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The definition of pointer arithmetic from the C++ Standard, [expr.add], paragraph 7 [ISO/IEC 14882-2014], states the following:

For addition or subtraction, if the expressions P or Q have type “pointer to cv T”, where T is different from the cv-unqualified array element type, the behavior is undefined. [Note: In particular, a pointer to a base class cannot be used for pointer arithmetic when the array contains objects of a derived class type. —end note]

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The C++ Standard, [expr.sub], paragraph 11 [ISO/IEC 14882-2014], defines array subscripting as being identical to pointer arithmetic. Specifically, it states the following:

The expression E1[E2] is identical (by definition) to *((E1)+(E2)).

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The following code examples assume the following static variables and class definitions:.

int globI;
double globD;

struct S {
  int i;
  
  S() : i(globI++) {}
};

struct T : S {
  double d;
  
  T() : S(), d(globD++) {}
};

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