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<h4 class="subsection">11.8.1 Declare <em>and</em> Define Static Members</h4>

<p><a name="index-C_002b_002b-static-data_002c-declaring-and-defining-3233"></a><a name="index-static-data-in-C_002b_002b_002c-declaring-and-defining-3234"></a><a name="index-declaring-static-data-in-C_002b_002b-3235"></a><a name="index-defining-static-data-in-C_002b_002b-3236"></a>When a class has static data members, it is not enough to <em>declare</em>
the static member; you must also <em>define</em> it.  For example:

<pre class="smallexample">     class Foo
     {
       ...
       void method();
       static int bar;
     };
</pre>
 <p>This declaration only establishes that the class <code>Foo</code> has an
<code>int</code> named <code>Foo::bar</code>, and a member function named
<code>Foo::method</code>.  But you still need to define <em>both</em>
<code>method</code> and <code>bar</code> elsewhere.  According to the ISO
standard, you must supply an initializer in one (and only one) source
file, such as:

<pre class="smallexample">     int Foo::bar = 0;
</pre>
 <p>Other C++ compilers may not correctly implement the standard behavior. 
As a result, when you switch to <samp><span class="command">g++</span></samp> from one of these compilers,
you may discover that a program that appeared to work correctly in fact
does not conform to the standard: <samp><span class="command">g++</span></samp> reports as undefined
symbols any static data members that lack definitions.

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