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POPopulate An Array Using Constexpr at Compile-time
 

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  1. POPopulate An Array Using Constexpr at Compile-time
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    <p>I would like to populate an array of enum using constexpr. The content of the array follows a certain pattern.</p> <p>I have an enum separating ASCII character set into four categories.</p> <pre><code>enum Type { Alphabet, Number, Symbol, Other, }; constexpr Type table[128] = /* blah blah */; </code></pre> <p>I would like to have an array of 128 <code>Type</code>. They can be in a structure. The index of the array will be corresponding to the ASCII characters and the value will be the <code>Type</code> of each character.</p> <p>So I can query this array to find out which category an ASCII character belongs to. Something like</p> <pre><code>char c = RandomFunction(); if (table[c] == Alphabet) DoSomething(); </code></pre> <p>I would like to know if this is possible without some lengthy macro hacks.</p> <p>Currently, I initialize the table by doing the following.</p> <pre><code>constexpr bool IsAlphabet (char c) { return ((c &gt;= 0x41 &amp;&amp; c &lt;= 0x5A) || (c &gt;= 0x61 &amp;&amp; c &lt;= 0x7A)); } constexpr bool IsNumber (char c) { /* blah blah */ } constexpr bool IsSymbol (char c) { /* blah blah */ } constexpr Type whichCategory (char c) { /* blah blah */ } constexpr Type table[128] = { INITIALIZE }; </code></pre> <p>where <code>INITIALIZE</code> is the entry point of some very lengthy macro hacks. Something like</p> <pre><code>#define INITIALIZE INIT(0) #define INIT(N) INIT_##N #define INIT_0 whichCategory(0), INIT_1 #define INIT_1 whichCategory(1), INIT_2 //... #define INIT_127 whichCategory(127) </code></pre> <p>I would like a way to populate this array or a structure containing the array without the need for this macro hack...</p> <p>Maybe something like</p> <pre><code>struct Table { Type _[128]; }; constexpr Table table = MagicFunction(); </code></pre> <p>So, the question is how to write this <code>MagicFunction</code>?</p> <p><strong>Note:</strong> I am aware of cctype and likes, this question is more of a <code>Is this possible?</code> rather than <code>Is this the best way to do it?</code>.</p> <p>Any help would be appreciated.</p> <p>Thanks,</p>
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    1. USJimmy Lu
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    1. COYou do know that ASCII only ranges `[0 .. 127]`? And that `char`'s signedness is implementation defined? Your current approach is very dangerous. Oh, and last but not least, the C++ standard doesn't demand ASCII encoding at all. It might aswell be EBCDIC.
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    2. COThe good news is that because arrays can be initialized with pack expansions, what you ask for is indeed feasible. You just need to invoke the function plenty of times :p
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      1. USMatthieu M.
    3. COMost likely not possible because C++ doesn't require ASCII representation for characters. Also, in the strictest sense, ASCII character set comprises of only 128 characters.
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      1. USHappy Green Kid Naps
 

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