C++函数模板中使用输出流出现 operator<< 不明确错误

Question

 #include<iostream>
#include<set>
#include<string>
using namespace std;
template<class T>
void print(T first, T last)
{
    for (; first != last; ++first)
    {
        cout<<*first<<" ";

        /*在类IntSet中重载了<<，为什么还会出现operator<<不明确错误 ? ? ?*/

    }
    cout << endl;
}

class IntSet
{
public:
        int x;
        bool exist;
        IntSet(int Integer) :x(Integer)
        {
            exist = true;
        }
        friend bool operator <(const IntSet &t1,const IntSet &t2);
        friend ostream & operator <<(ostream o, const IntSet &t);
        void setfalse()
        {
            exist = false;
        }
};

IntSet & operator *(IntSet &t)
{
    return t;
}

bool operator <(const IntSet &t1,const IntSet &t2)
{
    return t1.x < t2.x;
}
ostream & operator <<(ostream &o, const IntSet &t)
{
    if (t.exist)
    {
        o << t.x;
        return o;
    }
    else
    {
        return o;
    }
}

typedef multiset<IntSet> MY_SET;
typedef multiset<IntSet>::iterator my_iter;

int main()
{
    MY_SET ms;
    int Integer;
    string cmd;
    int iter;
    cin >> iter;
    while (iter)
    {
        cin >> cmd;
        if (cmd == "add")
        {
            cin >> Integer;
            ms.insert(IntSet(Integer));
            cout << ms.count(IntSet(Integer)) << endl;

            print(ms.begin(),ms.end());
        }
        else if (cmd == "del")
        {
            cin >> Integer;
            cout << ms.count(IntSet(Integer)) << endl;
            my_iter p;

            pair<my_iter, my_iter> bounds = ms.equal_range(IntSet(Integer));

            for (p = bounds.first; p != bounds.second; p++)  
                 const_cast<IntSet&>(*p).setfalse();
            print(ms.begin(), ms.end());
        }
        else if (cmd == "ask")
        {
            cin >> Integer;
            my_iter p;
            pair<my_iter, my_iter> bounds = ms.equal_range(IntSet(Integer));
            int num=0;
            for (p = bounds.first; p != bounds.second; p++) 
            {
                if (p->exist) num++;
            }
            if (ms.count(Integer))
            {
                cout << 1 << " "<<num<<endl;
            }
            else
            {
                cout << 0 << " "<<0<<endl;
            }
        }
        iter--;
    }
    return 0;
}

Answer 1

IntSet & operator *(IntSet &t)
{
return t;
}
你这个重载的是乘法，不是取地址