Re: specializing swap for nested iterator class

From:
=?UTF-8?B?RGFuaWVsIEtyw7xnbGVy?= <daniel.kruegler@googlemail.com>
Newsgroups:
comp.lang.c++.moderated
Date:
Sat, 21 Apr 2012 13:27:13 -0700 (PDT)
Message-ID:
<jmucdc$70f$1@dont-email.me>
Am 21.04.2012 09:51, schrieb galexander2@nc.rr.com:

I'm developing my own container class and have implemented the
container's iterator as a nested class. I want to include a
specialized version of swap for the iterator. Here's a small
example of what I tried:

     #include<cstdlib>
     #include<functional>
     #include<iostream>
     #include<memory>
     #include<utility>

     namespace ns {
       template<class Key, class Compare = std::less<Key>, class
Allocator = std::allocator<Key> >
       class myContainer {
         public:
           class iterator_implementation {
             public:
               inline iterator_implementation() {
               }
           };
         public:
           typedef iterator_implementation iterator;
       };
       template<class Key, class Compare, class Allocator>
       inline void swap(typename myContainer<Key, Compare,
Allocator>::iterator& x,
                        typename myContainer<Key, Compare,
Allocator>::iterator& y) {
         std::cout<< "**** specialized iterator swap ****"<< std::endl;
       }
     }


The problem is located here in the signature of your function
template: A parameter like

typename myContainer<Key, Compare,Allocator>::iterator

is always very suspicious unless designed carefully: It does not allow
by-argument deduction of the template parameters. I did not check the
FAQ, but this should actually belong to the FAQ list: A
type-construction like

typename /some_template/</some_arguments/>::/some_type/

where some_arguments needs to be deduced is always an undeduced
context, because in general the compiler cannot find the set of
some_arguments given a dependent type some_type: This is like finding
an inverse function: To find the inverse of a mathematical function
requires that this function is invertible (i.e. there are some
constraints imposed), which is a unique inverse relation. C++
templates are not (in general) invertible type/value functions.

     int main(int argc, char* argv[]) {
       ns::myContainer<int>::iterator x =

ns::myContainer<int>::iterator();

       ns::myContainer<int>::iterator y =

ns::myContainer<int>::iterator();

       using std::swap;
       std::cout<< "before swap(x, y)"<< std::endl;
       swap(x, y);
       std::cout<< "after swap(x, y)"<< std::endl;
       std::cout<< std::endl;
       std::cout<< "before swap<int, std::less<int>, std::allocator<int>

(x, y)"<< std::endl;

       swap<int, std::less<int>, std::allocator<int> >(x, y);
       std::cout<< "after swap<int, std::less<int>, std::allocator<int>

(x, y)"<< std::endl;

       exit(0);
     }

The problem is that swap(x, y) calls std::swap rather than the
specialized ns:swap. It seems that ADL isn't finding ns:swap. To
call ns:swap, one has to specify template arguments as in for
example swap<int, std::less<int>, std::allocator<int> >(x, y), which
isn't what I want. Is this correct behavior according to the C++
standard?


Yes, that is to be expected: You swap overload can never be called by
argument deduction.

Or is it a compiler bug? If this is correct standard behavior, can
someone point to where this behavior is described?


This is described in 14.8.2.5 [temp.deduct.type] p4-6, in particular
see p5:

"The non-deduced contexts are:
? The nested-name-specifier of a type that was specified using a
qualified-id."

combined with p6:

"When a type name is specified in a way that includes a non-deduced
context, all of the types that comprise that type name are also
non-deduced."

In your example "myContainer<Key, Compare, Allocator>" is the above
mentioned nested-name-specifier and the referred to type is
"myContainer<Key, Compare, Allocator>::iterator".

In this case is there a standard way to define a specialize swap for
iterator short of making the iterator_implementation class its own
stand-alone class rather than a nested class?


The most simple one is to add a defining in-class friend function to
your iterator type such as:

      namespace ns {
        template<class Key, class Compare = std::less<Key>, class
Allocator = std::allocator<Key> >
        class myContainer {
          public:
            class iterator_implementation {
              public:
                inline iterator_implementation() {
                }

                friend void swap(iterator_implementation& x,
                                 iterator_implementation& y) {
                  std::cout << "**** specialized iterator swap ****" <<
std::endl;
                }
            };
          public:
            typedef iterator_implementation iterator;
        };
      }

HTH & Greetings from Bremen,

Daniel Kr?gler

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