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POsimulate C++ function template instantiation with implicit conversion
 

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  1. POsimulate C++ function template instantiation with implicit conversion
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    <p>I already asked <a href="https://stackoverflow.com/questions/1741840/c-implicit-template-instantiation">two</a> <a href="https://stackoverflow.com/questions/1746352/how-to-have-templated-function-overloads-accept-derived-classes-from-different-ba">questions</a> related to what I'm trying to do (one resolved, one of which I will close soon). I know that C++ template instantiation does not allow any implicit conversions (see for example <a href="https://stackoverflow.com/questions/867462/c-implicit-conversions/868398#868398">this comment</a>), but I would like to simulate it.</p> <p>Suppose I have the following skeleton code:</p> <pre><code>template &lt;class T&gt; struct Base_A{ virtual void interface_func() const = 0; }; template &lt;class T&gt; struct Derived_A : public Base_A&lt;T&gt;{ typedef T value_type; void interface_func() const{} }; template &lt;class T&gt; struct Base_B{ virtual void interface_func() = 0; // note: non-const }; template &lt;class T&gt; struct Derived_B : public Base_B&lt;T&gt;{ typedef T value_type; void interface_func(){} }; template &lt;class BType&gt; struct Adapter : public Base_A&lt;typename BType::value_type&gt;{ BType &amp;ref_B; Adapter(BType &amp;inst_B):ref_B(B_inst){} void interface_func() const{} // does stuff with ref_B to simulate an A }; template &lt;class Should_Always_Be_Base_A&gt; void f(const Should_Always_Be_Base_A &amp;arg){ // Only Base_A can be passed in by const ref // Passing in a Base_B by const ref would not work. } Derived_A&lt;int&gt; A; Derived_B&lt;int&gt; B; f(A); // passes in A by const ref f(B); // I want to pass in Adapter&lt;Derived_B&lt;int&gt; &gt;(B) </code></pre> <p>I want the template parameter for function <code>f</code> to always be a derived class of <code>Base_A</code> or an <code>Adapter</code>. The answer to restricting the type of <code>arg</code> <a href="https://stackoverflow.com/questions/1746352/how-to-have-templated-function-overloads-accept-derived-classes-from-different-ba">can be done</a>, but the implicit conversion to an Adapter cannot. Is there any way to do this? The net result is I want to be able to call <code>f</code> as <code>f(A)</code> or <code>f(B)</code>, and in both cases I need to know the actual derived type of A or B within <code>f</code> (<code>f</code> cannot just see a reference to the base class).</p> <p><strong>Aside:</strong></p> <p>Presently, I just have <code>f(A)</code> working, and <code>f(B)</code> actually calls an overload which performs the Adapter construction, but I have other functions which take N arguments, each of which can be A or B, so I would need 2^N overloads.</p> <p>For the curious, this is in application to the matrix library I'm working on. <code>Base_A</code> represents the base matrix type, and <code>Base_B</code> represents the base matrix-view type. For operations which will modify a matrix argument, I need to pass in the matrix by non-const reference or a modifiable matrix-view by const-ref. The adapter is just a trivial matrix-to-view adapter. So for example, I currently have a function like</p> <pre><code>Scale(const MatrixViewBase&lt;T&gt; &amp;Mview, const T &amp;scale_factor){ // does the actual work } Scale(MatrixBase&lt;T&gt; &amp;M, const T &amp;scale_factor){ Scale(Adapter&lt;MatrixBase&lt;T&gt; &gt;(M), scale_factor); } </code></pre> <p>It's tedious and error prone to make 2^N copies of all these functions just to create the needed overloads to handle both views and non-views. As is, this is not good enough since I want Scale to be able to know the full derived type of Mview, not just the base class, because I will potentially generate instances of types dependent on Mview.</p> <p><strong>Edit 1:</strong> Changed all B types to have non-const interface functions. This was the original intent, so apologies for any confusion.</p> <p><strong>Edit 2:</strong> Have this working code, still requires 2^N overloads, but I can live with it unless someone suggests how to deal with that.</p> <pre><code>#include &lt;iostream&gt; template &lt;class T&gt; struct ReadableMatrix{ typedef T value_type; }; template &lt;class T&gt; struct WritableMatrix{ typedef T value_type; }; template &lt;class T&gt; struct WritableMatrixView{ typedef T value_type; }; template &lt;class T&gt; struct Matrix : public WritableMatrix&lt;T&gt;{ typedef T value_type; typedef ReadableMatrix&lt;T&gt; readable_matrix; typedef WritableMatrix&lt;T&gt; writable_matrix; }; template &lt;class T&gt; struct MatrixView : public WritableMatrixView&lt;T&gt;{ typedef T value_type; typedef ReadableMatrix&lt;T&gt; readable_matrix; // not really used; needs an adapter before using typedef WritableMatrixView&lt;T&gt; writable_matrix; }; template &lt;class T, class R&gt; struct IsReadableMatrix{ }; template &lt;class T, class R&gt; struct IsReadableMatrix&lt;ReadableMatrix&lt;T&gt;, R&gt;{ typedef R type; }; template &lt;class T, class R&gt; struct IsWritableMatrix{ }; template &lt;class T, class R&gt; struct IsWritableMatrix&lt;WritableMatrix&lt;T&gt;, R&gt;{ typedef R type; }; template &lt;class T, class R&gt; struct IsWritableMatrixView{ }; template &lt;class T, class R&gt; struct IsWritableMatrixView&lt;WritableMatrixView&lt;T&gt;, R&gt;{ typedef R type; }; template &lt;class TA, class TB&gt; typename IsReadableMatrix&lt;typename TA::readable_matrix, typename IsWritableMatrixView&lt;typename TB::writable_matrix, void &gt;::type&gt;::type Copy(const TA &amp;A, const TB &amp;B){ std::cout &lt;&lt; "Here" &lt;&lt; std::endl; } template &lt;class TA, class TB&gt; typename IsReadableMatrix&lt;typename TA::readable_matrix, typename IsWritableMatrix&lt;typename TB::writable_matrix, void &gt;::type&gt;::type Copy(const TA &amp;A, TB &amp;B){ std::cout &lt;&lt; "Here2" &lt;&lt; std::endl; } int main(){ Matrix&lt;int&gt; M, M2; MatrixView&lt;int&gt; V, V2; Copy(M, M2); Copy(V, V2); Copy(M, V); Copy(V, M); } </code></pre>
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    1. USCommunity
    1. USVictor Liu
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      1. POsimulate C++ function template instantiation with implicit conversion
      1. USDavid Rodríguez - dribeas
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    1. COI'm a bit confused. In your actual implementation, how do you figure 2^N functions? It would be more helpful if you could elaborate an example that shows where you are having exponential functions
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      1. UScoppro
    2. CO@coppro: The `Scale` example is an example. I need to have one version which works on the view, and a second one which adapts the non-view into a view. If I have a function Mult(A, B, C), where C = A*B, I need 2^3 = 8 overloads where each of A, B, and C could be a view or a non-view.
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      1. POsimulate C++ function template instantiation with implicit conversion
      1. USVictor Liu
 

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