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    <p>The reason is that your <code>K</code> and <code>V</code> type parameters are in a non-deducible context, so your function template is never even instantiated during overload resolution. </p> <p>Look at it again:</p> <pre><code>template &lt;typename K, typename V&gt; map_keys_iterator&lt;K,V&gt; map_keys(const typename std::map&lt;K,V&gt;::iterator &amp;i) </code></pre> <p>For this to work, the C++ compiler would somehow have to walk from a specific <code>iterator</code> class to its "parent container" type - <code>map</code> in this case - to get its <code>K</code> and <code>V</code>. In general, this is impossible - after all, a particular <code>iterator</code> might be a typedef for some other class, and the actual type of argument in the call is that other class; there's no way the compiler can "retrace" it. So, per C++ standard, it doesn't even try in this case - and, more generally, in any case where you have <code>typename SomeType&lt;T&gt;::OtherType</code>, and <code>T</code> is a type parameter.</p> <p>What you can do is make the entire parameter type a template type parameter. This requires some trickery to derive <code>K</code> and <code>V</code>, though.</p> <pre><code>template &lt;typename Iterator&gt; map_keys_iterator&lt; typename std::iterator_traits&lt;Iterator&gt;::value_type::first_type, typename std::iterator_traits&lt;Iterator&gt;::value_type::second_type &gt; map_keys(Iterator i) </code></pre> <p>Unfortunately, you'll have to repeat those two in the body of the function as well, when invoking the constructor of your type.</p> <p>As a side note, iterators are generally passed by value (they're meant to be lightweight).</p>
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    1. POImplementing a map_keys_iterator by derivation: a single compiler error
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    1. USPavel Minaev
    1. USPavel Minaev
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    1. POImplementing a map_keys_iterator by derivation: a single compiler error
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    1. COAha.... so if I understand correctly, it was the "one extra layer of indirection" issue, huh? _I_ can go up and down a composite expression's types, but the compiler can only go down. I hadn't thought of checking `iterator_traits` and see it provided access to the data types. A good one. I checked it overnight and it allowed me to create a `map_keys_iterator` that worked at the first try with map, multimap and unordered_map so I'm happy now.
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      1. USLuis Machuca
    2. COYou can't really go up and down either, in more general cases. In this case, for example, the actual type of argument - judging from your error message - was `std::_Rb_tree_iterator`. Now imagine that all you have is that, and try to logically deduce the associated `std::map` type, and from it `K` and `V`, without guesswork :) and that's not even to mention that I can always throw a wrench into there by e.g. specializing a particular instantiation of `map` to reuse iterator type from a different one - so there'll be more than one "parent" map type for some iterator type...
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    3. COOK. That clears it a lot. Fixed the code and it works as said above. I would post it but I'm sure it would be "the same but less" than what `boost::something` has brought here.
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