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| <a name="Declare-and-Define-Static-Members"></a> |
| <h4 class="subsection">11.7.1 Declare <em>and</em> Define Static Members</h4> |
| |
| <a name="index-C_002b_002b-static-data_002c-declaring-and-defining"></a> |
| <a name="index-static-data-in-C_002b_002b_002c-declaring-and-defining"></a> |
| <a name="index-declaring-static-data-in-C_002b_002b"></a> |
| <a name="index-defining-static-data-in-C_002b_002b"></a> |
| <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 |
| { |
| … |
| 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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