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POStrange behavior of operators with CRTP and template specialization
 

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  1. POStrange behavior of operators with CRTP and template specialization
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    <p>I'm writing a vector template with a few specializations so that there are named accessors for the first few items of the vector (such x, y, z, w) depending on the size. <a href="https://stackoverflow.com/a/8645315/140367">KennyTM's answer</a> was a great help with this. I also added a form of <a href="http://en.wikipedia.org/wiki/Curiously_recurring_template_pattern" rel="nofollow noreferrer">Curiously recurring template pattern</a> to be able to use the operators naturally. Here is the base class and a specialization with added functions (omitted most member implementations for brevity): </p> <pre><code>template&lt;int Dim, typename V, typename ItemType = float&gt; class VectorImpl { public: typedef ItemType value_type; VectorImpl() { elements.fill(0); } VectorImpl(std::initializer_list&lt;ItemType&gt; init_list); V operator+(const V&amp; other) { V result; for (int i = 0; i &lt; Dim; ++i) result.elements[i] = elements[i] + other.elements[i]; return result; } QString toString(); // ... other members ... protected: VectorImpl(const VectorImpl&amp; other) = default; protected: std::array&lt;ItemType, Dim&gt; elements; }; template&lt;int Dim, typename ItemType = float&gt; class Vector : public VectorImpl&lt;Dim, Vector&lt;Dim, ItemType&gt;, ItemType&gt; { typedef VectorImpl&lt;Dim, Vector&lt;Dim, ItemType&gt;, ItemType&gt; ParentType; public: Vector() : ParentType() {} Vector(const ParentType&amp; other) : ParentType(other) {} Vector(std::initializer_list&lt;ItemType&gt; init_list) : ParentType(init_list) {} }; template&lt;typename ItemType&gt; class Vector&lt;3, ItemType&gt; : public VectorImpl&lt;3, Vector&lt;3, ItemType&gt;, ItemType&gt; { typedef VectorImpl&lt;3, Vector&lt;3, ItemType&gt;, ItemType&gt; ParentType; public: Vector() : ParentType() {} Vector(const ParentType&amp; other) : ParentType(other) {} Vector(std::initializer_list&lt;ItemType&gt; init_list) : ParentType(init_list) {} ItemType x() const { return this-&gt;elements[0]; } ItemType y() const { return this-&gt;elements[1]; } ItemType z() const { return this-&gt;elements[2]; } }; </code></pre> <p>And I wanted to add <code>qDebug()&lt;&lt;</code> support, so I did this:</p> <pre><code>template&lt;int Dim, typename ItemType&gt; QDebug operator&lt;&lt;(QDebug dbg, Vector&lt;Dim, ItemType&gt;&amp; v) { dbg.nospace() &lt;&lt; v.toString(); return dbg.space(); } </code></pre> <p>Now, the following code compiles and works:</p> <pre><code>Vector&lt;3&gt; v1 = { 3,4,5 }; qDebug() &lt;&lt; v1; </code></pre> <p>This one does, too:</p> <pre><code>Vector&lt;3&gt; v1 = { 3,4,5 }; Vector&lt;3&gt; v2 = { 1,-1,1 }; qDebug() &lt;&lt; v1; auto v3 = v1 + v2; qDebug() &lt;&lt; v3; </code></pre> <p>But this one does not:</p> <pre><code>Vector&lt;3&gt; v1 = { 3,4,5 }; Vector&lt;3&gt; v2 = { 1,-1,1 }; qDebug() &lt;&lt; (v1 + v2); </code></pre> <p>The compiler says: </p> <blockquote> <p>error: no match for 'operator&lt;&lt;' in 'qDebug()() &lt;&lt; v1.Vector&lt;3>::.VectorImpl::operator+ [with int Dim = 3, V = Vector&lt;3>, ItemType = float]((*(const Vector&lt;3>*)(&amp; v2)))'</p> </blockquote> <p>What is going on? Why is the type of <code>v1 + v2</code> different when assigning to a variable? What should I do to make this compile?</p>
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