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<a name="Declare-and-Define-Static-Members"></a>
<h4 class="subsection">14.7.1 Declare <em>and</em> Define Static Members</h4>

<a name="index-C_002b_002b-static-data_002c-declaring-and-defining"></a>
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<p>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:
</p>
<div class="smallexample">
<pre class="smallexample">class Foo
{
  &hellip;
  void method();
  static int bar;
};
</pre></div>

<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:
</p>
<div class="smallexample">
<pre class="smallexample">int Foo::bar = 0;
</pre></div>

<p>Other C++ compilers may not correctly implement the standard behavior.
As a result, when you switch to <code>g++</code> from one of these compilers,
you may discover that a program that appeared to work correctly in fact
does not conform to the standard: <code>g++</code> reports as undefined
symbols any static data members that lack definitions.
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