No match for 'operator<<' in '((HttpRequest*

Dear advanced c/g++ programers:

  I have a simple program from book C++ cookbook, page 291, 8.3, Using
Constructors and Destructors to manage
resources (or RAII), but it can not get compiled in my g++
------------------------------------------------------------------------------------------------
// Example 8-3. Using constructors and destructors
#include <iostream>
#include <string>
using namespace std;
class Socket {
public:
   Socket(const string& hostname) {}
};
class HttpRequest {
public:
  HttpRequest (const string& hostname) :
     sock_(new Socket(hostname)) {}
  void send(string soapMsg) {sock_ << soapMsg; }
  ~HttpRequest () {delete sock_;}
private:
   Socket* sock_;
};
void sendMyData(string soapMsg, string host) {
   HttpRequest req(host);
   req.send(soapMsg);
   // Nothing to do here, because when req goes out of scope
   // everything is cleaned up.
}
int main() {
   string s = "xml";
   sendMyData(s, "www.oreilly.com");
}
------------------------------------------------------------------------------------------------------------------------
my test compile fail as
----------------------------------
eric@eric-laptop:~/cppcookbook/ch8$ g++ Example8-3.cpp
Example8-3.cpp: In member function ‘void
HttpRequest::send(std::string)’:
Example8-3.cpp:13:39: error: no match for ‘operator<<’ in
‘((HttpRequest*)this)->HttpRequest::sock_ << soapMsg’
-------------------------------------------------------------
on both g++ 4.3.4(cygwin on windown xp) and 4.5.2(ubuntu10.04)
you can get its source code from
http://examples.oreilly.com/9780596007614/
to test by yourself

looking and thanks your help a lot in advance, Eric