Re: operator< for algorithms

From:
Victor Bazarov <v.Abazarov@comAcast.net>
Newsgroups:
comp.lang.c++
Date:
Thu, 02 Apr 2009 11:14:45 -0400
Message-ID:
<gr2kp5$i4s$1@news.datemas.de>
Hicham Mouline wrote:

"Victor Bazarov" <v.Abazarov@comAcast.net> wrote in message
news:gr2jto$g24$1@news.datemas.de...

Hicham Mouline wrote:

"Victor Bazarov" <v.Abazarov@comAcast.net> wrote in message
news:gr2bjd$st4$1@news.datemas.de...

Hicham Mouline wrote:

namespace NS1 { namespace NS2 {

template <typename T1, typename T2>
class C {
public:
  typedef T1::xType xType;
  typedef T1::yType yType;
...
private:
  typedef std::pair< xType, yType> EntryType;
  boost::array< EntryType , size > ContainerType;
  ....
};
...
}}

I need to use std::lower_bound and sorting algorithms on my
ContainerType.

Where (which namespace) and how can I define the operator< comparing 2
EntryType
for the std:: algorithms to use that operator?

Anywhere appropriate. Any namespace/class where your compiler is
capable of finding it.

Note I use these std::algorithms only in member functions of the C
template.
So I wish to define the operator< in a way that is specific and visible
only to C template.

Have you tried defining it as a static member of the C template?

Post the rest of your code, with only the operator< missing, and we can
probably figure it out. I am just too lazy to provide all the necessary
driver code for what you've described, I consider it your job if you
want my help. Sorry, such a PITA I am.

V

Not at all. Thank you for your contrib!

I put the code here
http://codepad.org/Gwfm36GL

It fails to compile under g++4
"operator< needs to be must be either a non-static member function or a
non-member function"

if it is member non-static, it is interpreted as applying to C<T1,T2> not
to EntryType

As EntryType is private, I don't know how to define outside of C<T1,T2>

I managed to make your code from the 'codepad.org' to compile when I
removed the operator< definition from the 'C' template (it wasn't the
brightest suggestion, I guess) and put it right after the 'C' as this:

  template<class T, class U>
  bool operator<(const typename C<T,U>::EntryType& lhs,
                 const typename C<T,U>::EntryType& rhs)
  {
    return true;
  }

It is strange that this compiles.
EntryType is private, why does operator< outside manage to access it?


It is quite possible that since all instantiations of that operator are
made from inside a member function of 'C', where access to 'EntryType'
is granted, the compiler considers it an implicit permission...

V
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