Question

I have unconstrained generic type Atomic which implements an initializer (details in my previous question).

type
  Atomic<T> = class
    type TFactory = reference to function: T;
    class function Initialize(var storage: T; factory: TFactory): T;
  end;

Now I want to write simplified Initialize function which would take the type information from T (provided that typeof(T) is tkClass) and create new instance (when necessary) with the default constructor.

Sadly, this fails:

class function Atomic<T>.Initialize(var storage: T): T;
begin
  if not assigned(PPointer(@storage)^) then begin
    if PTypeInfo(TypeInfo(T))^.Kind  <> tkClass then
      raise Exception.Create('Atomic<T>.Initialize: Unsupported type');
    Result := Atomic<T>.Initialize(storage,
      function: T
      begin
        Result := TClass(T).Create; // <-- E2571
      end);
  end;
end;

Compiler reports error E2571 Type parameter 'T' doesn't have class or interface constraint.

How can I trick the compiler to create an instance of class T?

Answer 1

You can use the new Delphi Rtti to make this task. The drawback of given solution is that it won’t work if the constructor isn’t named as Create. If you need to make it work all the time, just enumerate your type methods, check if it’s a constructor and have 0 parameters and then invoke it. Works in Delphi XE. Sample code:

class function TTest.CreateInstance<T>: T;
var
  AValue: TValue;
  ctx: TRttiContext;
  rType: TRttiType;
  AMethCreate: TRttiMethod;
  instanceType: TRttiInstanceType;
begin
  ctx := TRttiContext.Create;
  rType := ctx.GetType(TypeInfo(T));
  AMethCreate := rType.GetMethod('Create');

  if Assigned(AMethCreate) and rType.IsInstance then
  begin
    instanceType := rType.AsInstance;

    AValue := AMethCreate.Invoke(instanceType.MetaclassType, []);// create parameters

    Result := AValue.AsType<T>;
  end;
end;

Updated solution:

class function TTest.CreateInstance<T>: T;
var
  AValue: TValue;
  ctx: TRttiContext;
  rType: TRttiType;
  AMethCreate: TRttiMethod;
  instanceType: TRttiInstanceType;
begin
  ctx := TRttiContext.Create;
  rType := ctx.GetType(TypeInfo(T));
  for AMethCreate in rType.GetMethods do
  begin
    if (AMethCreate.IsConstructor) and (Length(AMethCreate.GetParameters) = 0) then
    begin
      instanceType := rType.AsInstance;

      AValue := AMethCreate.Invoke(instanceType.MetaclassType, []);

      Result := AValue.AsType<T>;

      Exit;
    end;
  end;
end;

And call it like this:

var
  obj: TTestObj;
begin
  obj := TTest.CreateType<TTestObj>;