这个错误怎么解决，求助，error C2679: binary '<<'

[fancy320]

#include<map>
#include<algorithm>
#include<iterator>
#include<string>
#include<iostream>
using namespace std;
typedef map<string, int> months_type;
template <class First,class Second>
ostream& operator<<(ostream& out,const pair<First,Second> &p)
{
cout<<p->first<<" has "<<p->second<<" days";
return out;
}
ostream& operator<<(ostream& out,const months_type & l)
{
copy(l.begin(),l.end(),ostream_iterator<months_type::value_type>(cout,";"));
return out;
}
void main()
{
months_type months;
typedef months_type::value_type value_type;
months.insert(value_type(string("January"),31));
months.insert(value_type(string("February"),28));
months.insert(value_type(string("April"),30));
months.insert(value_type(string("March"),31));
cout<<months<<endl;

}
//编译时的错误
//error C2679: binary '<<' : no operator defined which takes a right-hand operand of type 'const struct std::pair<class std::basic_string<char,struct std::char_traits<char>,class st

[taodm]

cout<<p.first

[Jim_King_2000]

因为ostream_iterator会寻找operator<<，而std namespace里面有很多operator<<，因此编译器不会去全局空间查找operator<<。也就是说，std::operator<<隐藏了::operator<<。把operator<<放入std namespace即可编译通过。

#include<map>
#include<algorithm>
#include<iterator>
#include<string>
#include<iostream>

using namespace std;

typedef map<string, int> months_type;

namespace std
{
	template <class First,class Second>
	ostream& operator<<(ostream& out, const pair<First,Second> &p)
	{
		cout<<p.first<<" has "<<p.second<<" days";
		return out;
	}

	ostream& operator<<(ostream& out,const months_type & l)
	{
		copy(l.begin(), l.end(), ostream_iterator<months_type::value_type>(out, ";\n"));
		return out;
	}
}

void main()
{
	months_type months;
	typedef months_type::value_type value_type;
	months.insert(value_type(string("January"),31));
	months.insert(value_type(string("February"),28));
	months.insert(value_type(string("April"),30));
	months.insert(value_type(string("March"),31));
	cout<<months<<endl;
}

但是往std namespace里面加东西是不太好的行为，会引起冲突。因此LZ还是不要这样用为好。