Pass argument by reference.

Hello,

I am new in luabind. I am trying to pass argument by a reference to std::string.
When I use const reference to std::string. It works.
The following code works:
--------------------------------
.lua
function test(a)
 c_ref(a)
end

.cpp
void c_ref(const std::string& val) 
{ 
    std::cout << val;
}
module(L)
[
    def("c_ref", &c_ref)
];
luabind::call_function< void >(a_state, "test", str);
--------------------------------

But as soon as I change 'const std::string&' to 'std::string&' and 'str' to 'boost::ref(str)', it does not work. I got "you are trying to use an unregistered type".
--------------------------------
void c_ref(std::string& val) {}
luabind::call_function< void >(a_state, "test", boost::ref(str));
--------------------------------

It is not clear for me why std::string is known class, but std::string& does not.

Could you please explain? And how to fix my second sample?

Thanks,
Valery.

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