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POIssue with std::shared_ptr, inheritance, and template argument deduction
 

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  1. POIssue with std::shared_ptr, inheritance, and template argument deduction
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    <p>I'm trying to use template argument deduction with inheritance and <code>std::shared_ptr</code>. As you can see in the sample code below, I'm passing a <code>shared_ptr&lt;Derived&gt;</code> to a templated non-member function, which should do template argument deduction. If I manually name the type everything works, and if I let it do template argument deduction it doesn't. It would <em>seem</em> as though the compiler couldn't figure out the type, however the error message shows that it did. I'm not sure what's going on here, and I would appreciate any input. (Visual Studio 2010)</p> <pre><code>#include &lt;memory&gt; template &lt;typename T&gt; class Base {}; class Derived : public Base&lt;int&gt; {}; template &lt;typename T&gt; void func(std::shared_ptr&lt;Base&lt;T&gt; &gt; ptr) {}; int main(int argc, char* argv[]) { std::shared_ptr&lt;Base&lt;int&gt;&gt; myfoo(std::shared_ptr&lt;Derived&gt;(new Derived)); // Compiles func(myfoo); // Compiles func&lt;int&gt;(std::shared_ptr&lt;Derived&gt;(new Derived)); // Compiles func(std::shared_ptr&lt;Derived&gt;(new Derived)); // Doesn't compile. The error message suggests it did deduce the template argument. return 0; } </code></pre> <p>The error message:</p> <pre><code>5&gt; error C2664: 'func' : cannot convert parameter 1 from 'std::tr1::shared_ptr&lt;_Ty&gt;' to 'std::tr1::shared_ptr&lt;_Ty&gt;' 5&gt; with 5&gt; [ 5&gt; _Ty=Derived 5&gt; ] 5&gt; and 5&gt; [ 5&gt; _Ty=Base&lt;int&gt; 5&gt; ] 5&gt; Binding to reference 5&gt; followed by 5&gt; Call to constructor 'std::tr1::shared_ptr&lt;_Ty&gt;::shared_ptr&lt;Derived&gt;(std::tr1::shared_ptr&lt;Derived&gt; &amp;&amp;,void **)' 5&gt; with 5&gt; [ 5&gt; _Ty=Base&lt;int&gt; 5&gt; ] 5&gt; c:\Program Files (x86)\Microsoft Visual Studio 10.0\VC\include\memory(1532) : see declaration of 'std::tr1::shared_ptr&lt;_Ty&gt;::shared_ptr' 5&gt; with 5&gt; [ 5&gt; _Ty=Base&lt;int&gt; 5&gt; ] 5&gt; </code></pre>
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    1. USAndy Prowl
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    1. COHere's what GCC 4.7.3 has to say about it, FYI: `t.cpp:9:6: note: template argument deduction/substitution failed: t.cpp:16:47: note: mismatched types 'Base<T>' and 'Derived' t.cpp:16:47: note: 'std::shared_ptr<Derived>' is not derived from 'std::shared_ptr<Base<T> >'`
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      1. POIssue with std::shared_ptr, inheritance, and template argument deduction
      1. USJohn Zwinck
 

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