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POUnderstanding C++ member function template specialization
 

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  1. POUnderstanding C++ member function template specialization
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    <p>I have the following class:</p> <pre><code>#pragma once #include &lt;string&gt; #include &lt;iostream&gt; class testclass { public: template &lt;class T&gt; T item(const std::string&amp; key) { std::cout &lt;&lt; "non-specialized\n"; return T(); } }; </code></pre> <p>For the item method I would like to provide a specialization for strings. I try to do this the following way (in testclass.cpp):</p> <pre><code>#include "testclass.h" #include &lt;iostream&gt; template&lt;&gt; std::string testclass::item(const std::string&amp; key) { std::cout &lt;&lt; "specialized\n"; return std::reverse(key.begin(), key.end()); } </code></pre> <p>And then I try to call the function like this:</p> <pre><code>#include &lt;iostream&gt; #include "testclass.h" int main() { testclass t; std::string key = "foo"; t.item&lt;int&gt;(key); std::string s = t.item&lt;std::string&gt;(key); std::cout &lt;&lt; s &lt;&lt; std::endl; } </code></pre> <p>However, the output is</p> <pre><code>$ ./a.out non-specialized non-specialized (empty line) </code></pre> <p>What I excepted was</p> <pre><code>$ ./a.out non-specialized specialized oof </code></pre> <p>How can I do this properly? I am using g++ 4.5.2 to compile the program. </p> <p><strong>Edit</strong>:</p> <p>The solution is the move the whole definition of the specialization of <code>item</code> to testclass.h (but not into the class). I had other mistakes in the program, such as not including <code>&lt;algorithm&gt;</code> (for reverse), and incorrectly thinking that it would return the reversed string. To achieve the excepted behaviour, the .cpp file is left empty, and the header contents are the following:</p> <pre><code>#pragma once #include &lt;string&gt; #include &lt;iostream&gt; #include &lt;algorithm&gt; class testclass { public: template &lt;class T&gt; T item(const std::string&amp; key) { std::cout &lt;&lt; "non-specialized\n"; return T(); } }; template&lt;&gt; std::string testclass::item(const std::string&amp; key) { std::cout &lt;&lt; "specialized\n"; std::string s = key; std::reverse(s.begin(), s.end()); return s; } </code></pre>
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    1. CODon't you have to forward declare the specialized version? You have no way to discover this implementation from main.cpp (it only includes testclass.h). I write it here, cause I'm not sure.
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    2. COexactly. Compiler doesn't see the specialization while compiling main.cpp
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