Re: map vs. set (stl)

From:
Qwavel <qwavel@gmail.com>
Newsgroups:
comp.lang.c++
Date:
23 May 2007 16:57:34 -0700
Message-ID:
<1179964654.736652.98270@o5g2000hsb.googlegroups.com>
On May 23, 7:39 pm, Markus Schoder <a3vr6dsg-use...@yahoo.de> wrote:

On Wed, 23 May 2007 15:29:54 -0700, Qwavel wrote:

On May 23, 4:59 pm, Markus Schoder <a3vr6dsg-use...@yahoo.de> wrote:

On Wed, 23 May 2007 12:41:18 -0700, Qwavel wrote:

Let's say I have something like this, where 'name' is a POD type, and
'value' is a class.

std::map< name, value >

But then I realize that 'name' should actually be one of the members
of 'value' class, so I have a redundancy. I then switch and start
using std::set< value >. To make 'value' suitable for this purpose,
I make it look like this...

class value {
  const int name;
  bool operator<( const value& rhs ) const
     { return name < rhs.name; }
  void operator=( const value& rhs );
  ...
};

This now satisfies the requirements of a set, and it works. Great.
But I feel as though I have really strayed far from the idea of a
set. For example, the key part of my value is constant, but the
whole value is not.

Should I really be using a set like this?


The problem you might be facing is that you cannot (without casting)
modify the objects in the set through a set iterator. A set iterator is
basically always a const iterator to prevent breaking the ordering of
the set.


Yes, that is what you would expect.

However, in my STL, the set::find function returns a non-const iterator,
so I can modify the elements of the set. Of course, I must be careful
not to change the key value.

I'm using the STL that comes with MS VC8. I don't know if this behavior
conforms to the standard or not.


The current wording of the standard surprisingly _requires_ this behavior
but there is a defect report pending (103) that proposes to change this
and make keys in associative containers immutable.

--
Markus Schoder


Yikes! That change would break some of my code.

Does that mean that the best solution for me is to use
std::map<key,value>, in spite of the fact that I would then have two
copies of the key (since 'value' contains the key)?

It would not be efficient for me to erase and re-add the value just to
modify it.

Qwavel (OP)

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