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<h3 class="section">7.12 Backwards Compatibility</h3>
<a name="index-Backwards-Compatibility"></a>
<a name="index-ARM-_005bAnnotated-C_002b_002b-Reference-Manual_005d"></a>

<p>Now that there is a definitive ISO standard C++, G++ has a specification
to adhere to.  The C++ language evolved over time, and features that
used to be acceptable in previous drafts of the standard, such as the ARM
[Annotated C++ Reference Manual], are no longer accepted.  In order to allow
compilation of C++ written to such drafts, G++ contains some backwards
compatibilities.  <em>All such backwards compatibility features are
liable to disappear in future versions of G++.</em> They should be considered
deprecated.   See <a href="Deprecated-Features.html#Deprecated-Features">Deprecated Features</a>.
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<dt><code>Implicit C language</code></dt>
<dd><p>Old C system header files did not contain an <code>extern &quot;C&quot; {&hellip;}</code>
scope to set the language.  On such systems, all system header files are
implicitly scoped inside a C language scope.  Such headers must
correctly prototype function argument types, there is no leeway for
<code>()</code> to indicate an unspecified set of arguments.
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