Annex F (informative) Core undefined behavior [ub]

F.6 [class]: Classes [ub.class]

F.6.4[ub:class.cdtor.before.ctor]
Specified in: [class.cdtor]

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.
2
#
[Example 1: struct W { int j; }; struct X : public virtual W {}; struct Y { int *p; X x; Y() : p(&x.j) { // undefined behavior, x is not yet constructed } }; — end example]