Lazy initialization of const attributes in c++

I would like to carry out a lazy initialization of a set of (std::vector) attributes in c++. They have to be const, in the sense that after the first time they are initialized (via a get method), their values cannot be modified. What is the cleanest way to do this?

I tried to define the attributes as const, but in this way they must be initialized in the construntor initialization list, so no lay initialization seems to be possible.

You might do something like:

class MyClass
{
public:
    const std::vector<int>& getV()
    {
        if (!v) {
            v = std::vector{4, 8, 15, 16, 23, 42};
        }
        return *v;
    }
private:
    std::optional<std::vector<int>> v;
};

If some specific value is "prohibited" from vector (as empty vector), you might remove optional and use that value instead.