Safe C++

Chapter 4. Index Out of Bounds

There are several ways in C++ to create an array of objects of some type T. Three common methods are:

#define N 10  // array size N is known at compile time
  T static_array[N];

  int n = 20; // array size n is calculated at runtime
  T* dynamic_array = new T[n];

  std::vector<T> vector_array; // array size can be changed at runtime

Of course, you can still use the calloc() and malloc() functions and your program will compile and run, but it’s not a good idea to mix C and C++ unless you have to because you’re relying on legacy C libraries. However you allocate the array, you can access an element in it using an unsigned integer index:

const T& element_of_static_array  = static_array[index];
const T& element_of_dynamic_array = dynamic_array[index];
const T& element_of_vector_array  = vector_array[index];

Let’s deal with dynamic arrays and vectors first, and return to the static array later in this chapter.

Dynamic Arrays

What would happen if we provide an index value that is larger than or equal to the array size? In all three of the preceding examples, the code will silently return garbage. (The exception to this rule for Microsoft Visual Studio 2010 is discussed later.) The situation is even worse if you decide to use the operator [] in the left-hand side of an assignment:

some_array[index] = x;

Depending on your luck (or lack of thereof) you might overwrite some other unrelated variable, an element of another array, or even a program instruction, and in the latter ...