Re: Iterator inheritance

From:

"Alf P. Steinbach" <alfps@start.no>

Newsgroups:

comp.lang.c++

Date:

Wed, 30 Jan 2008 19:49:25 +0100

Message-ID:

<13q1hhvmkc11n6d@corp.supernews.com>

* Henrik Goldman:

Using std::map I'm creating a class which adds a thread safety layer on top
of map.
However on some compilers I receive compiler errors which are not easily
decipherable.

I'm pretty sure that it's not a compiler specific problem but rather a lack
of understaning on my part of how to do the deriving correctly.

Here is the short version of the problem:

#include <map>

using namespace std;

template <class T, class U>

class CMap

{

public:

virtual ~CMap() {}

typedef typename map<T,U>::value_type value_type;

typedef typename map<T,U>::iterator iterator;

... omitted code

protected:

map<T, U> m_Map;

};

template <class T, class U>

class CThrSafeMap: public CMap<T, U>

{

public:

virtual iterator Insert(const value_type &V) // <<---- problem is here

{

CMutexGuard MtxGuard(&m_Mutex);

return m_Map.insert(V).first;

}

While it works perfectly fine with Visual Studio I'm receiving errors with
g++ 3.4.6:

../../shared/generic/mymap.h:51: error: `iterator' does not name a type
../../shared/generic/mymap.h:51: error: (perhaps `typename CMap<T,
U>::iterator' was intended)
../../shared/generic/mymap.h: In member function `virtual bool
CThrSafeMap<T, U>::IsEmpty()':
../../shared/generic/mymap.h:62: error: `m_Map' was not declared in this
scope

What am I missing here?

Should I go further with the typename keyword?

Yes and no. You just need to refer to the base class' typedef. It
might seem like the language is not perfectly designed in this respect
(and I would agree!), but, what you need is an abundance of redundance:

   template< classs T, class U >
   class TreadSafeMap: public Map<T, U>
   {
   public:
       typedef Map<T, U> Base;
       typedef typename Base::value_type value_type;
       typedef typename Base::iterator iterator;

       //...
   };

Cheers, & hth.,

- Alf

--
A: Because it messes up the order in which people normally read text.
Q: Why is it such a bad thing?
A: Top-posting.
Q: What is the most annoying thing on usenet and in e-mail?