Re: Using a std::set ordering predicate with ctor arguments

From:

Carl Barron <cbarron413@adelphia.net>

Newsgroups:

comp.lang.c++.moderated

Date:

Tue, 6 Feb 2007 02:15:09 CST

Message-ID:

<050220072143408755%cbarron413@adelphia.net>

In article <Xns98CEB5EA9A312wrgrpde@news.albasani.net>, Wolfram Roesler
<wr@spam.la> wrote:

Hello all,

I know how to write a functor class to specify the ordering predicate
used by a std::set, something like this:

  class Order
  {
  public:
    bool operator()(MyType const &a,MyType const &b) const;
  };

  std::set<MyType,Order> MySet;

My question is, is it possible to use an ordering class that requires
constructor arguments? As in,

  class Order2
  {
  public:
    Order2(SomeType const &s);
    bool operator()(MyType const &a,MyType const &b) const;
  };

In my application, the Order2 ctor would store some information from
the SomeType object in a private class member in order to make it
available to the operator() function.

Is there any way to get this done?

   looks like:
class MyOrder
{
    // stuff to copy from SomeType
public:
    explicit Myorder(const SomeType &st)
    {
       // copy data from st to the private data
    }
    bool operator () (MyType const &a,MyType &b)const
    {
       // ...
    }
};
SomeType some;
// initailize some.
std::set<MyType,MyOrder> foo(MyOrder(some));

// ...

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