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POWhat are the expression syntax over types C++ support?
 

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  1. POWhat are the expression syntax over types C++ support?
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    <p>I was working with a templated class which takes a set of integers. The code was like,</p> <pre><code>template&lt;unsigned... Idx&gt; struct work{ ... }; </code></pre> <p>Then I realized, user may need to provide either a set of integers, or a range of integers. So, I changed the syntax little to support instantiation like,</p> <pre><code>work&lt;indexes&lt;1,3,2,4&gt; &gt; //instead of work&lt;1,3,2,4&gt; work&lt;span&lt;1,4&gt; &gt; //same as work&lt;1,2,3,4&gt; </code></pre> <p>While, in C++ we have large number of operators, and can be used to formulate exotic expression templates (say <code>boost::xpressive</code>, <code>boost::lambda</code>, <code>boost::spirit</code> etc) , possibilities for type manipulation is much less.</p> <p>In a boostcon keynote by Sean Parent, he noted one still can not write <code>pair&lt;int&gt;</code> to denote <code>a pair of integers</code>. In my persinal library, I made a syntax like <code>tuple&lt;int[3]&gt;</code> to denote a tuple of 3 integers, instead of writing a tuple with 3 int in the type arguments, noting that I do not write a raw array as tuple argument anywhere! (note: <code>std::array&lt;int,3&gt;</code> is not same as the above, as std::array can not store references while <code>tuple</code> can, say <code>std::tuple&lt;int&amp;,int&amp;,int&amp;&gt;</code> is possible)</p> <p>So, I want to know what are the different kind of "type expressions" I can write? </p> <p>So far I can think of object type, function type, reference type, with/without cv modifiers, pointers etc. e.g</p> <pre><code> template&lt;class T&gt; struct tpl; using t1 = tpl&lt;int&gt;;//simple type //function (can have function pointer/reference also) // e.g. void(*)(int,float) or void(&amp;)(int,float) using t2 = tpl&lt;void(int,float)&gt;; //array can have pointer / reference also //e.g. int(&amp;)[4] or int (*)[4] using t3 = tpl&lt;int[4]&gt;; using t4 = tpl&lt;int[]&gt;; using t5 = tpl&lt;int const&gt;;//with cv modifiers using t6 = tpl&lt;int*&gt;;//with pointer using t7 = tpl&lt;int&amp;&gt;;//with reference (&amp; or &amp;&amp;) using t8 = tpl&lt;tpl&lt;int&gt; &gt;; //template itself using t9 = tpl&lt;void(...)&gt;; //variadic functions using t10 = tpl&lt;R C::*&gt;; //pointer to member </code></pre> <p>But I believe, many more are possible.</p> <p>NOTE: This question is purely theoretical, <strong>I just want to know all kinds of syntax I can write inside &lt;> as type argument</strong>, and not about the readability/morality aspect of it, or even how can I implement some of the examples I had given, like the work class. </p>
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    1. COThere is an infinity of answers to this question, for example the infinite series `char, char*, char**, ...`. It cannot therefore be answered in a finite way.
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    2. COYes, yes, I know! But if they are regular, I can still express them as `a type followed by zero or more *`, or any other way, as long as it expresses the intent.
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    3. COLook for `type_id` in the standard.
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