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POC++ Constant preprocessor macros and templates
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  1. POC++ Constant preprocessor macros and templates
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    copied!<p>So say I have the following very simple macro, along with a bit of code to output it:</p> <pre><code>#define SIMPLEHASH(STRING) STRING[1] + STRING[2] + STRING[3] std::cout &lt;&lt; SIMPLEHASH("Blah"); </code></pre> <p>This outputs 309, and if you view the assembly you can see:</p> <pre><code>00131094 mov ecx,dword ptr [__imp_std::cout (132050h)] 0013109A push 135h 0013109F call dword ptr [__imp_std::basic_ostream&lt;char,std::char_traits&lt;char&gt; &gt;::operator&lt;&lt; (13203Ch)] </code></pre> <p>Where <code>135h</code> translates nicely into 309 decimal. Everything was compiled away to a constant.</p> <hr> <p>Now, say you have a template class as such:</p> <pre><code>template&lt;int X&gt; class Printer{ public: void Print(){ std::cout &lt;&lt; X; } }; </code></pre> <p>Then the following will nicely print the number 32:</p> <pre><code>Printer&lt;32&gt; p; p.Print(); </code></pre> <hr> <p>Both these things work individually, the problem comes when you try to combine them:</p> <pre><code>#define SIMPLEHASH(STRING) STRING[1] + STRING[2] + STRING[3] Printer&lt;SIMPLEHASH("Blah")&gt; p; p.Print(); </code></pre> <p>In visual studio this gives:</p> <blockquote> <p>1>.\ShiftCompare.cpp(187) : error C2975: 'X' : invalid template argument for 'Printer', expected compile-time constant expression<br> 1> .\ShiftCompare.cpp(127) : see declaration of 'X'</p> </blockquote> <p>Despite the fact that <code>SIMPLEHASH("Blah")</code> can be reduced to a constant at compile time, as seen in the first example.</p> <p>So, what gives, is there a way to tell the compiler "evaluate this first"? are templates naturally "before" macros in preprocessor evaluation?</p> <p>Does anyone see any way I can get these two to work together?</p>
 

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