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<h3 class="section">7.4 #pragma interface and implementation</h3>

<p><a name="index-interface-and-implementation-headers_002c-C_002b_002b-3170"></a><a name="index-C_002b_002b-interface-and-implementation-headers-3171"></a><a name="index-pragmas_002c-interface-and-implementation-3172"></a>
<code>#pragma interface</code> and <code>#pragma implementation</code> provide the
user with a way of explicitly directing the compiler to emit entities
with vague linkage (and debugging information) in a particular
translation unit.

 <p><em>Note:</em> As of GCC 2.7.2, these <code>#pragma</code>s are not useful in
most cases, because of COMDAT support and the &ldquo;key method&rdquo; heuristic
mentioned in <a href="Vague-Linkage.html#Vague-Linkage">Vague Linkage</a>.  Using them can actually cause your
program to grow due to unnecessary out-of-line copies of inline
functions.  Currently (3.4) the only benefit of these
<code>#pragma</code>s is reduced duplication of debugging information, and
that should be addressed soon on DWARF 2 targets with the use of
COMDAT groups.

     <dl>
<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-3173"></a>Use this directive in <em>header files</em> that define object classes, to save
space in most of the object files that use those classes.  Normally,
local copies of certain information (backup copies of inline member
functions, debugging information, and the internal tables that implement
virtual functions) must be kept in each object file that includes class
definitions.  You can use this pragma to avoid such duplication.  When a
header file containing &lsquo;<samp><span class="samp">#pragma interface</span></samp>&rsquo; is included in a
compilation, this auxiliary information will not be generated (unless
the main input source file itself uses &lsquo;<samp><span class="samp">#pragma implementation</span></samp>&rsquo;). 
Instead, the object files will contain references to be resolved at link
time.

     <p>The second form of this directive is useful for the case where you have
multiple headers with the same name in different directories.  If you
use this form, you must specify the same string to &lsquo;<samp><span class="samp">#pragma
implementation</span></samp>&rsquo;.

     <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-3174"></a>Use this pragma in a <em>main input file</em>, when you want full output from
included header files to be generated (and made globally visible).  The
included header file, in turn, should use &lsquo;<samp><span class="samp">#pragma interface</span></samp>&rsquo;. 
Backup copies of inline member functions, debugging information, and the
internal tables used to implement virtual functions are all generated in
implementation files.

     <p><a name="index-implied-_0040code_007b_0023pragma-implementation_007d-3175"></a><a name="index-g_t_0040code_007b_0023pragma-implementation_007d_002c-implied-3176"></a><a name="index-naming-convention_002c-implementation-headers-3177"></a>If you use &lsquo;<samp><span class="samp">#pragma implementation</span></samp>&rsquo; with no argument, it applies to
an include file with the same basename<a rel="footnote" href="#fn-1" name="fnd-1"><sup>1</sup></a> as your source
file.  For example, in <samp><span class="file">allclass.cc</span></samp>, giving just
&lsquo;<samp><span class="samp">#pragma implementation</span></samp>&rsquo;
by itself is equivalent to &lsquo;<samp><span class="samp">#pragma implementation "allclass.h"</span></samp>&rsquo;.

     <p>In versions of GNU C++ prior to 2.6.0 <samp><span class="file">allclass.h</span></samp> was treated as
an implementation file whenever you would include it from
<samp><span class="file">allclass.cc</span></samp> even if you never specified &lsquo;<samp><span class="samp">#pragma
implementation</span></samp>&rsquo;.  This was deemed to be more trouble than it was worth,
however, and disabled.

     <p>Use the string argument if you want a single implementation file to
include code from multiple header files.  (You must also use
&lsquo;<samp><span class="samp">#include</span></samp>&rsquo; to include the header file; &lsquo;<samp><span class="samp">#pragma
implementation</span></samp>&rsquo; only specifies how to use the file&mdash;it doesn't actually
include it.)

     <p>There is no way to split up the contents of a single header file into
multiple implementation files. 
</dl>

 <p><a name="index-inlining-and-C_002b_002b-pragmas-3178"></a><a name="index-C_002b_002b-pragmas_002c-effect-on-inlining-3179"></a><a name="index-pragmas-in-C_002b_002b_002c-effect-on-inlining-3180"></a>&lsquo;<samp><span class="samp">#pragma implementation</span></samp>&rsquo; and &lsquo;<samp><span class="samp">#pragma interface</span></samp>&rsquo; also have an
effect on function inlining.

 <p>If you define a class in a header file marked with &lsquo;<samp><span class="samp">#pragma
interface</span></samp>&rsquo;, the effect on an inline function defined in that class is
similar to an explicit <code>extern</code> declaration&mdash;the compiler emits
no code at all to define an independent version of the function.  Its
definition is used only for inlining with its callers.

 <p><a name="index-fno_002dimplement_002dinlines-3181"></a>Conversely, when you include the same header file in a main source file
that declares it as &lsquo;<samp><span class="samp">#pragma implementation</span></samp>&rsquo;, the compiler emits
code for the function itself; this defines a version of the function
that can be found via pointers (or by callers compiled without
inlining).  If all calls to the function can be inlined, you can avoid
emitting the function by compiling with <samp><span class="option">-fno-implement-inlines</span></samp>. 
If any calls were not inlined, you will get linker errors.

 <div class="footnote">
<hr>
<h4>Footnotes</h4><p class="footnote"><small>[<a name="fn-1" href="#fnd-1">1</a>]</small> A file's <dfn>basename</dfn>
was the name stripped of all leading path information and of trailing
suffixes, such as &lsquo;<samp><span class="samp">.h</span></samp>&rsquo; or &lsquo;<samp><span class="samp">.C</span></samp>&rsquo; or &lsquo;<samp><span class="samp">.cc</span></samp>&rsquo;.</p>

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