Template class creating object

jcga

Hello,

My question concerns C++ template classes rather than specific Qt points, so I'm wondering if I can post it here. Sorry if not.

Here is my problem. I have a template class derived from C++ 'list':

Template <class T>
class ObjListP : public list<T>
{
  // class implementation
};

I want this class to store only pointers to objects of class Obj (or its derivate classes), so I declare a list object

ObjListP<Obj*> mylist();

so that T is in fact a pointer Obj*.

Now, in a method of the above class, I want to create a new object of class T. Obviously

T obj=new T();

is wrong, because it creates a pointer to a pointer, that is an Obj** not an Obj*. I can create it as

Obj* obj=new Obj();

but this will not create pointers to objects in the classes derived from Obj if T is in fact a derived class.

Is there a way to create an object that is a pointer to an object of class T?

Thanks, and sorry if my question is misplaced.

JClaude

jsulm

This post is deleted!

jsulm

@jcga said in Template class creating object:

Now, in a method of the above class, I want to create a new object of class T. Obviously
T obj=new T();

Why do you want to do this? If you only want to manage pointers - why then do you create instances in this template?
For C++ questions you can use https://forum.qt.io/category/34/c-gurus

mrjj

Hi
Im not sure i understand question.
Say we have the following code

class Obj {
 public:
  virtual void print() { qDebug() << "obj"; }
};

class SubObj : public Obj {
 public:
  virtual void print() { qDebug() << "subobj"; }
};

template <class T>
class ObjListP : public std::vector<T> {
  // class implementation
};

and we call it like

  ObjListP<Obj*> mylist;
  mylist.push_back( new Obj);
  mylist.push_back( new SubObj);

  mylist[0]->print();
  mylist[1]->print();

we get
obj
subobj

so not sure what you mean it wont create the right type.
Or what the actual issue it.

jsulm

@mrjj He wants to use a pointer as T, so

ObjListP<Obj*> mylist;

and he wants to create instances of T inside the ObjListP class. But this is bad design in my opinion, because ObjListP class is managing pointers to T and should not create instances of T by itself. This is something users of ObjListP will do (as with any other container).

VRonin

I want this class to store only pointers to objects of class Obj

Add, inside the class
static_assert(std::is_pointer<T>::value && std::is_base_of<Obj, std::remove_pointer<T>::type>::value,"T must be a pointer to an Obj");

Is there a way to create an object that is a pointer to an object of class T?

T obj = new std::remove_pointer<T>::type;

You should also have a think about what happens if the user const-qualifies T. For example, in case of ObjListP<const Obj*> mylist;