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POClass template specializations with shared functionality
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  1. POClass template specializations with shared functionality
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    copied!<p>I'm writing a simple maths library with a template vector type:</p> <pre><code>template&lt;typename T, size_t N&gt; class Vector { public: Vector&lt;T, N&gt; &amp;operator+=(Vector&lt;T, N&gt; const &amp;other); // ... more operators, functions ... }; </code></pre> <p>Now I want some <em>additional</em> functionality specifically for some of these. Let's say I want functions <code>x()</code> and <code>y()</code> on <code>Vector&lt;T, 2&gt;</code> to access particular coordinates. I could create a partial specialization for this:</p> <pre><code>template&lt;typename T&gt; class Vector&lt;T, 3&gt; { public: Vector&lt;T, 3&gt; &amp;operator+=(Vector&lt;T, 3&gt; const &amp;other); // ... and again all the operators and functions ... T x() const; T y() const; }; </code></pre> <p>But now I'm repeating everything that already existed in the generic template.</p> <p>I could also use inheritance. Renaming the generic template to <code>VectorBase</code>, I could do this:</p> <pre><code>template&lt;typename T, size_t N&gt; class Vector : public VectorBase&lt;T, N&gt; { }; template&lt;typename T&gt; class Vector&lt;T, 3&gt; : public VectorBase&lt;T, 3&gt; { public: T x() const; T y() const; }; </code></pre> <p>However, now the problem is that all operators are defined on <code>VectorBase</code>, so they return <code>VectorBase</code> instances. These cannot be assigned to <code>Vector</code> variables:</p> <pre><code>Vector&lt;float, 3&gt; v; Vector&lt;float, 3&gt; w; w = 5 * v; // error: no conversion from VectorBase&lt;float, 3&gt; to Vector&lt;float, 3&gt; </code></pre> <p>I could give <code>Vector</code> an implicit conversion constructor to make this possible:</p> <pre><code>template&lt;typename T, size_t N&gt; class Vector : public VectorBase&lt;T, N&gt; { public: Vector(VectorBase&lt;T, N&gt; const &amp;other); }; </code></pre> <p>However, now I'm converting from <code>Vector</code> to <code>VectorBase</code> and back again. Even though the types are the same in memory, and the compiler might optimize all this away, it feels clunky and I don't really like to have potential run-time overhead for what is essentially a compile-time problem.</p> <p>Is there any other way to solve this?</p>
 

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