STL之set && multiset

一、set

在了解关联容器set之前，让我们先来看看下面这个例子，并猜测该例子输出什么：

// stl/set1.cpp

   #include <iostream>
   #include <set>

   int main()
   {
       //type of the collection
       typedef std::set<int> IntSet;

       IntSet coll;        //set container for int values

       /* insert elements from 1 to 6 in arbitray order
        *- value 1 gets inserted twice
        */
       coll.insert(3);
       coll.insert(1);
       coll.insert(5);
       coll.insert(4);
       coll.insert(1);
       coll.insert(6);
       coll.insert(2);

       /* print all elements
        *- iterate over all elements
        */
       IntSet::const_iterator pos;
       for (pos = coll.begin(); pos != coll.end(); ++pos) {
           std::cout << *pos << ‘ ‘;
       }
       std::cout << std::endl;
   }

其中，输出的结果为：1 2 3 4 5 6

下面，我们根据该输出结果对关联容器set做一个分析：

1. set元素的唯一性；

2. set默认按照从小到大排序；This type uses the default sorting criterion, which sorts the elements by using operator <.

　　如果你想要改变它的排序方法，需要传递额外的参数，例如：

 typedef set<int,greater<int> > IntSet;

　　Note that you have to put a space between the two ">" characters. ">>" would be parsed as shift operator, which would result in a syntax error.

二、multiset

If you want to use a multiset rather than a set, you need only change the type of the container (the header file remains the same):

   typedef multiset<int> IntSet;

A multiset allows duplicates, so it would contain two elements that have value 1. Thus, the output of the program would change to the following:

   1 1 2 3 4 5 6

例如：

// stl/mmap1.cpp

   #include <iostream>
   #include <map>
   #include <string>
   using namespace std;

   int main()
   {
       //type of the collection
       typedef multimap<int, string> IntStringMMap;

       IntStringMMap coll;       //set container for int/string values

       //insert some elements in arbitrary order
       //- a value with key 1 gets inserted twice
       coll.insert(make_pair(5,"tagged"));
       coll.insert(make_pair(2,"a"));
       coll.insert(make_pair(1,"this"));
       coll.insert(make_pair(4,"of"));
       coll.insert(make_pair(6,"strings"));
       coll.insert(make_pair(1,"is"));
       coll.insert(make_pair(3,"multimap"));

       /* print all element values
        *- iterate over all elements
        *- element member second is the value
        */
       IntStringMMap::iterator pos;
       for (pos = coll.begin(); pos != coll.end(); ++pos) {
           cout << pos->second << ‘ ‘;
       }
       cout << endl;
   }