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; 

Answer 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").

Answer 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.