ptr_container and pointers

Hello

I have a ptr_container:

Code:

---

typedef boost::ptr_vector<some_object> my_type;
my_type m_vector;

---

now I make pointer to it:

Code:

---

my_type *p = &m_vector;

---

Now I want to have random access to m_vector by using pointer p:

Code:

---

my_type::size_type st;
for( st = 0u; st != p->size(); ++st ) {
//now how to call a function inside some_object ?
      p[st]->some_function();
}

---

How should I do this?
Thanks for help

I believe operator[] returns a reference for ptr_vector. Since p is a pointer, dereference it first before using the [] operator, then use the . to access the function of the object it returns.

Code:

---

(*p)[st].some_function();

// or

p->operator[](st).some_function();

---

That's the gist of it. Has nothing to do with ptr_vector specifically, just generally with the fact that overloaded operators apply to the direct type of their arguments, and in the case of p, the direct type is my_type*, which is not what you want.

The fact that overloaded operators do not apply to pointers (and that it would be unintuitive and error-prone if they did) was one of the main reasons references were introduced to the language.

Hello

I have one more question:

In case I have:

Code:

---

class base {
public:
  virtual void function() {
  //do something
  }
};

class object : public base {
  virtual void function() {
  //do something
  }
};

typedef boost::ptr_vector<base> my_type;
my_type m_vector;
m_vector.push_back(new object());
my_type *p = &m_vector;

my_type::size_type st;
for( st = 0u; st != p->size(); ++st ) {
      object *ptr = static_cast<object *>((*p)[st]);
}

---

How should I cast base pointer to object pointer?
Is there any website where I can read more about that kind of dereferencing and casting? Im really not good with it.

Thanks for help again

Why do you insist in having a pointer to the ptr_vector, anyway? It's just inconvenient and distracts from the important part of the problem.

OK, the rules are:
1) If you're absolutely, absolutely sure that the object in question is of the derived class, use a static_cast. Actually, Boost has a BOOST_STATIC_DOWNCAST or something like that that does a static_cast in release mode and a checked dynamic_cast in debug mode, which is a better idea.
2) If you're not sure, do a dynamic_cast and check the result. (A dynamic_cast on references throws bad_cast if the cast is disallowed; on pointers it gives a NULL pointer.)
3) In general, try to avoid downcasting and prefer virtual functions.

The library I have to work with returns a pointer to ptr_vector instead of a reference. No idea why though.

So dereference it and assign it to a reference. After you've made sure you're not responsible for deleting the thing, that is.

Doesnt that mean copying it?

There is no copying involved if you assign it to a reference.

What is the right way to assign it to a reference?
my_type ref = &object; ?

Quote:

---

Originally Posted by l2u

Code:

---

my_type::size_type st;
for( st = 0u; st != p->size(); ++st ) {
//now how to call a function inside some_object ?
      p[st]->some_function();
}

---

---

Use p[st].some_function() instead.

Quote:

---

Originally Posted by l2u

What is the right way to assign it to a reference?
my_type ref = &object; ?

---

No.

Code:

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my_type &ref = object;

---

Is this case, object is a pointer, right?

no.

Code:

---

my_type object;
my_type &ref = object;

---

http://www.cprogramming.com/tutorial/references.html

If you have a pointer, dereference the pointer. You dereference with * not &.