C++c重载出错

我写了个Book的类
一开始如下：

class Book {public:Book(string name,string author,string pub,string isbn, int remain, int borrowed):_bookName(name),_bookAuthor(author),_bookPub(pub),_bookISBN(isbn),_bookRemain(remain),_bookBorrowed(borrowed){}//...中间省略}ofstream& operator<<(ofstream& out, const Book& book){     out << book._bookName << ',' << book._bookAuthor << ',' << book._bookISBN << ',' << book._bookPub << ',' << book._bookRemain << ',' << book._bookBorrowed << '\n';     return out;}

一开始这个<<操作符是可以用的。
然后我往里面多加了一个构造函数，代码如下

class Book {public:Book(string name,string author,string pub,string isbn, int remain, int borrowed):_bookName(name),_bookAuthor(author),_bookPub(pub),_bookISBN(isbn),_bookRemain(remain),_bookBorrowed(borrowed){}     Book(string isbn){     _bookAuthor = "";     _bookName = "";     _bookISBN = isbn;     _bookPub = "";     _bookRemain = 0;     _bookBorrowed = 0;}//..}

加上一个构造后，就开始报错：

./Book.h:79:10: error: use of overloaded operator '<<' is ambiguous (with operand types 'ofstream' (aka 'basic_ofstream<char>') and 'const string'      (aka 'const basic_string<char>'))     out << book._bookName << ',' << book._bookAuthor << ',' << book._bookISBN << ',' << book._bookPub << ',' << book._bookRemain << ',' << book._bookBorrowed << '\n';     ~~~ ^  ~~~~~~~~~~~~~~/usr/lib/gcc/x86_64-redhat-linux/4.7.2/../../../../include/c++/4.7.2/bits/basic_string.h:2749:5: note: candidate function [with _CharT = char, _Traits =      std::char_traits<char>, _Alloc = std::allocator<char>]    operator<<(basic_ostream<_CharT, _Traits>& __os,

我google了一下错误，找到了这个帖子http://stackoverflow.com/questions/33...
但是依旧不懂，怎么出现的冲突？我没有用模板什么的。
然后我,尝试放入一个没有参数的构造函数：

Book(){     _bookAuthor = "";     _bookName = "";     _bookISBN = "";     _bookPub = "";     _bookRemain = 0;     _bookBorrowed = 0;}

编译通过了。
这是为什么阿？