What are pros and cons of duplication an object instance with constructor or instance function?
Example A:
type
TMyObject = class
strict private
FField: integer;
public
constructor Create(srcObj: TMyObject); overload;
//alternatively:
//constructor CreateFrom(srcObj: TMyObject);
property Field: integer read FField;
end;
constructor TMyObject.Create(srcObj: TMyObject);
begin
inherited Create;
FField := srcObj.Field;
end;
Example B:
type
TMyObject = class
strict private
FField: integer;
public
function Clone: TMyObject;
property Field: integer read FField;
end;
function TMyObject.Clone: TMyObject;
begin
Result := TMyObject.Create;
Result.FField := FField;
end;
One major difference immediately springs to mind - in the latter case the Create constructor would have to be virtual so that a class hierarchy supporting Clone could be built basing on the TMyObject.
Assume that this is not a problem - that TMyObject and everything based on it is entirely under my control. What is your preferred way of doing copy constructor in Delphi? Which version do you find more readable? When would you use former or latter approach? Discuss. :)
EDIT: My main concern with the first example is that the usage is very heavy compared to the second approach, i.e.
newObj := TMyObject.Create(oldObj)
vs.
newObj := oldObj.Clone;
EDIT2 or "Why I want single-line operation"
I agree that Assign is a reasonable approach in most cases. It's even reasonable to implement 'copy constructor' internally by simply using assign.
I'm usually creating such copies when multithreading and passing objects through the message queue. If object creation is fast, I usually pass a copy of the original object because that really simplifies the issues of object ownership.
IOW, I prefer to write
Send(TMyObject.Create(obj));
or
Send(obj.Clone);
to
newObj := TMyObject.Create;
newObj.Assign(obj);
Send(newObj);
The first adds information about which object to want to create, the second not. This can be used to instantiate e.g. a descendant or an ancestor of a class
The Delphi way (TPersistent
) separates creation and cloning:
dest := TSomeClass.Create;
dest.Assign(source);
and has this same property that you explicitly choose the class to instantiate. But you don't need two constructors, one for normal use, and one where you want to clone.
edit due to oneline requirement You can mix it of course using Delphi metaclasses (untested)
type
TBaseSomeObject = class;
TBaseObjectClass = class of TBaseSomeObject;
TBaseSomeObject = class(TPersistent)
function Clone(t: TBaseObjectClass = nil): TBaseSomeObject; virtual;
end;
...
function TBaseSomeObject.Clone(t: TBaseObjectClass = nil): TBaseSomeObject;
begin
if Assigned(t) then
Result := t.Create
else
Result := TBaseObjectClass(Self.ClassType).Create;
Result.Assign(Self);
end;
SendObject(obj.Clone); // full clone.
SendObject(obj.Clone(TDescandantObject)); // Cloned into Descendant object
For the rest, just implement your assign()
operators, and you can mix multiple ways.
edit2
I replaced the code above with code tested in D2009. There are some dependencies of the types that might have confused you, hope it is clearer this way. Of course you'll have to study the assign mechanism. I also tested the metaclass=nil
default parameter and it works, so I added it.