noahsw@cyberdude.com (Noah Spitzer-Williams) wrote in message news:<3dc5ea8a.0309231810.2a89c55b@posting.google. com>...
[color=blue]
> I would like to do something seemingly simple: find out if an
> element in an array that is passed to my function exists.[/color]
The general rule is that, if you don't already know the size of the
array, you cannot determine the size of the array. The rule works as
follows:
std::vector (the standard C++ resizeable array) and boost::array (a
third-party fixed-size array class, available from boost.org) both
know their own size and can supply it on request:
#include <vector>
template <typename T>
std::size_t size (const std::vector<T> &v) { return v.size(); }
#include <boost/array.hpp>
template <typename T, std::size_t N>
std::size_t size (const boost::array<T, N> &v) { return v.size(); }
A named array also knows its own size, but a function can only
determine that size if the array is passed by reference:
template <typename T, std::size_t N>
std::size_t size (T (&v) [N]) { return N; }
template <typename T, std::size_t N>
std::size_t size (const T (&v) [N]) { return N; }
Besides that (or the rare pointer-to-array), arrays are passed by
pointer to the first element, never by value. That means that the
following prototypes are identical:
void f (int *array);
void g (int array[]);
void h (int array[20]);
(Note that all three functions accept any array of ints, be it int[50]
or int[5]. The "20" is misleading, allowed but ignored by C++. Don't
put it in your code.)
Since a pointer does not maintain the size of the array into which it
points (if it points into one at all), you must pass that in yourself:
template <typename T> void f (const T *v, std::size_t n);
int main () {
int array[20];
f(array, size(array)); // use the by-reference size() function
}
Also, since pointers do not maintain sizes, it is impossible to
calculate the size of an array allocated with new[] or similar. The
solution is to use std::vector instead, or to maintain the size
yourself. (I go with std::vector in almost all cases.)
[color=blue]
> I used to think I could just do: if (arr[i]) ...
> However, if this element's value happens to be 0, the conditional
> will return false, even though the element actually exists.[/color]
All elements of an array exist.
[color=blue]
> Any ideas? Can you check if the element == null?[/color]
Not unless that element is a pointer. But that's not what you meant.
:)
- Shane
