Element count of an array in C

Question

Let s say I have an array arr  When would the following not give the number of elements of the array  sizeof arr    sizeof arr 0     I can thing of only one case  the array contains elements that are of different derived types of the type of the array   Am I right and are there  I am almost positive there must be  other such cases   Sorry for the trivial question  I am a Java dev and I am rather new to C     Thanks

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countof my array  in MSVC     I can thing of only one case  the array contains elements that are of different derived types of the type of the array    Elements of an array in C   are objects  not pointers  so you cannot have derived type object as an element   And like mentioned above  sizeof my array   like  countof   as well  will work just in the scope of array definition

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No that would still produce the right value because you must define the array to be either all elements of a single type or pointers to a type   In either case the array size is known at compile time so sizeof arr    sizeof arr 0   always returns the element count   Here is an example of how to use this correctly   int nonDynamicArray  4      define nonDynamicArrayElementCount   sizeof nonDynamicArray    sizeof nonDynamicArray  0        I ll go one further here to show when to use this properly   You won t use it very often   It is primarily useful when you want to define an array specifically so you can add elements to it without changing a lot of code later   It is a construct that is primarily useful for maintenance   The canonical example  when I think about it anyway  -  is building a table of commands for some program that you intend to add more commands to later   In this example to maintain improve your program all you need to do is add another command to the array and then add the command handler   char         commands            lt --- note intentional lack of explicit array size      open        close        abort        crash       define kCommandsCount    sizeof commands    sizeof commands  0       void processCommand  char  command         int i       for   i   0  i  lt  kCommandsCount    i                if command    commands  i   do something  be sure to compare full string

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Since C  17 you can also use the standardized free function  std  size container  which will return the amount of elements in that container  example  std  vector lt int gt  vec     1  2  3  4  8    std  cout  lt  lt  std  size vec   lt  lt   quot  n n quot         5  int A      40 10 20   std  cout  lt  lt  std  size A   lt  lt    n           3

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Arrays in C   are very different from those in Java in that they are completely unmanaged  The compiler or run-time have no idea whatsoever what size the array is   The information is only known at compile-time if the size is defined in the declaration   char array 256     In this case  sizeof array  gives you the proper size   If you use a pointer as an array however  the  array  will just be a pointer  and sizeof will not give you any information about the actual size of the array   STL offers a lot of templates that allow you to have arrays  some of them with size information  some of them with variable sizes  and most of them with good accessors and bounds checking

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Use the Microsoft   countof array   Macro  This link to the Microsoft Developer Network explains it and offers an example that demonstrates the difference between  sizeof array   and the   countof array   macro   Microsoft and the   countof array   Macro

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Let s say I have an array arr  When   would the following not give the   number of elements of the array    sizeof arr    sizeof arr 0      One thing I ve often seen new programmers doing this   void f Sample  arr       int count   sizeof arr  sizeof arr 0      what would be count  10     Sample arr 10   f arr     So new programmers think the value of count will be 10  But that s wrong    Even this is wrong   void g Sample arr      even more deceptive form       int count   sizeof arr  sizeof arr 0      count would not be 10       It s all because once you pass an array to any of these functions  it becomes pointer type  and so sizeof arr  would give the size of pointer  not array     EDIT   The following is an elegant way you can pass an array to a function  without letting it to decay into pointer type   template lt size t N gt  void h Sample   amp arr  N         size t count   N    N is 10  so would be count        you can even do this now        size t count   sizeof arr  sizeof arr 0     it ll return 10    Sample arr 10   h arr     pass   same as before

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There are no cases where  given an array arr  that the value of sizeof arr    sizeof arr 0   is not the count of elements  by the definition of array and sizeof   In fact  it s even directly mentioned    5 3 3 2            When applied to an array  the result is the total number of bytes in the array  This implies that the size of an array of n elements is n times the size of an element    Emphasis mine  Divide by the size of an element  sizeof arr 0    to obtain n

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I know is old topic but what about simple solution like while loop   int function count array           int i   0       while array i     NULL             i                return i       I know that is slower than sizeof   but this is another example of array count

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It seems that if you know the type of elements in the array you can also use that to your advantage with sizeof   int numList       0  1  2  3  4     cout  lt  lt  sizeof numList    sizeof int         gt  5

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First off  you can circumvent that problem by using std  vector instead of an array  Second  if you put objects of a derived class into an array of a super class  you will experience slicing  but the good news is  your formula will work  Polymorphic collections in C   are achieved using pointers  There are three major options here    normal pointers a collection of boost  shared ptr a Boost Pointer Container

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Let s say I have an array arr  When would the following not give the number of elements of the array  sizeof arr    sizeof arr 0      In contexts where arr is not actually the array  but instead a pointer to the initial element   Other answers explain how this happens      I can thing of only one case  the array contains elements that are of different derived types of the type of the array    This cannot happen  for  fundamentally  the same reason that Java arrays don t play nicely with generics   The array is statically typed  it reserves  slots  of memory that are sized for a specific type  the base type       Sorry for the trivial question  I am a Java dev and I am rather new to C      C   arrays are not first-class objects  You can use boost  array to make them behave more like Java arrays  but keep in mind that you will still have value semantics rather than reference semantics  just like with everything else   In particular  this means that you cannot really declare a variable of type analogous to Foo   in Java  nor replace an array with another one of a different size  the array size is a part of the type   Use  size   with this class where you would use  length in Java   It also supplies iterators that provide the usual interface for C   iterators

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