Tutorial :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;  


Solution:5

You can create an abstract method in base class, and call it in the constructor, and override in child classes to be executed when created from class reference.


Note:If u also have question or solution just comment us below or mail us on toontricks1994@gmail.com
Previous
Next Post »