set和multiset的内部通常是采用平衡二叉树来实现。当放入元素时，会按照一定的排序方法自动排序，默认是按照less<>排序规则来排序。这种自动排序的特性加速了元素查找的过程，但问题是：不可以直接修改set或multiset容器中的元素值，因为这样就违反了元素自动排序的规则。如果想修改一个元素的值，则必须先删除原有的元素，再插入新的元素。
multiset容器的集合并、交、差运算（注意：并集运算的结果）
#include "stdafx.h"
#include <iostream>
#include <set>
#include <algorithm>
using namespace std;
int_tmain(int argc, _TCHAR* argv[])
{
const int N=10;
int a[N] = {4,1,1,1,1,1,0,5,1,0};
int b[N] = {4,4,2,4,2,4,0,1,5,5};
multiset<int> A(a,a+N);
multiset<int> B(b,b+N);
multiset<int> C,D;
cout<<"Set A:";
copy(A.begin(),A.end(),ostream_iterator<int>(cout,""));
cout<<endl;
cout<<"Set B: ";
copy(B.begin(),B.end(),ostream_iterator<int>(cout,""));
cout<<endl;
cout<<"Union (A and B): ";
set_union(A.begin(),A.end(),B.begin(),B.end(),ostream_iterator<int>(cout,""));
cout<<endl;
cout<<"Union (B and A): ";
set_union(B.begin(),B.end(),A.begin(),A.end(),ostream_iterator<int>(cout,""));
cout<<endl;
cout<<"Intersection (A and B): ";
set_intersection(A.begin(),A.end(),B.begin(),B.end(),ostream_iterator<int>(cout,""));
cout<<endl;
cout<<"Intersection (B and A): ";
set_intersection(B.begin(),B.end(),A.begin(),A.end(),ostream_iterator<int>(cout,""));
cout<<endl;
set_difference(A.begin(),A.end(),B.begin(),B.end(),inserter(C,C.begin()));
cout<<"Set C(difference of A and B): ";
copy(C.begin(),C.end(),ostream_iterator<int>(cout,""));
cout<<endl;
set_difference(B.begin(),B.end(),A.begin(),A.end(),inserter(D,D.begin()));
cout<<"Set D(difference of B and A): ";
copy(D.begin(),D.end(),ostream_iterator<int>(cout,""));
cout<<endl;
return 0;
}
执行结果：
注意并集：最终结果中的相同元素的个数取集合中最多的数目，如此处求并集时0的个数取的是集合A的个数，为两个；5的个数取的是B中的数目，为两个。
因此，两个集合的并、交满足交换率，而差则不满足交换律，符合数学原理。