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Expressions that have an integral type can be added to or subtracted from a pointer, resulting in a value of the pointer type. If the resulting pointer is not a valid member of the container, or one past the last element of the container, the behavior of the additive operator is undefined. The C++ Standard, [expr.add], paragraph 5 [ISO/IEC 14882-2014], states, in part in part, states the following:

If both the pointer operand and the result point to elements of the same array object, or one past the last element of the array object, the evaluation shall not produce an overflow; otherwise, the behavior is undefined.

Because iterators are a generalization of pointers, the same constraints apply to additive operators with random access iterators. Specifically, the C++ Standard, [iterator.requirements.general], paragraph 5, states the following:

Just as a regular pointer to an array guarantees that there is a pointer value pointing past the last element of the array, so for any iterator type there is an iterator value that points past the last element of a corresponding sequence. These values are called past-the-end values. Values of an iterator i for which the expression *i is defined are called dereferenceable. The library never assumes that past-the-end values are dereferenceable.

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In this noncompliant code example, a random access iterator from a std::vector is used in an additive expression, but the resulting value could be outside the bounds of the container rather than a past-the-end value:.

#include <iostream>
#include <vector>
 
void f(const std::vector<int> &c) {
  for (auto i = c.begin(), e = i + 20; i != e; ++i) {
    std::cout << *i << std::endl;
  }
}

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If adding or subtracting an integer to a pointer results in a reference to an element outside the array or one past the last element of the array object, the behavior is undefined but frequently leads to a buffer overflow or buffer underrun, which can often be exploited to run arbitrary code. Iterators and standard template library containers exhibit the same behavior and caveats as pointers and arrays.

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