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(gcc-300.info)Static Definitions

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Declare _and_ Define Static Members
-----------------------------------

   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 ISO 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.