By Reference Parameter Passing
MAXScript supports by-reference parameter passing. This allows a reference to be passed
to a variable, property, or array element in the caller's context as a parameter to
a function and any assignments to that parameter in the called function will be assigned
directly to the caller's variable, property or array element. This is often used as
a mechanism for passing multiple results back from a function.
To define by-reference parameters for a function, you prefix them with an '&' character.
System Exception Note:
The following example is an error:
ERROR EXAMPLE
|
fn test1 &p3 = return &p3.x
p=[1,2,3]
r=test1 p
*r
|
Result: access violation!
It is an error to pass back a reference to a local that has gone from the call stack, but it is
very difficult to detect this efficiently. Be careful not to pass back &refs to local variables as it may cause system exceptions.
EXAMPLE
|
function foo x y &z =
(
...
z = x * y
...
return true
)
|
Declares a function taking three parameters, x, y and z. The &z in the parameter list indicates the z parameter is to be passed by-reference. To
call this function, you provide a reference value for the z parameter using the new
'&' reference operator.
|
local var1, var2
...
var1 = foo 2 3 &var2
|
The above example calls the foo function with the arguments 2, 3 and &var2 . The &var2 is a reference to the var2 local variable. When the function is called, the z parameter
in the body is a reference to var2 local in the caller, and the
|
|
in the body of foo will cause x * y to be assigned to the var2 local in the caller. After
|
|
var1 will contain true and var2 will contain x * y or 6 .
|
Any number of by-reference parameters can be used and they can be keyword parameters
if needed:
The ' & ' reference operator used to supply references in a call can be applied to any <accessor>
construct in MAXScript. These are:
variables (locals, globals, rollout locals, plug-in locals, etc.)
property access
array index
parameter (of a scripted plug-in)
EXAMPLE
|
p = [1, 2, 3]
foo 2 3 &p.x
|
would cause the x coordinate in the point to be passed by-reference and so after the
call, the value in p would be
|
|
EXAMPLE
|
a = #(1, 2, 3, 4, 5)
foo 2 3 &a[3]
|
would cause the 3rd element in the array to be passed by-reference and so after the
call, the value in a would be #(1, 2, 6, 4, 5)
|
NOTE:
This mechanism is used by the Function Publishing interface in MAXScript to support
BY-REF interface method parameters and results. Any parameter defined as by-ref (or
with a type code suffix _BR) in the published interface, requires an '&' reference
value.