for-in loop
"for-in" loop exists in delphi starting from 2005 version.
It has the next syntax:
String loop:
<delphi> procedure StringLoop(S: String); var
C: Char;
begin
for C in S do DoSomething(C);
end; </delphi>
Array loop:
<delphi> procedure ArrayLoop(A: Array of Byte); var
B: Byte;
begin
for B in A do DoSomething(B);
end; </delphi>
Set loop:
<delphi> type
TColor = (cRed, cGren, cBlue); TColors = set of TColor;
procedure SetLoop(Colors: TColors); var
Color: TColor;
begin
for Color in Colors do DoSomething(Color);
end; </delphi>
To traverse some container class you need to add an enumerator for it. Enumerator is a class built by the next template:
<delphi> TSomeEnumerator = class public
function MoveNext: Boolean; property Current: TSomeType;
end; </delphi>
There are only 2 things required for the enumerator: MoveNext method which asks enumerator to step forward and property Current which can return any desired type.
Next thing is to add magic GetEnumerator method to the container class which returns an enumerator instance.
For example: <delphi> type
TEnumerableTree = class;
TTreeEnumerator = class private FTree: TEnumerableTree; FCurrent: TNode; public constructor Create(ATree: TEnumerableTree); function MoveNext: Boolean; property Current: TNode read FCurrent; end;
TEnumerableTree = class public function GetEnumerator: TTreeEnumerator; end;
constructor TTreeEnumerator.Create(ATree: TEnumerableTree); begin
inherited Create; FTree := ATree; FCurrent := nil;
end;
function TTreeEnumerator.MoveNext: Boolean; begin
// some logic to get the next node from a tree if FCurrent = nil then FCurrent := FTree.GetFirstNode else FCurrent := FTree.GetNextNode(FCurrent); Result := FCurrent <> FTree.GetLastNode;
end;
function TEnumerableTree.GetEnumerator: TTreeEnumerator; begin
Result := TTreeEnumerator.Create(Self);
end;
</delphi>