Static members of a class:

_Static_ has a third meaning inside a class. A static data member of a
class is a data member that belongs to the class, not to an individual
object within the class. For example, in an Order class, a static
data member could be used to represent the total number of orders.

class Order:
{
...
private:
  static int order_count;
...
}

A static function member in a class can only be used to update static
data members.

Since static data members belong to the class rather than to a
specific object, they are referred to as classname::name rather than
as object.name.

For the same reason, a static data member must be initialized globally
rather than in a function updating a particular object, for example:

  int Order::order_count = 0;

A good place to put this initialization is directly above the function
definitions in the Order.cc file.

--

Constant members of a class:

When constants are related to a specific class, they can be defined
inside the class. Constant data members of a class should always be
declared as static, since only one copy of a constant (as opposed to
one per object) is needed. For example:

class Order:
{
...
private:
  static float tax_rate;
...
}

A static const data member must be initialized globally like any other
static data member. In addition, since it is a constant, its value
cannot later be changed. For example:

  const float Order::tax_rate = .0575;

Note that 'const' is included in the declaration and the
initialization, but 'static' is only included in the declaration.

If the constant item is an array, all of the values needed must be
given, as in the following example:

In Order.h:
  static const float tax_rates[3];

In Order.cc:
  const float Order::tax_rates[3] = {.0575, .0675, .0875};