When a class has static data members, it is not enough to declare the static member; you must also define it. For example:
class Foo { ... void method(); static int bar; };
This declaration only establishes that the class Foo
has an
int
named Foo::bar
, and a member function named
Foo::method
. But you still need to define both
method
and bar
elsewhere. According to the draft ANSI
standard, you must supply an initializer in one (and only one) source
file, such as:
int Foo::bar = 0;
Other C++ compilers may not correctly implement the standard behavior.
As a result, when you switch to g++
from one of these compilers,
you may discover that a program that appeared to work correctly in fact
does not conform to the standard: g++
reports as undefined
symbols any static data members that lack definitions.