Use of std::optional to pass a std::vector<int> to a functional by reference

Question

I am unclear if the correct code to pass an optional vector of ints to a function by reference is :

void test_func(std::optional<std::vector<int>&> vec)

or

void test_func(std::optional<std::vector<int>>& vec)

Any help much appreciated.

Answer 1

A non-owning pointer is a nullable reference type.

void test_func(std::vector<int>* vec)