters.

You add an asterix (*) before the parameter name to signify that it holds multiple parameter values. If there is no 'as type', the type is assumed to be an array of objects: (object). You can declare the type as any array type. For example (int) if your method only accepts int parameters.

Here is an example:

def mymethod(x as int, *rest):
    print "first arg:", x
    for item in rest:
        print "extra param:", item
    print
 
mymethod(1, "a", "b", "c")
mymethod(2, 3, 4, 5, 6, 7)

Some boo builtins accept a variable number of parameters, like matrix() and ICallable.Call.

ByRef Parameters

Add a "ref" keyword before the parameter name to make a parameter be passed by reference instead of by value. This allows you to change a variable's value outside of the context where it is being used. Some examples:

Basic byref example:

def dobyref(ref x as int):
        x = 4
 
x = 1
print x //-->1
dobyref(x)
print x //-->4

DllImport Example: Wrapping a native method that takes a parameter by reference:

import System.Windows.Forms from System.Windows.Forms
import System.Drawing from System.Drawing
import System.Runtime.InteropServices
 
class ExtTextBox(TextBox):
    //must be static
    [DllImport("user32")]
    static def GetCaretPos(ref p as Point):
        pass
 
 
f = Form(Text: "byref test")
t = ExtTextBox()
f.Controls.Add(t)
 
b = Button(Text: "GetCaretPos")
b.Click += do:
    p = Point(0,0)
    t.GetCaretPos(p)
    MessageBox.Show(p.X.ToString())
b.Location = Point(0,100)
f.Controls.Add(b)
Application.Run(f)

See also tests/testcases/integration/callables/byref*.boo in the boo source distribution.