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POC++ template-function -> passing a template-class as template-argument
 

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    <p>i try to make intensive use of templates to wrap a factory class:</p> <p>The wrapping class (i.e. classA) gets the wrapped class (i.e. classB) via an template-argument to provide 'pluggability'.</p> <p>Additionally i have to provide an inner-class (innerA) that inherits from the wrapped inner-class (innerB).</p> <p>The problem is the following error-message of the g++ "gcc version 4.4.3 (Ubuntu 4.4.3-4ubuntu5)":</p> <pre><code>sebastian@tecuhtli:~/Development/cppExercises/functionTemplate$ g++ -o test test.cpp test.cpp: In static member function ‘static classA&lt;A&gt;::innerA&lt;iB&gt;* classA&lt;A&gt;::createInnerAs(iB&amp;) [with iB = int, A = classB]’: test.cpp:39: instantiated from here test.cpp:32: error: dependent-name ‘classA::innerA&lt;&gt;’ is parsed as a non-type, but instantiation yields a type test.cpp:32: note: say ‘typename classA::innerA&lt;&gt;’ if a type is meant </code></pre> <p>As you can see in the definition of method createInnerBs, i intend to pass a non-type argument. So the use of typename is wrong!</p> <p>The code of test.cpp is below:</p> <pre><code>class classB{ public: template &lt; class iB&gt; class innerB{ iB&amp; ib; innerB(iB&amp; b) :ib(b){} }; template&lt;template &lt;class&gt; class classShell, class iB&gt; static classShell&lt;iB&gt;* createInnerBs(iB&amp; b){ // this function creates instances of innerB and its subclasses, // because B holds a certain allocator return new classShell&lt;iB&gt;(b); } }; template&lt;class A&gt; class classA{ // intention of this class is meant to be a pluggable interface // using templates for compile-time checking public: template &lt;class iB&gt; class innerA: A::template innerB&lt;iB&gt;{ innerA(iB&amp; b) :A::template innerB&lt;iB&gt;(b){} }; template&lt;class iB&gt; static inline innerA&lt;iB&gt;* createInnerAs(iB&amp; b){ return A::createInnerBs&lt;classA&lt;A&gt;::template innerA&lt;&gt; &gt;(b); // line 32: error occurs here } }; typedef classA&lt;classB&gt; usable; int main (int argc, char* argv[]){ int a = 5; usable::innerA&lt;int&gt;* myVar = usable::createInnerAs(a); return 0; } </code></pre> <p>Please help me, i have been faced to this problem for several days. Is it just impossible, what i'm trying to do? Or did i forgot something?</p> <p>Thanks, Sema</p>
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