for-in loop
"for-in" loop exists in delphi starting from 2005 version.
Delphi implementation
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>
Traversing container
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>
After this you are able to execute the next code:
<delphi> procedure TreeLoop(ATree: TEnumerableTree); var
ANode: TNode;
begin
for ANode in ATree do DoSomething(ANode);
end; </delphi>
Of course enumerator support is built into the basic classes: TList, TStrings, TCollection, TComponent, ...
For-in loop can be easily translated into the while loop. Two next examples doing same things:
Example 1. <delphi> procedure TraverseStrings(AStrings: TStrings); var
S: String;
begin
for S in AStrings do DoSomething(S);
end; </delphi>
Example 2. <delphi> procedure TraverseStrings(AStrings: TStrings); var
S: String; Enumerator: TStringsEnumerator;
begin
Enumerator := AStrings.GetEnumerator; while Enumerator.MoveNext do DoSomething(Enumerator.Current);
end; </delphi>
It is also possible to make some class enumerable if you implement the next interface for the container: <delphi>
IEnumerable = interface(IInterface) function GetEnumerator: IEnumerator; end;
</delphi>
Where IEnumerator is declared as: <delphi>
IEnumerator = interface(IInterface) function GetCurrent: TObject; function MoveNext: Boolean; procedure Reset; property Current: TObject read GetCurrent; end;
</delphi>
Proposed extensions
The following examples are not supported by Delphi, and proposed for FPC mode only.
In Delphi, it is impossible to traverse neither enumerated nor ranged type:
<delphi> type
TColor = (clRed, clBlue, clBlack);
var
Color: TColor; ch: Char;
begin
for Color in TColor do DoSomething(Color); for ch in 'a'..'z' do DoSomethingOther(ch);
end. </delphi>
Although you can traverse a set. So the next code is valid even in Delphi: <delphi> type
TColor = (clRed, clBlue, clBlack);
var
Color: TColor;
begin
for Color in [clRed..clBlack] do DoSomething(Color);
end. </delphi>
It is impossible to choose among different possible enumerators. For example you can traverse a tree using different orders. The well known algorithms are: preorder, postorder, inorder and breadth‑first traversals. Therefore it would be useful to have an ability to choose an enumerator. For example using one of the following syntax variants:
<delphi> type
TTreeEnumeratorType = (tePreOrder, tePostOrder, teInOrder, teBreadthFirst)
var
Node: TNode;
begin
for Node in Tree using GetEnumerator(teInOrder) do // Variant 1 for Node using Tree.GetInOrderEnumerator do // Variant 2 for Node in GetEnumerator(Tree, teInOrder) do // Variant 3 Dosomething(Node);
end. </delphi>
It is also not possible to add an enumerator without modifying the class, as well as add enumerator to non-class type. It is proposed to allow this like in this example:
<delphi> type
TMyRecord = record F1: Integer; F2: array of TMyType; end; TMyArrayEnumerator = class constructor Create(const A: TMyRecord); function Current: TMyType; function MoveNext: Boolean; end;
// This is new built-in operator. operator GetEnumerator(const A: TMyRecord): TMyArrayEnumerator; begin Result := TMyArrayEnumerator.Create(A); end;
var
A: MyRecord; V: TMyType
begin
for V in A do DoSomething(V);
end. </delphi>