Is the multiplication operator documented for sets?

Question

I noticed this block of code today and was wondering if these type of operations are documented somewhere and why it was done this way (Performance, etc).

var
  Shift: TShiftState
begin
  if [ssShift, ssCtrl] * Shift <> [] then
  begin
    ...
  end;
end;

It looks to me from my testing and just looking at the code like this is checking if Shift contains either ssShift or ssCtrl. Is this documented behavior or is it just leveraging the fact that the set is actually stored as an integer internally?

Answer 1

That's not the multiplication operator when used with sets; it's the intersection operator. This is documented, and has been since pre-Delphi days. See Expressions, particularly the section on set operations. They're the standard mathematical operators for union, intersection, sub- and super-sets, equality, inequality, and membership.

Here's a summary of the table from the docs:

Operator  Operation     Operand Types  Result Type  Example  
--------  ---------     -------------  -----------  -------
+          union        set            set          Set1 + Set2 
-          difference   set            set          S - T 
*          intersection set            set          S * T 
<=         subset       set            Boolean      Q <= MySet 
>=         superset     set            Boolean      S1 >= S2 
=          equality     set            Boolean      S2 = MySet 
<>         inequality   set            Boolean      MySet <> S1 
in         membership   ordinal, set   Boolean      A in Set1