How can I create an Delphi object from a class reference and ensure constructor execution?

Question

How can I create an instance of an object using a class reference, and ensure that the constructor is executed?

In this code example, the constructor of TMyClass will not be called:

type
   TMyClass = class(TObject)
     MyStrings: TStrings;
     constructor Create; virtual;
   end;

constructor TMyClass.Create;
begin
   MyStrings := TStringList.Create;
end;

procedure Test;
var
   Clazz: TClass;
   Instance: TObject;
begin
   Clazz := TMyClass;
   Instance := Clazz.Create;
end;

Solution 1

Use this:

type
  TMyClass = class(TObject)
    MyStrings: TStrings;
    constructor Create; virtual;
  end;
  TMyClassClass = class of TMyClass; // <- add this definition

constructor TMyClass.Create;
begin
   MyStrings := TStringList.Create;
end;

procedure Test;
var
  Clazz: TMyClassClass; // <- change TClass to TMyClassClass
  Instance: TObject;
begin
   Clazz := TMyClass; // <- you can use TMyClass or any of its child classes. 
   Instance := Clazz.Create; // <- virtual constructor will be used
end;

Alternatively, you can use a type-casts to TMyClass (instead of "class of TMyClass").

Solution 2

Alexander's solution is a fine one but does not suffice in certain situations. Suppose you wish to set up a TClassFactory class where TClass references can be stored during runtime and an arbitrary number of instances retrieved later on.

Such a class factory would never know anything about the actual types of the classes it holds and thus cannot cast them into their according meta classes. To invoke the correct constructors in such cases, the following approach will work.

First, we need a simple demo class (don't mind the public fields, it's just for demonstration purposes).

interface

uses
  RTTI;

type
  THuman = class(TObject)
  public
    Name: string;
    Age: Integer;

    constructor Create(); virtual;
  end;

implementation

constructor THuman.Create();
begin
  Name:= 'John Doe';
  Age:= -1;
end;

Now we instantiate an object of type THuman purely by RTTI and with the correct constructor call.

procedure CreateInstance();
var
  someclass: TClass;
  c: TRttiContext;
  t: TRttiType;
  v: TValue;
  human1, human2, human3: THuman;
begin
  someclass:= THuman;

  // Invoke RTTI
  c:= TRttiContext.Create;
  t:= c.GetType(someclass);

  // Variant 1a - instantiates a THuman object but calls constructor of TObject
  human1:= t.AsInstance.MetaclassType.Create;

  // Variant 1b - same result as 1a
  human2:= THuman(someclass.Create);

  // Variant 2 - works fine
  v:= t.GetMethod('Create').Invoke(t.AsInstance.MetaclassType,[]);
  human3:= THuman(v.AsObject);

  // free RttiContext record (see text below) and the rest
  c.Free;

  human1.Destroy;
  human2.Destroy;
  human3.Destroy;
end;

You will find that the objects "human1" and "human2" have been initialized to zero, i.e., Name='' and Age=0, which is not what we want. The object human3 instead holds the default values provided in the constructor of THuman.

Note, however, that this method requires your classes to have constructor methods with not parameters. All the above was not conceived by me but explained brillantly and in much more detail (e.g., the c.Free part) in Rob Love's Tech Corner.

Solution 3

Please check if overriding AfterConstruction is an option.

Solution 4

Your code slightly modified:

type
  TMyObject = class(TObject)
    MyStrings: TStrings;
    constructor Create; virtual;
  end;
  TMyClass = class of TMyObject;

constructor TMyObject.Create;
begin
  inherited Create;
  MyStrings := TStringList.Create;
end;

procedure Test; 
var
  C: TMyClass;
  Instance: TObject;
begin
   C := TMyObject;
   Instance := C.Create;
end;

View more solutions

Comments

[mjn]

Ok, if I understand correctly this means that if I want to build a generic object factory with Delphi, I need to assign "class of TMyClass" to a variable - but this seems not possible.

[Alex]

If you want to construct object of certain type, then you need to have class type information. If you have no class info - you can not construct an object of this type. Quite obvious ;)

[mjn]

Very good idea, it is a virtual method in TObject so I do not need to add any synthetic new root class. +1 for this idea.

[bummi]

This is not exactly what the question is asking for.

[alijunior]

I was faced with this same problem, when I pass the class reference in the constructor of some list manager, specifying what kind of child it was populate the list, and when the list created the children, only the parent base class of children's contructor was called... the contructor of children items wasn't... the only solution for me, because constructor must have parameters, was declare an abstract method, call it in parent constructor, and override in child classes. This workaround may can help someone faced with our same problem... Best regards!