What's the meaning of #define when there is only 2 part?
<p>I know the following 3 parts <code>#define</code>:</p>

<pre><code>#define PI 3.4
</code></pre>

<p>which mean it will replace <code>PI</code> with <code>3.4</code>.</p>

<p>But that's the meaning of 2 parts <code>#define</code> like this:</p>

<pre><code>#define something
</code></pre>

<p>Will it replace <code>something</code> with null/empty string?</p>

<p>The following is the code example, I searched the file, only list the related lines</p>

<pre><code>  D:\mariadb\storage\pbxt\src\cache_xt.cc (23 hits)  
    Line 172: #ifdef xtPublic  
    Line 173: #undef xtPublic  
    Line 188: #define xtPublic  
    Line 325: xtPublic XTIndHandlePtr xt_ind_get_handle(..)  
    Line 378: xtPublic void xt_ind_release_handle(XTIndHandlePtr..)  
    Line 516: xtPublic xtBool xt_ind_copy_on_write(XTIndReferencePtr iref)  
    Line 597: xtPublic void xt_ind_lock_handle(XTIndHandlePtr handle) 
</code></pre>
