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POPassing object of a template class as a function parameter which expects base template class
 

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  1. POPassing object of a template class as a function parameter which expects base template class
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    <p>I have a template class which parametrized by another class with deep hierarchy. I want to pass to a function base template class parametrized by another base class. Here is the example: </p> <pre><code>// Base template class test #include &lt;stdio.h&gt; // Base classes template&lt;class T&gt; class Method { public: Method&lt;T&gt; (T * t) { this-&gt;t = t; } virtual int solve() = 0; protected: T * t; }; class Abstract { public: virtual int get() = 0; virtual void set(int a) = 0; }; // Concrete classes, there might be a few of them template&lt;class T&gt; class MethodImpl : public Method&lt;T&gt; { public: MethodImpl&lt;T&gt; (T * t) : Method&lt;T&gt;(t) {} int solve() { return this-&gt;t-&gt;get() + 1; } }; class Concrete : public Abstract { public: int get() { return this-&gt;a; } void set(int a) { this-&gt;a = a; } protected: int a; }; // Uses methods of Base classes only class User { public: int do_stuff(Abstract &amp; a, Method&lt;Abstract&gt; &amp; ma) { a.set(2); return ma.solve(); } }; // Example usage int main () { Concrete * c = new Concrete(); MethodImpl&lt;Concrete&gt; * mc = new MethodImpl&lt;Concrete&gt;(c); User * u = new User(); int result = u-&gt;do_stuff(*c, *mc); printf("%i", result); return 0; } </code></pre> <p>I get this error during compilation:</p> <pre><code>btc.cpp: In function 'int main()': btc.cpp:62: error: no matching function for call to 'User::do_stuff(Concrete&amp;, MethodImpl&lt;Concrete&gt;&amp;)' btc.cpp:50: note: candidates are: int User::do_stuff(Abstract&amp;, Method&lt;Abstract&gt;&amp;) </code></pre> <p>However it works fine if I create local variables with the same logic:</p> <pre><code>int main () { Abstract * a = new Concrete(); Method&lt;Abstract&gt; * ma = new MethodImpl&lt;Abstract&gt;(a); a-&gt;set(2); int result = ma-&gt;solve(); printf("%i", result); return 0; } </code></pre>
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    1. USAndrew
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      1. VTUpMod
    1. COAhh, covariance. That was the magic word I was trying to think of, ta.
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      1. USRook
    2. CO[C++ template are not covariant](http://stackoverflow.com/questions/2203388/c-templates-polymorphism?lq=1): `Method<Concrete>` is not a derived class of `Method<Base>`.
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      1. USLuc Touraille
 

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