Referring to non-static members in initializer lists before the constructor has started executing can result in undefined behavior. This results since not all members are constructed at this time. From the standard draft:

§12.7.1: For an object with a non-trivial constructor, referring to any non-static member or base class of the object before the constructor begins execution results in undefined behavior.

Example

struct W { int j; };
struct X : public virtual W { };
struct Y {
   int *p;
   X x;
   Y() : p(&x.j) { // undefined, x is not yet constructed
   }
};