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PODeducing template argument for nested template fails
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  1. PODeducing template argument for nested template fails
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    copied!<p>Ok, I read throught quite some “could not deduce template argument” questions but none seems to match my case — or I don't understand the answer…</p> <p>There's <a href="https://stackoverflow.com/questions/4630941/failed-to-specialize-function-template" title="Question 4630941">one</a> that I feel goes in the right direction, but I failed extracting the solution for my problem.</p> <p>The stripped down code in my header looks like this:</p> <pre><code>template&lt;typename T&gt; class TemplateProblem { public: // Do I really need this or did I miss something from the STL? template&lt;typename Tin, typename Tout&gt; struct UnaryFunction : public std::unary_function&lt;Tin, Tout&gt; { virtual Tout operator()(Tin input) = 0; }; template&lt;typename Tin, typename Tout&gt; struct StaticCast : public UnaryFunction&lt;Tin, Tout&gt; { virtual Tout operator()(Tin input) { return static_cast&lt;Tout&gt;(input); } }; private: T * const _data; T const _bias; template&lt;typename Tin&gt; void Init(Tin * data, int length, UnaryFunction&lt;Tin, T&gt; mapper, Tin bias); public: template&lt;typename Tin&gt; TemplateProblem(Tin * data, int length, Tin bias = Tin()); template&lt;typename Tin&gt; TemplateProblem(Tin * data, int length, UnaryFunction&lt;Tin, T&gt; mapper, Tin bias = T()); }; template&lt;typename T&gt; template&lt;typename Tin&gt; void TemplateProblem&lt;T&gt;::Init(Tin * data, int length, UnaryFunction&lt;Tin, T&gt; mapper, Tin bias) { T mappedBias = mapper(bias); for (int i = 0; i &lt; length; i++) { _data[i] = data[i] + mappedBias; } } template&lt;typename T&gt; template&lt;typename Tin&gt; TemplateProblem&lt;T&gt;::TemplateProblem(Tin * data, int length, UnaryFunction&lt;Tin, T&gt; mapper, Tin bias = T()) : _data(new T[length]), _bias(bias) { Init(data, length, mapper, bias); } template&lt;typename T&gt; template&lt;typename Tin&gt; TemplateProblem&lt;T&gt;::TemplateProblem(Tin * data, int length, Tin bias = T()) : _data(new T[length]), _bias(bias) { StaticCast&lt;Tin, T&gt; cast; Init(data, length, cast, bias); } </code></pre> <p>I instantiate it like this:</p> <pre><code>unsigned char pixels[] = {23, 42, 65, 97}; TemplateProblem&lt;int&gt; tp(pixels, 4); </code></pre> <p>From VS2012 I get these messages:</p> <pre><code>Error 1 error C2784: 'void TemplateProblem&lt;T&gt;::Init(Tin *,int,TemplateProblem&lt;T&gt;::UnaryFunction&lt;Tin,T&gt;,Tin)' : could not deduce template argument for 'TemplateProblem&lt;T&gt;::UnaryFunction&lt;Tin,T&gt;' from 'TemplateProblem&lt;T&gt;::StaticCast&lt;Tin,Tout&gt;' ...\templateproblem.h 62 1 TemplateProblem Error 2 error C2893: Failed to specialize function template 'void TemplateProblem&lt;T&gt;::Init(Tin *,int,TemplateProblem&lt;T&gt;::UnaryFunction&lt;Tin,T&gt;,Tin)' ...\templateproblem.h 62 1 TemplateProblem </code></pre> <p>The error also occurs when I move the two <code>struct</code>s out of the <code>class</code> as <a href="https://stackoverflow.com/questions/12640808/nested-template-and-parameter-deducing" title="Question 12640808">this answer</a> suggests.</p>
 

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