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POFunctional-Programming Style in C++ Macros: Is this documented anywhere?
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  1. POFunctional-Programming Style in C++ Macros: Is this documented anywhere?
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    copied!<p>Reading some C++ code I came across what I'll call a "functional" use of function Macros roughly as follows (this is a totally stylized example to make the point):</p> <pre><code>#define TOP_LEVEL(ARG1) \ ARG1("foo1","bar1") \ ARG1("foo2","bar2") #define NEXT_LEVEL(ARG2A, ARG2B) \ cout &lt;&lt; ARG2A &lt;&lt; " and " &lt;&lt; ARG2B; TOP_LEVEL(NEXT_LEVEL) </code></pre> <p>I'm relatively new to the language and at first I couldn't figure this out, but then I ran it through just the preprocessor (<code>g++ -E</code>) and lo and behold it resolves to:</p> <pre><code>cout &lt;&lt; "foo1" &lt;&lt; " and " &lt;&lt; "bar1"; cout &lt;&lt; "foo2" &lt;&lt; " and " &lt;&lt; "bar2"; </code></pre> <p>Do you see what it did there? It passed the Macro NEXT_LEVEL <strong>like a function pointer</strong> to the Macro TOP_LEVEL. Seeing how useful this could potentially be, I wanted to learn more about it: passing around functions to other functions is <a href="http://en.wikipedia.org/wiki/First-class_function" rel="nofollow">pretty sophisticated stuff</a> and there must be at least something more to say about the technique. </p> <p>Yet despite a ton of Googling I can't find evidence that this feature of the preprocessor even exists, let alone anything approaching documentation: <a href="http://www.cplusplus.com/doc/tutorial/preprocessor/" rel="nofollow">here</a>, <a href="http://gcc.gnu.org/onlinedocs/cpp/Macros.html" rel="nofollow">here</a>, <a href="http://publib.boulder.ibm.com/infocenter/macxhelp/v6v81/index.jsp?topic=%2Fcom.ibm.vacpp6m.doc%2Flanguage%2Fref%2Fclrc09cpxmac.htm" rel="nofollow">here</a> and <a href="http://www.cprogramming.com/tutorial/cpreprocessor.html" rel="nofollow">here</a> are just four examples of Macro tutorials that skip right past this; the last even has a section called "Advanced Macro tricks" - surely this qualifies!?</p> <p>(Please note this is totally different than simply calling a function macro with another <strong>evaluated</strong> function macro as an argument- FOO(BAR(2)) is much more straightforward.) </p> <p>My questions are:</p> <ul> <li>Is there an actual name for this behavior?</li> <li>Is it documented anywhere?</li> <li>It is commonly used, or are there well known pitfalls, etc.?</li> </ul>
 

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