Not able to access inherited protected member?

Newsgroup: comp.lang.c++

Subject: Not able to access inherited protected member?

From: Shriramana Sharma <samjnaa@...>

Date: Thu, 27 Jun 2013 11:10:19 -0700 (PDT)



Hello. Please consider the following minimized example for the problem I am facing.



>>>-------------------------------

template < typename T >

struct item {

typedef item<T> * pointer ;

pointer prev, next ;

} ;



template < typename ItemPtr >

struct iterator_base {

public:

iterator_base () : ptr(0) {}

protected:

ItemPtr ptr ;

explicit iterator_base ( ItemPtr p ) : ptr(p) {}

} ;



template < typename T >

struct iterator : public iterator_base < item<T> * > {

template<typename> friend class const_iterator ;

} ;



template < typename T >

struct const_iterator : public iterator_base < const item<T> * > {

const_iterator () {}

const_iterator ( const iterator<T> & it ) : ptr(it.ptr) {}

} ;



int main () {

iterator<int> it ;

const_iterator<int> cit ;

cit = it ;

}

<<<------------------------



I get the error messages:



>>>-------------------------------

$ g++ -o iterator-base-minimal-2 iterator-base-minimal-2.cpp

iterator-base-minimal-2.cpp: In constructor ?const_iterator<T>::const_iterator(const iterator<T>&)?:

iterator-base-minimal-2.cpp:28:46: error: class ?const_iterator<T>? does not have any field named ?ptr?

const_iterator ( const iterator<T> & it ) : ptr(it.ptr) {}

^

$ clang++ -o iterator-base-minimal-2 iterator-base-minimal-2.cpp

iterator-base-minimal-2.cpp:28:46: error: member initializer 'ptr' does not name a non-static data member or base class

const_iterator ( const iterator<T> & it ) : ptr(it.ptr) {}

^~~~~~~~~~~

1 error generated.

<<<-------------------------------



I am not sure why I can't access ptr which is a protected member of a parent class. OK so anyhow I thought I'll explicitly scope it by the base type and so I changed the const_iterator definition to:



>>>-------------------------------

template < typename T >

struct const_iterator : public iterator_base < const item<T> * > {

typedef iterator_base < const item<T> * > base ;

const_iterator () {}

const_iterator ( const iterator<T> & it ) : base::ptr(it.ptr) {}

} ;

<<<-------------------------------



But now I get even more cryptic error messages:



>>>-------------------------------

$ g++ -o iterator-base-minimal-2 iterator-base-minimal-2.cpp

iterator-base-minimal-2.cpp: In instantiation of ?const_iterator<T>::const_iterator(const iterator<T>&) [with T = int]?:

iterator-base-minimal-2.cpp:35:6: required from here

iterator-base-minimal-2.cpp:28:62: error: no type named ?ptr? in ?struct iterator_base<const item<int>*>?

const_iterator ( const iterator<T> & it ) : base::ptr(it.ptr) {}

^

$ clang++ -o iterator-base-minimal-2 iterator-base-minimal-2.cpp

iterator-base-minimal-2.cpp:28:52: error: typename specifier refers to non-type member 'ptr' in 'iterator_base<const item<int> *>'

const_iterator ( const iterator<T> & it ) : base::ptr(it.ptr) {}

^~~

iterator-base-minimal-2.cpp:35:8: note: in instantiation of member function 'const_iterator<int>::const_iterator' requested here

cit = it ;

^

iterator-base-minimal-2.cpp:13:10: note: referenced member 'ptr' is declared here

ItemPtr ptr ;

^

iterator-base-minimal-2.cpp:28:52: error: 'ptr' is a protected member of 'iterator_base<const item<int> *>'

const_iterator ( const iterator<T> & it ) : base::ptr(it.ptr) {}

^

iterator-base-minimal-2.cpp:13:10: note: must name member using the type of the current context 'const_iterator<int>'

ItemPtr ptr ;

^

2 errors generated.

<<<-------------------------------



I'm using GCC 4.8.1 and Clang 3.2 on Kubuntu Raring 64-bit. Can anyone please tell me what the problem is and how I can solve it? Thank you very much!

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