sol2/examples/source/docs/as_function.cpp

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

int main() {
	struct callable {
		int operator()(int a, bool b) {
			return a + (b ? 10 : 20);
		}
	};


	sol::state lua;
	// Binds struct as userdata
	// can still be callable, but beware
	// caveats
	lua.set("not_func", callable());
	// Binds struct as function
	lua.set("func", sol::as_function(callable()));
	// equivalent: lua.set_function( "func", callable() );
	// equivalent: lua["func"] = callable();
}