Question

Sometimes we want an optional parameter

function doSomething(foo:Integer; bar:TObject=nil)
begin
    if bar <> nil then // do something optional with bar
    ....
end

How do I do the equivalent with a boolean, that allows me to differentiate between the two boolean values and “no value”?

function doSomething(foo:Integer; bar:Boolean=nil) // Won't compile
begin
    if bar <> nil then // do something optional with bar
end

Obviously this won’t compile as a Boolean cannot be nil.

Basically, I want a parameter with three possible states; true, false or “unspecified”.

Answer 1

You can do this other way, using overloading:

function doSomething(foo:Integer): Boolean; overload;
begin
  // do something without bar
end;

function doSomething(foo:Integer; bar:Boolean): Boolean; overload
begin
  // do something optional with bar
end;

Then you can use it as doSomething(1) , as well as doSomething(1, true)

Using your example, it will be equivalent to:

function doSomething(foo:Integer; bar:Boolean=nil): Boolean; // Won't compile
begin
    if bar <> nil then 
      // do something optional with bar
    else
      // do something without bar
end;