sol2/examples/source/tutorials/erase_demo.cpp

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

#include <iostream>

int main() {

	sol::state lua;
	lua["bark"] = 50;
	sol::optional<int> x = lua["bark"];
	// x will have a value
	if (x) {
		std::cout << "x has no value, as expected"
		          << std::endl;
	}
	else {
		return -1;
	}

	lua["bark"] = sol::lua_nil;
	sol::optional<int> y = lua["bark"];
	// y will not have a value
	if (y) {
		return -1;
	}
	else {
		std::cout << "y has no value, as expected"
		          << std::endl;
	}

	return 0;
}