Re: reference to pointer used in ctor - using 'this' in ctor (repost - corrected sample code)

From:

Barry <dhb52@126.com>

Newsgroups:

comp.lang.c++

Date:

Wed, 29 Aug 2007 14:26:57 +0800

Message-ID:

<fb33jl$j46$1@aioe.org>

Anonymous wrote:

template<class T>
class A
{
public:
   A() {
     T * object = new T(this); //passing instance as parent of nested class
     m_obj.push_back(object);
   }

   ~A() {
       for (std::vector<T>::iterator it= m_obj.begin();
            it != m_obj.end(); it++) delete *it ;
    }

private:
   std::vector<T*> m_objs ;
};

class B
{
public:
   friend class A;
   ~B(){};

private:
   B(A<B>* parent):m_parent(parent){}

A<B>* m_parent ;
};

class C
{
public:
   friend class A;
   ~C(){};

private:
   C(A<C>*& parent):m_parent(parent){}

A<C>* m_parent ;
};

int main( int argc, char* argv[])
{
    A<B> ab ; // Ok
    A<C> ac ; // Croaks here ...
};

Class A instantiated with B compiles ok, but when instantiated with
class C (that accepts a *&), I get the error message - cannot convert
parameter 1 from A const* to A* - why is that ?

Why is a reference to a ptr being passed as a const ptr ?

If 'this' is a const ptr during construction (I doubt this assertion
very much), then why is it ok to use as a non-const pointer in class B?

I attempted to compile your code, but found the template class friend
problem, as Victor marked else-thread.
You inject a friend A into B, actually A is a template class, so you
have to write A<T> as a friend of B(so B have also to be templated),

template <class T>
class B
{
public:
friend class A<T>;
....

when come to declaration of A's object,
A<B> ab;
B have to be changed into B<X>, where X is recursively dependent on T of
A<T>, which is to be a templated B.
So the recursive declaration looks like this:
A<B<B<B....> > >

So the job couldn't be done. Probably you have to redesign your classes.

--
Thanks
Barry