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<title>C++ Interface - Using the GNU Compiler Collection (GCC)</title>
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<link rel="up" href="C_002b_002b-Extensions.html#C_002b_002b-Extensions" title="C++ Extensions">
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(see below). A copy of the license is included in the section entitled
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``GNU Free Documentation License''.
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A GNU Manual
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<a name="C++-Interface"></a>
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<a name="C_002b_002b-Interface"></a>
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<p>
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Next: <a rel="next" accesskey="n" href="Template-Instantiation.html#Template-Instantiation">Template Instantiation</a>,
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Previous: <a rel="previous" accesskey="p" href="Vague-Linkage.html#Vague-Linkage">Vague Linkage</a>,
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Up: <a rel="up" accesskey="u" href="C_002b_002b-Extensions.html#C_002b_002b-Extensions">C++ Extensions</a>
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</div>
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<h3 class="section">7.4 C++ Interface and Implementation Pragmas</h3>
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<p><a name="index-interface-and-implementation-headers_002c-C_002b_002b-4210"></a><a name="index-C_002b_002b-interface-and-implementation-headers-4211"></a><a name="index-pragmas_002c-interface-and-implementation-4212"></a>
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<code>#pragma interface</code> and <code>#pragma implementation</code> provide the
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user with a way of explicitly directing the compiler to emit entities
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with vague linkage (and debugging information) in a particular
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translation unit.
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<p><em>Note:</em> These <code>#pragma</code>s have been superceded as of GCC 2.7.2
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by COMDAT support and the “key method” heuristic
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mentioned in <a href="Vague-Linkage.html#Vague-Linkage">Vague Linkage</a>. Using them can actually cause your
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program to grow due to unnecessary out-of-line copies of inline
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functions.
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<dl>
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<dt><code>#pragma interface</code><dt><code>#pragma interface "</code><var>subdir</var><code>/</code><var>objects</var><code>.h"</code><dd><a name="index-g_t_0023pragma-interface-4213"></a>Use this directive in <em>header files</em> that define object classes, to save
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space in most of the object files that use those classes. Normally,
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local copies of certain information (backup copies of inline member
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functions, debugging information, and the internal tables that implement
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virtual functions) must be kept in each object file that includes class
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definitions. You can use this pragma to avoid such duplication. When a
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header file containing ‘<samp><span class="samp">#pragma interface</span></samp>’ is included in a
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compilation, this auxiliary information is not generated (unless
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the main input source file itself uses ‘<samp><span class="samp">#pragma implementation</span></samp>’).
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Instead, the object files contain references to be resolved at link
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time.
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<p>The second form of this directive is useful for the case where you have
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multiple headers with the same name in different directories. If you
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use this form, you must specify the same string to ‘<samp><span class="samp">#pragma
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implementation</span></samp>’.
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<br><dt><code>#pragma implementation</code><dt><code>#pragma implementation "</code><var>objects</var><code>.h"</code><dd><a name="index-g_t_0023pragma-implementation-4214"></a>Use this pragma in a <em>main input file</em>, when you want full output from
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included header files to be generated (and made globally visible). The
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included header file, in turn, should use ‘<samp><span class="samp">#pragma interface</span></samp>’.
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Backup copies of inline member functions, debugging information, and the
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internal tables used to implement virtual functions are all generated in
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implementation files.
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<p><a name="index-implied-_0040code_007b_0023pragma-implementation_007d-4215"></a><a name="index-g_t_0040code_007b_0023pragma-implementation_007d_002c-implied-4216"></a><a name="index-naming-convention_002c-implementation-headers-4217"></a>If you use ‘<samp><span class="samp">#pragma implementation</span></samp>’ with no argument, it applies to
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an include file with the same basename<a rel="footnote" href="#fn-1" name="fnd-1"><sup>1</sup></a> as your source
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file. For example, in <samp><span class="file">allclass.cc</span></samp>, giving just
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‘<samp><span class="samp">#pragma implementation</span></samp>’
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by itself is equivalent to ‘<samp><span class="samp">#pragma implementation "allclass.h"</span></samp>’.
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<p>Use the string argument if you want a single implementation file to
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include code from multiple header files. (You must also use
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‘<samp><span class="samp">#include</span></samp>’ to include the header file; ‘<samp><span class="samp">#pragma
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implementation</span></samp>’ only specifies how to use the file—it doesn't actually
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include it.)
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<p>There is no way to split up the contents of a single header file into
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multiple implementation files.
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</dl>
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<p><a name="index-inlining-and-C_002b_002b-pragmas-4218"></a><a name="index-C_002b_002b-pragmas_002c-effect-on-inlining-4219"></a><a name="index-pragmas-in-C_002b_002b_002c-effect-on-inlining-4220"></a>‘<samp><span class="samp">#pragma implementation</span></samp>’ and ‘<samp><span class="samp">#pragma interface</span></samp>’ also have an
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effect on function inlining.
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<p>If you define a class in a header file marked with ‘<samp><span class="samp">#pragma
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interface</span></samp>’, the effect on an inline function defined in that class is
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similar to an explicit <code>extern</code> declaration—the compiler emits
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no code at all to define an independent version of the function. Its
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definition is used only for inlining with its callers.
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<p><a name="index-fno_002dimplement_002dinlines-4221"></a>Conversely, when you include the same header file in a main source file
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that declares it as ‘<samp><span class="samp">#pragma implementation</span></samp>’, the compiler emits
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code for the function itself; this defines a version of the function
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that can be found via pointers (or by callers compiled without
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inlining). If all calls to the function can be inlined, you can avoid
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emitting the function by compiling with <samp><span class="option">-fno-implement-inlines</span></samp>.
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If any calls are not inlined, you will get linker errors.
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<div class="footnote">
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<hr>
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<h4>Footnotes</h4><p class="footnote"><small>[<a name="fn-1" href="#fnd-1">1</a>]</small> A file's <dfn>basename</dfn>
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is the name stripped of all leading path information and of trailing
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suffixes, such as ‘<samp><span class="samp">.h</span></samp>’ or ‘<samp><span class="samp">.C</span></samp>’ or ‘<samp><span class="samp">.cc</span></samp>’.</p>
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<hr></div>
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