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POImplementing a map_keys_iterator by derivation: a single compiler error
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  1. POImplementing a map_keys_iterator by derivation: a single compiler error
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    copied!<p>I was working again with C++ during the weekend and came to notice something that I'm not sure where does it come from.</p> <p><a href="https://stackoverflow.com/questions/1443793/iterate-keys-in-a-c-map">Following the advice in this thread</a>, I decided to implement a <code>map_keys_iterator</code> and <code>map_values_iterator</code>. I took the -- I think -- recommended-against approach of deriving a class from <code>std::map&lt;K,V&gt;::iterator</code> and implementing it as such:</p> <pre><code>template &lt;typename K, typename V&gt; struct map_values_iterator: public std::map&lt;K,V&gt;::iterator { // explicitly call base's constructor typedef typename std::map&lt;K,V&gt;::iterator mIterator; map_values_iterator (const mIterator&amp; mi) : mIterator(mi) {}; const V&amp; operator* () const { return (*this)-&gt;second; } }; </code></pre> <p>So far, so good, and the following code works (nvm the Unicode, I default to work with i18n-capable terminals):</p> <pre><code>typedef std::map&lt;double,string&gt; Map; Map constants; constants[M_PI] = "π"; constants[(1+sqrt(5))/2] = "φ"; constants[exp(M_PI)-M_PI] = "fake_20"; // ... fill map with more constants! map_values_iterator&lt;double, std::string&gt; vs(constants.begin()); for (; vs != m.end(); ++vs) { cout&lt;&lt; (vs != m.begin() ? ", " : "")&lt;&lt; *vs; } cout&lt;&lt; endl; </code></pre> <p>This code prints the expected result, something like (because a Map's elements are ordered):</p> <pre><code>..., φ, ..., π, ...., fake_20, .... </code></pre> <p>So I'd guess a <code>map_keys_iterator</code> would work in a similar way as well. I took the care that a Map's value_type is actually <code>pair&lt;const K, V&gt;</code> so the <em>keys</em> version will return a value.</p> <p>However, it is unwieldly to have to declare the iterator's type so I wanted to create a caller with the classical <code>make_pair</code>-like idiom. And this is where trouble begins:</p> <pre><code>template &lt;typename K, typename V&gt; map_values_iterator&lt;K,V&gt; map_values(const typename std::map&lt;K,V&gt;::iterator &amp;i) { return lpp::map_values_iterator&lt;K,V&gt; (i); } template &lt;typename K, typename V&gt; map_values_iterator&lt;K,V&gt; map_values(const typename std::map&lt;K,V&gt;::const_iterator &amp;i) { return lpp::map_values_iterator&lt;K,V&gt; (i); } </code></pre> <p>I'm relatively sure this function has the right signature and constructor invocation. However if I attempt to call the function from code:</p> <pre><code>auto vs= map_values(constants.begin()); </code></pre> <p>I get a single STL compiler error of the form:</p> <pre><code>error: no matching function for call to ‘map_values(std::_Rb_tree_iterator&lt;std::pair&lt;const double, std::basic_string&lt;char, std::char_traits&lt;char&gt;, std::allocator&lt;char&gt; &gt; &gt; &gt;)’ </code></pre> <p>I'm assuming here that in this particular case the whole <code>_Rb_tree_iterator</code> is actually the correct iterator for which <code>map::iterator</code> is typedefed; I'm not completely sure however. I've tried to provide more overloads to see if one of them matches (drop the reference, drop the const, use only non-const_iterator variants, etc) but so far nothing allows the signature I'm interested in.</p> <p>If I store the base iterator in a variable before calling the function (as in <code>auto begin= constans.begin(); auto vs= map_values(begin);</code>) I get the exact same base error, only the description of the unmatched call is obviously different (in that it is a "const blah&amp;").</p> <p>My first attempt at implementing this sort of iterator was by creating a base class that aggregated <code>map::iterator</code> instead of inheriting, and deriving two classes, each with the adequate <code>operator*</code>, but that version ran into many more problems than the above and still forced me to replicate too much of the interface. So I tried this option for code-expedition.</p> <p>I've tried to look for answers to this issue but my Google-fu isn't very strong today. Maybe I am missing something obvious, maybe I forgot something with the derivation (although I'm almost sure I didn't -- iterators are unlike containers), maybe I am actually required to specify all the template parameters for the map, or maybe my compiler is broken, but whatever it is I can't find it and I am having real trouble understanding what is the <em>actual</em> thing the compiler is complaining about here. In my previous experience, if you are doing something wrong with the STL you are supposed to see a diarrhoea of errors, not only <strong>one</strong> (which isn't STL to boot).</p> <p>So... any (well-encapsulated) pointers would be appreciated.</p>
 

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