TA = class(TObject)
public
constructor Create; virtual;
end;

TB = class(TA)
public
constructor Create; override;
end;

TClass = class of TA;

function: CreateClass(AType: TClass): TA;
begin
Result := AType.Create;
end;

...
begin
CreateClass(TB); // 只有虚的构造函数才能准确的调用到TB的构造函数。
end;