Re: "One"-to-one associations and equality operators

From:

=?ISO-8859-1?Q?Daniel_Kr=FCgler?= <daniel.kruegler@googlemail.com>

Newsgroups:

comp.lang.c++.moderated

Date:

Sun, 27 Nov 2011 14:19:16 -0800 (PST)

Message-ID:

<jatjip$psj$1@dont-email.me>

Am 27.11.2011 13:59, schrieb kelvSYC:

I'm having a conundrum: it seems I can't reconcile equality operators
and (pseudo) one-to-one associations. Suppose we have two classes,
Key and Value, like so:

struct Key {
   bool operator==(const Key& key) const;
   const Value& getValue() const;
};

struct Value {
   boost::weak_ptr<Key> key;
};

So each Value is tied to one (specific) Key among those that are
equal. The challenge is: how do I make a central Key-Value repository
using C++03, such that every Value in the repository is linked to its
Key? std::map<Key, Value> doesn't work (I don't think, since I think
the Key would be copied, and thus the Value loses its association with
the Key).

You only mention ==, which is not used by std::map and would not induce a strictly weak ordering (at least one that is reasonably written ;-)). In the following I assume that your problem exists for your operator< or the actual ordering criterion used or that you will use boost's unordered containers.

Would std::map<boost::shared_ptr<Key>,
boost::shared_ptr<Value>> do the trick? Is there an easier solution?

If I understand you correctly, your criterion depends on identities of keys, not of their values. Most smart pointers, like boost::shared_ptr, will satisfy this. But any other identity wrapper will do so as well. In theory you could use boost::reference_wrapper, for example. But this means that you need to take care for the life-cycle management of the referenced keys "manually". If you can ensure that the keys have a life-time that is not shorter than that of the container, there should be no problem with using something like boost::reference_wrapper.

HTH & Greetings from Bremen,

Daniel Kr?gler

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