(How) can I count the items in an enum?

How about traits, in an STL fashion? For instance:

enum Foo
{
    Bar,
    Baz
};

write an

std::numeric_limits<enum Foo>::max()

specialization (possibly constexpr if you use c++11). Then, in your test code provide any static assertions to maintain the constraints that std::numeric_limits::max() = last_item.

For C++, there are various type-safe enum techniques available, and some of those (such as the proposed-but-never-submitted Boost.Enum) include support for getting the size of a enum.

The simplest approach, which works in C as well as C++, is to adopt a convention of declaring a ...MAX value for each of your enum types:

enum Folders { FA, FB, FC, Folders_MAX = FC };
ContainerClass *m_containers[Folders_MAX + 1];
....
m_containers[FA] = ...; // etc.

Edit: Regarding { FA, FB, FC, Folders_MAX = FC} versus {FA, FB, FC, Folders_MAX]: I prefer setting the ...MAX value to the last legal value of the enum for a few reasons:

1. The constant's name is technically more accurate (since Folders_MAX gives the maximum possible enum value).
2. Personally, I feel like Folders_MAX = FC stands out from other entries out a bit more (making it a bit harder to accidentally add enum values without updating the max value, a problem Martin York referenced).
3. GCC includes helpful warnings like "enumeration value not included in switch" for code such as the following. Letting Folders_MAX == FC + 1 breaks those warnings, since you end up with a bunch of ...MAX enumeration values that should never be included in switch.

switch (folder) 
{
  case FA: ...;
  case FB: ...;
  // Oops, forgot FC!
}

Add a entry, at the end of your enum, called Folders_MAX or something similar and use this value when initializing your arrays.

ContainerClass* m_containers[Folders_MAX];

There's not really a good way to do this, usually you see an extra item in the enum, i.e.

enum foobar {foo, bar, baz, quz, FOOBAR_NR_ITEMS};

So then you can do:

int fuz[FOOBAR_NR_ITEMS];

Still not very nice though.

But of course you do realize that just the number of items in an enum is not safe, given e.g.

enum foobar {foo, bar = 5, baz, quz = 20};

the number of items would be 4, but the integer values of the enum values would be way out of the array index range. Using enum values for array indexing is not safe, you should consider other options.

edit: as requested, made the special entry stick out more.