discussion of protected in Lippman

[t]

Lippman's C++ Primer, 4th ed., p562, dicussion of protected members
seems to be wrong, unless I am misinterpreting things. He says:

"A derived class object may access the protected members of its base
class only through a derived object. The derived class has no special
access to the protected members of base type objects."

He gives the following example:

================================================== =======
class Item_base
{
public:
// stuff omitted
protected:
double price;
private:
std::string isbn;
};

class Bulk_item : public Item_base {
// stuff omitted
};

void Bulk_item::memfcn(const Bulk_item&d, const Item_Base& b)
{
// attempt to use protected member
double ret = price; // ok: uses this->price
ret = d.price; // ok: uses price from a Bulk_item object
ret = b.price; // error: no access to price from an Item_base
}

================================================== =======

Am I right?

[t]

Nevermind. I just tried it (which I should have done before
posting). Lippman is correct.

I was confusing:

(1) how a derived class inherits all the members of the base class and
can use the public and protected members, yet cannot use the inherited
private members; in particular, an inherited protected member becomes
a member of the derived class itself;

and

(2) if a derived class object has access to a base class object (this
doesn't include the base class PART of a derived class object), then
it can only access the base class object's public members and not its
protected or private members.

===

Is this correct? This was a bit too subtle for me to catch at first.

[Neelesh Bodas]

On Sep 21, 12:07 pm, t <[e-mail]:

Lippman's C++ Primer, 4th ed., p562, dicussion of protected members
seems to be wrong, unless I am misinterpreting things. He says:

"A derived class object may access the protected members of its base
class only through a derived object. The derived class has no special
access to the protected members of base type objects."

He gives the following example:

================================================== =======
class Item_base
{
public:
// stuff omitted
protected:
double price;
private:
std::string isbn;

};

class Bulk_item : public Item_base {
// stuff omitted

};

void Bulk_item::memfcn(const Bulk_item&d, const Item_Base& b)
{
// attempt to use protected member
double ret = price; // ok: uses this->price
ret = d.price; // ok: uses price from a Bulk_item object
ret = b.price; // error: no access to price from an Item_base

}

================================================== =======

Am I right?

No. The description and the example are both correct.
The point to note is this : "a member function can access all (even
private) members of its arguments of the same type"
In your example:

void Bulk_item::memfcn(const Bulk_item&d, const Item_base& b)
{

double ret = price; // uses this->price

ret = d.price; // The argument d is of type Bulk_item which is
same as type of *this. Hence, all members of d are accessible to every
member function of Bulk_item. Hence d.price works.
(Recollect first part of the sentence from book : A derived class
object may access the protected members of its base class only through
a derived object. Thats other part of reason why d.price works. Note
that d.price would not work if you try to access it from a general
global function like "main")

ret = b.price; // (Recollect second part of sentence from the
book : The derived class has no special access to the protected
members of base type objects. Thats why member 'price' of object b,
which is protected, cannot be accessed by general code.)
}
-Neelesh