tostring

I'm having a problem mapping an operator<< to tostring for my class.
My environment is GCC 4.0 on Mac OS X 10.4+.  Here is the code.

  1 #include <iostream>
  2
  3 #include "lua.hpp"
  4 #include "luabind/luabind.hpp"
  5
  6 struct test_t
  7 {
  8     test_t(const std::string& name)
  9         :
 10             name_(name)
 11     {
 12     }
 13     friend std::ostream& operator<<(
 14         std::ostream& os, const test_t& test
 15     );
 16
 17 private:
 18     std::string name_;
 19 };
 20
 21 std::ostream& operator<<(std::ostream& os, const test_t& test)
 22 {
 23     os
 24         << "test_t: "
 25         << test.name_;
 26
 27     return os;
 28 }
 29
 30 int main()
 31 {
 32     lua_State* L = luaL_newstate();
 33     luaL_openlibs(L);
 34     using namespace luabind;
 35     open(L);
 36     module(L)
 37     [
 38         class_<test_t>("test_t")
 39             .def(constructor<const std::string&>())
 40             .def(tostring(self))
 41     ];
 42     if (luaL_dostring(L, "t = test_t('hi') print(t)") == 1)
 43     {
 44         std::cout
 45             << "Couldn't run Lua program: "
 46             << lua_tostring(L, -1)
 47             << std::endl;
 48     }
 49
 50 }

Compiling this yields the following error.

src/main.cpp:40: error: 'self' was not declared in this scope
src/main.cpp:40: error: 'tostring' was not declared in this scope

Commenting out line 40 causes the sample to compile but of course, the
call to print(t) fails with:

Couldn't run Lua program: No such operator defined

How should I change this example to map test_t's operator<< to its
__tostring metamethod in Lua?

   Regards,
   Ken

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