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+// -*- C++ -*-
+
+// Copyright (C) 2005, 2006, 2009 Free Software Foundation, Inc.
+//
+// This file is part of the GNU ISO C++ Library. This library is free
+// software; you can redistribute it and/or modify it under the terms
+// of the GNU General Public License as published by the Free Software
+// Foundation; either version 3, or (at your option) any later
+// version.
+
+// This library is distributed in the hope that it will be useful, but
+// WITHOUT ANY WARRANTY; without even the implied warranty of
+// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU
+// General Public License for more details.
+
+// You should have received a copy of the GNU General Public License
+// along with this library; see the file COPYING3. If not see
+// .
+
+
+// Copyright (C) 2004 Ami Tavory and Vladimir Dreizin, IBM-HRL.
+
+// Permission to use, copy, modify, sell, and distribute this software
+// is hereby granted without fee, provided that the above copyright
+// notice appears in all copies, and that both that copyright notice
+// and this permission notice appear in supporting documentation. None
+// of the above authors, nor IBM Haifa Research Laboratories, make any
+// representation about the suitability of this software for any
+// purpose. It is provided "as is" without express or implied
+// warranty.
+
+/**
+ * @file basic_multimap_example.cpp
+ * A basic example showing how to use multimaps.
+ */
+
+/**
+ * This example shows how to use "multimaps" in the context of a simple
+ * bank account application. Each customer holds a bank account
+ * (or more than one) which holds some balance.
+ */
+
+#include
+#include
+#include
+#include
+
+using namespace std;
+using namespace __gnu_pbds;
+
+// A simple hash functor.
+// hash could serve instead of this functor, but it is not yet
+// standard everywhere.
+struct string_hash : public unary_function
+{
+ inline size_t
+ operator()(const string& r_s) const
+ {
+ size_t ret = 0;
+ string::const_iterator b = r_s.begin();
+ string::const_iterator e = r_s.end();
+ while (b != e)
+ {
+ ret *= 5;
+ ret += static_cast(*(b++));
+ }
+ return ret;
+ }
+};
+
+int main()
+{
+ // Each customer is identified by a string.
+ typedef string customer;
+
+ // Each account is identified by an unsigned long.
+ typedef unsigned long account_id;
+
+ // The balance in the account is a floating point.
+ typedef float balance_t;
+
+ /*
+ * This is the data structure type used for storing information
+ * about accounts. In this case the primary key is the customer,
+ * and the secondary key is the account id.
+ *
+ * A hash-based container maps each customer to a list-based
+ * container that maps each account to the balance it holds.
+ *
+ * Note that we could use any combination of primary and secondary
+ * associative-containers. In this case we choose a hash-based
+ * container for the primary keys, since we do not need to store
+ * customers in a sorted order; we choos a list-based container for
+ * the secondary keys, since we expect that the average number of
+ * accounts per customer will be small.
+ */
+ typedef
+ cc_hash_table<
+ customer,
+ list_update<
+ account_id,
+ balance_t>,
+ string_hash>
+ accounts_t;
+
+ // This object will hold all information.
+ accounts_t acc;
+
+ // Customer "a" opens empty account 12.
+ acc["a"][12] = 0;
+
+ // Customer "a" deposits 45 into account 12.
+ acc["a"][12] += 45;
+
+ // Customer "b" opens account 13 with balance 12.3.
+ acc["b"][13] = 12.3;
+
+ // Customer "c" opens empty account 14.
+ acc["c"][14] = 0;
+
+ // Customer "a" opens account 160 with balance 142.
+ // Note that "a" already holds account 12.
+ acc["a"][160] = 142;
+
+ // Verify the number of accounts that "a" holds.
+ accounts_t::const_point_iterator it = acc.find("a");
+ assert(it != acc.end());
+ assert(it->second.size() == 2);
+
+ // The begining of the month has arrived. We need to give a 3%
+ // interest to all accounts with a positive balance.
+
+ // First we loop over all customers.
+ accounts_t::iterator cust_it;
+ for (cust_it = acc.begin(); cust_it != acc.end(); ++cust_it)
+ {
+ // For each customer, we loop over the customer's accounts.
+ accounts_t::mapped_type::iterator it;
+ for (it = cust_it->second.begin(); it != cust_it->second.end(); ++it)
+ if (it->second > 0)
+ it->second *= 1.03;
+ }
+
+ // Customer "a" closes all accounts.
+ acc.erase("a");
+
+ // The bank now has only 2 customers.
+ assert(acc.size() == 2);
+
+ return 0;
+}
+