Daily Archives: November 25, 2017

Understanding the difference between recognized arrays and pointers 'Recognized' means sizeof(array_name) gives the underlying allocated size

Posted on by

array≠ pointer:

a pointer only contains a memory location,
while an array already has memory allocated to hold the data.

The confusion comes from the fact that array names are always seen as pointers anywhere in C, but when an array name is referred in places that the scope happens know the allocated size, namely

  • Global arrays: everybody knows the size
  • Local arrays: only the instantiating function knows its size.

, the array name itself has a superpower that pointers lack: report the underlying allocated data size (NOT pointer size) using sizeof(array).

Definition: An array is ‘recognized‘ if the array name is used in the scope that knows the underlying data size.

Corollary: Calling the array name with sizeof() gives the underlying allocated data size.

Examples of consequences that can be derived from the definition above:

  • Heap allocations always return a pointer type, NOT an array name!
    So heap arrays are never recognized.
  • VLA in C99 are considered local stack arrays, so it’s recognized
  • x[] is just a cosmetic shorthand for *x: it doesn’t prevent any recognized array from decaying into a pointer across boundary.
  • The storage duration (static or not) does not matter. e.g.
    • Heap pointers at global level are not recognized arrays
    • Static local array still loses the recognition across function boundaries
      (unless passed carefully by data type T (&array)[N]).

Most often recognized arrays cannot be aliased without decaying into a pointer. However, we can bind a recognized array to a reference to an array, which is a completely different type. Example:

int v[]{1,2,3,4};
int (&w)[4]=v;  // w is a reference to an array of size 4

int* p = v;     // Decays v to a pointer. Size information lost.
// int &w[4]=v; // Does not compile: this means an array of 4 references.

Note that the syntax requires a bracket for reference name. Omitting it will lead the compiler to misinterpret it as an array of references, which cannot* be compiled.

This means contrary to common beliefs, you can pass a recognized array across functions through reference, but this is rarely done because of the hassle of explicitly entering the number of elements (4 for the example above) as part of the data type. This can still be done through templates/constexpr, but for such inconvenience, we’re better off using std::vector (or std::array if you want near zero overhead).

However, so far I haven’t found a way to re-recognize an array from a pointer. That means there is no way to keep a local array’s recognition across function boundaries in C since it does not have references like C++.

To summarize with a usage example: this post has described the entire logic needed to decide whether sizeof(x)/sizeof(x[0]) gives you the number of array elements, or how many times your machine pointer type is bigger than the element storage.

* references must be bound on creation. Declaring an array of references means you want to bound references in batches. There are no mechanisms to do so as of C++14.

 

Loading