[C++] pointer arithmetic

Refernece:
[C++] Object Lifetimes reading minute

class Vec {
  public:
    double* data() { return &x; }

  private:
    double x,y,z;
};

Eigen::Map<...>(absl::Span<Vec>);

*(Vect::data() + 1) // Does not give us y

Take away: 

* Even though x, y, and z are allocated sequentially in memory without padding,

physical layout does not supersede semantic rules.

* The layout guarantees mean you can safely memcpy the data, or cast a Vec* to a double* to access the first element (x). It does not grant permission to use pointer arithmetic on double* to slide across the members.

* The pointer arithmetic is only guaranteed within the type of array.

* Pointer to variable only is considered as pointer to array of size 1.

* Thus any pointer arithmetic on single variable is considered out-of-bound; compiler is free to assume anything.

(reference: https://timsong-cpp.github.io/cppwp/n4140/expr.add#5)

Explain:

Only char*, unsigned char*, and std::byte* are explicitly granted an exception in the standard to 

examine the raw object representation. double* enjoys no such privilege.

(reference: https://timsong-cpp.github.io/cppwp/n4861/basic.lval#11.3)

Fix:

class Vec {
 public:
  double* data() { return data_; } // Legal: returns pointer to element 0 of a 3-element array
 private:
  double data_[3]; // x=data_[0], y=data_[1], z=data_[2]
};