metatable_key

a key for setting and getting an object’s metatable

struct metatable_key_t {};
const metatable_key_t metatable_key;

You can use this in conjunction with sol::table to set/get a metatable. Lua metatables are powerful ways to override default behavior of objects for various kinds of operators, among other things. Here is an entirely complete example, showing getting and working with a usertype’s metatable defined by sol:

messing with metatables
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#define SOL_ALL_SAFETIES_ON 1
#include <sol/sol.hpp>

#include "assert.hpp"

int main(int, char* []) {

	struct bark {
		int operator()(int x) {
			return x;
		}
	};

	sol::state lua;
	lua.open_libraries(sol::lib::base);

	lua.new_usertype<bark>("bark",
		sol::meta_function::call_function, &bark::operator()
	);

	bark b;
	lua.set("b", &b);

	sol::table b_as_table = lua["b"];		
	sol::table b_metatable = b_as_table[sol::metatable_key];
	sol::function b_call = b_metatable["__call"];
	sol::function b_as_function = lua["b"];

	int result1 = b_as_function(1);
	// pass 'self' directly to argument
	int result2 = b_call(b, 1);
	c_assert(result1 == result2);
	c_assert(result1 == 1);
	c_assert(result2 == 1);
}