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 wi

Answer 1

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.