Re: hasMember

From:

James Kanze <james.kanze@gmail.com>

Newsgroups:

comp.lang.c++

Date:

Thu, 9 Dec 2010 03:54:56 -0800 (PST)

Message-ID:

<aa54412a-8091-436e-9507-70c6a479b99c@r16g2000prh.googlegroups.com>

On Dec 8, 12:51 pm, Jeff Flinn <TriumphSprint2...@hotmail.com> wrote:

Andrea Crotti wrote:

James Kanze <james.ka...@gmail.com> writes:

The simplest solution is to return a pointer to the map's
contents, or NULL if the value isn't there, e.g.:

    int const* Cont::getValue(int idx) const
    {
        std::map<int, int>::const_iterator result
                = content.find(idx);
        return result == content.end()
            ? NULL
            : &result->second;
    }

and:

    int const* i = obj.getValue(idx);
    if ( i != NULL ) {
        ...
    }

I would do that in C but I don't think it looks nice in C++...

To avoid the memory allocation and ownership issues use boost::optional.

There aren't any memory allocation and ownership issues. I use
the Fallible idiom (on which boost/optional is based) a lot, but
in this case, a pointer seems simpler and more appropriate:
a pointer is the standard idiom for a fallible reference.

   #include <boost/optional.hpp>

   typedef boost::optional<int> optional_int;

   optional_int Cont::getValue(int idx) const
   {
      std::map<int, int>::const_iterator itr = content.find(idx);
      return (itr != cont.end())? itr->second : optional_int();
   }

and:

   if(optional_int i = obj.getValue(idx)) // safe bool idiom
   {
      int x = *i + 123; // deref for value
   }

Which doesn't work in general, since the returned value (if
found) is supposed to be a reference to the element, not a copy
of it.

I don't know if boost would support boost::optional<int&>. When
I wrote my own Fallible, it didn't even try to support it,
since you already have fallible references, in the form of
pointers.

--
James Kanze