Can constructors be private? [duplicate] - delphi

Question

Take a look at this class:
TTest = class(TObject)
public
constructor Create(A:Integer);overload;
constructor Create(A,B:Integer);overload;
end;
Now when we want to use the class:
var
test: TTest;
begin
test:= TTest.Create; //this constructor is still visible and usable!
end;
Can anyone help me with hiding this constructor?

Answer 1

So long as you have overloaded constructors named Create, you cannot hide the parameterless TObject constructor when deriving from TObject.
This is discussed here: http://www.yanniel.info/2011/08/hide-tobject-create-constructor-delphi.html
If you are prepared to put another class between your class and TObject you can use Andy Hausladen's trick:
TNoParameterlessContructorObject = class(TObject)
strict private
constructor Create;
end;
TTest = class(TNoParameterlessContructorObject)
public
constructor Create(A:Integer);overload;
constructor Create(A,B:Integer);overload;
end;

Answer 2

You can hide the inherited Create by just introducing a non overloaded Create. As you need two overloaded Create, you can either merge those into one Create with an optional second parameter:
TTest = class(TObject)
public
constructor Create(A:Integer; B: Integer = 0);
end;
This will give a compiler warning, signalling that you're hiding the default parameterless constructor. To get rid of the warning you can declare the hiding constructor like so:
TTest = class(TObject)
public
constructor Create(A:Integer; B: Integer = 0); reintroduce;
end;
or, if this is not feasible, you can introduce an intermediate class introducing the first create and then the final class with the overloaded second one:
preTest = class(TObject)
public
constructor Create(A:Integer); reintroduce;
end;
TTest = class(preTest)
public
constructor Create(A,B:Integer);overload;
end;

Answer 3

Another option is to use the deprecated keyword and raise an exception at runtime.
TTest = class(TObject)
public
constructor Create; overload; deprecated 'Parameterless constructor is not Supported for a TTest class';
constructor Create(const A: Integer); overload;
constructor Create(const A, B: Integer); overload;
end;
implementation
constructor TTest.Create;
begin
raise Exception.Create('Parameterless constructor is not Supported for a TTest class.');
end;

Answer 4

Through the two inheritance, user creation of TMySingleton class can be prevented from design time rather than runtime.
unit MySingleton;
interface
uses System.Classes, System.SysUtils;
type
// Constructor Block external access
THideConstructor = class abstract
strict protected
constructor Create; virtual; abstract;
end;
// Switching the access to the Create function THideConstructor in TObject through the constructor Overloading
// Declaring Create Method as a procedure to prevent class call-TMySingle.Create('string') call impossible
TOverloadConstructor = class(THideConstructor)
public
procedure Create(s: string); reintroduce; overload; deprecated 'null method';
end;
TMySingleton = class sealed(TOverloadConstructor)
private
class var MyObj: TMySingleton;
strict protected
// Hiding TOverloadConstructor.Create(s: string);
// Implement THideConstructor.Create
constructor Create; override;
public
class function Obj: TMySingleton;
function Echo(const value: string): String;
destructor Destroy; override;
end;
implementation
{ TMySingleton }
constructor TMySingleton.Create;
begin
// TODO
end;
destructor TMySingleton.Destroy;
begin
Self.MyObj := nil;
inherited;
end;
function TMySingleton.Echo(const value: string): String;
begin
result := value;
end;
class function TMySingleton.Obj: TMySingleton;
begin
if MyObj = nil then
MyObj := Self.Create;
result := MyObj;
end;
{ TOverloadContructor }
procedure TOverloadConstructor.Create(s: string);
begin
// null method
end;
initialization
TMySingleton.MyObj := nil;
finalization
if Assigned(TMySingleton.MyObj) then
FreeAndNil(TMySingleton.MyObj);
end.
If the user
var
Singleton: TMySingleton;
begin
Singleton := TMySingleton.Create;
Design-time error occurs.
[dcc32 Error] Unit1.pas(33): E2625 Private member 'THideConstructor.Create' is inaccessible here MySingleton.pas(11): Related method: constructor Create;
enter image description here
Also, you can't see any autocomplete hints named Create.