sol2/examples/source/docs/runtime_extension.cpp

#define SOL_ALL_SAFETIES_ON 1
#include <sol/sol.hpp>

#include <iostream>

struct object {
	int value = 0;
};

int main(int, char*[]) {

	std::cout << "==== runtime_extension =====" << std::endl;

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

	lua.new_usertype<object>("object");

	// runtime additions: through the sol API
	lua["object"]["func"] = [](object& o) { return o.value; };
	// runtime additions: through a lua script
	lua.script(
	     "function object:print () print(self:func()) end");

	// see it work
	lua.script("local obj = object.new() \n obj:print()");

	return 0;
}