C++ Invalid pointer arithmetic


Example

The following uses of pointer arithmetic cause undefined behavior:

  • Addition or subtraction of an integer, if the result does not belong to the same array object as the pointer operand. (Here, the element one past the end is considered to still belong to the array.)

    int a[10];
    int* p1 = &a[5];
    int* p2 = p1 + 4; // ok; p2 points to a[9]
    int* p3 = p1 + 5; // ok; p2 points to one past the end of a
    int* p4 = p1 + 6; // UB
    int* p5 = p1 - 5; // ok; p2 points to a[0]
    int* p6 = p1 - 6; // UB
    int* p7 = p3 - 5; // ok; p7 points to a[5]
    
  • Subtraction of two pointers if they do not both belong to the same array object. (Again, the element one past the end is considered to belong to the array.) The exception is that two null pointers may be subtracted, yielding 0.

    int a[10];
    int b[10];
    int *p1 = &a[8], *p2 = &a[3];
    int d1 = p1 - p2; // yields 5
    int *p3 = p1 + 2; // ok; p3 points to one past the end of a
    int d2 = p3 - p2; // yields 7
    int *p4 = &b[0];
    int d3 = p4 - p1; // UB
    
  • Subtraction of two pointers if the result overflows std::ptrdiff_t.

  • Any pointer arithmetic where either operand's pointee type does not match the dynamic type of the object pointed to (ignoring cv-qualification). According to the standard, "[in] particular, a pointer to a base class cannot be used for pointer arithmetic when the array contains objects of a derived class type."

    struct Base { int x; };
    struct Derived : Base { int y; };
    Derived a[10];
    Base* p1 = &a[1];           // ok
    Base* p2 = p1 + 1;          // UB; p1 points to Derived
    Base* p3 = p1 - 1;          // likewise
    Base* p4 = &a[2];           // ok
    auto p5 = p4 - p1;          // UB; p4 and p1 point to Derived
    const Derived* p6 = &a[1];
    const Derived* p7 = p6 + 1; // ok; cv-qualifiers don't matter