How is luabind::object's assignment operator overloaded?

Question

I'm learning luabind and attempting to use luabind::object to access variables in Lua from C++.

When I assigned an int to an "object", the compilation failed.

The Code:

int main()
{
    using namespace luabind;
    lua_State *L;
    L = luaL_newstate();
    luaL_openlibs(L);
    open(L);
    luaL_dofile(L, "./test.lua"); // a = 5
    object g = globals(L);
    object num = g["a"];
    num = 6; // Failed to compile
    return 0;
}

The error messages:

luatets.cpp:21:6: error: no match for ‘operator=’ (operand types are ‘luabind::adl::object’ and ‘int’)
  num = 6;
/usr/include/luabind/object.hpp:731:9: note: luabind::adl::object& luabind::adl::object::operator=(const luabind::adl::object&)
   class object : public object_interface<object>
/usr/include/luabind/object.hpp:731:9: note:   no known conversion for argument 1 from ‘int’ to ‘const luabind::adl::object&’

But after I combined the two lines, the code worked:

g["a"] = 6;

I don't know why this happened. In luabind documentation, it's said that:

When you have a Lua object, you can assign it a new value with the assignment operator (=). When you do this, the default_policy will be used to make the conversion from C++ value to Lua.

And in the class declaration, the assignment operator is overloaded for arbitrary types, not only for object&:

template <class T>
object& operator=(T const&);

object& operator=(object const&); 

By the way, I found my problem similar to this, but nobody answered it.

I've had a look at the luabind headers, but could not find any clue inside those horrible proxies.

Could anyone tell me why the first code is not correct and whether the operator=(T const &) is overloaded ?

Thanks a lot!!

Answer 1

Luabind 0.9.1 has no such operator= overload in object, regardless of what the (probably outdated?!) documentation says. In fact, object has no operator= overloads at all (and neither has object_interface, which object is derived from).

However, it has:

template<class T>
    index_proxy<object> operator[](T const& key) const

Which is why g["a"] = 6; worked since index_proxy has:

template<class T>
    this_type& operator=(T const& value)

which can be instantiated for int.