Notes

A value that might or might not be there — without reaching for a null pointer or a magic sentinel value to mean “nothing.”

std::optional<int> findIndex(const std::vector<int>& v, int target) {
    for (size_t i = 0; i < v.size(); i++)
        if (v[i] == target) return i;
    return std::nullopt;   // explicitly "no value"
}

auto result = findIndex(v, 5);
if (result) {
    std::cout << *result;   // dereference like a pointer to get the value
}

Why this beats a sentinel value

int findIndex(const std::vector<int>& v, int target);   // returns -1 if not found?

A sentinel like -1 only works if it can never be a legitimate result — and it’s easy to forget to check for it. optional makes “might not have a value” part of the type itself, and forces a check before you can get at the value at all.

Related

API design guidelines