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POC++0x template function object inference
 

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    <p>I'm a Scala/Java programmer looking to reintroduce myself to C++ and learn some of the exciting features in C++0x. I wanted to start by designing my own slightly functional collections library, based on Scala's collections, so that I could get a solid understanding of templates. The problem I'm running into is that the compiler doesn't seem to be able to infer any type information for templated function objects. </p> <p><a href="http://www.cc.gatech.edu/~yannis/fc++/tutorial.html" rel="nofollow">FC++</a> seems to have solved this using "Signatures". These seem really similar to the result_type typename, and I thought I would get this using the new function syntax. Can anyone suggest a way to do this sort of thing in C++0x, if it's possible, or at least explain how FC++ was able to accomplish this? Here's a snippet of code I was playing around with</p> <pre><code>#include &lt;vector&gt; #include &lt;iostream&gt; #include &lt;algorithm&gt; using namespace std; template&lt;class T&gt; class ArrayBuffer { private: vector&lt;T&gt; array; public: ArrayBuffer(); ArrayBuffer(vector&lt;T&gt; a) : array(a) {} template&lt;typename Fn&gt; void foreach(Fn fn) { for(unsigned int i = 0; i &lt; array.size(); i++) fn(array[i]); } template&lt;typename Fn&gt; auto map(Fn fn) -&gt; ArrayBuffer&lt;decltype(fn(T()))&gt; { vector&lt;decltype(fn(T()))&gt; result(array.size()); for(int unsigned i = 0; i &lt; array.size(); i++) result[i] = fn(array[i]); return result; } }; template&lt;typename T&gt; class Print { public: void operator()(T elem) { cout&lt;&lt;elem&lt;&lt;endl; } }; template&lt;typename T&gt; class Square{ public: auto operator()(T elem) -&gt; T { return elem * elem; } }; int main() { vector&lt;int&gt; some_list = {5, 3, 1, 2, 4}; ArrayBuffer&lt;int&gt; iterable(some_list); ArrayBuffer&lt;int&gt; squared = iterable.map(Square&lt;int&gt;()); // works as expected iterable.foreach(Print&lt;int&gt;()); // Prints 25 9 1 4 16 as expected iterable.foreach(Print()); // Is there a way or syntax for the compiler to infer that the template must be an int? ArrayBuffer&lt;int&gt; squared2 = iterable.map(Square()); // Same as above - compiler should be able to infer the template. } </code></pre>
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