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POCan you use an undefined type in a C++ template function if the function is never used?
 

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  1. POCan you use an undefined type in a C++ template function if the function is never used?
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    <p>I am trying to create a conversion function between objects of two classes (Eigen::Vector3d and MyVector, a Protocol Buffers message), but I want to delay evaluation of the function body until the function is referenced (at which point both classes would be defined).</p> <p>The function should be callable in files that later define both classes, and it should not cause a compilation error if the function is never used.</p> <p>I have:</p> <pre><code>#include &lt;Eigen/Core&gt; // defines Eigen::Vector3d class MyVector { public: int set_x(int x) { x_ = x; } private: int x_; } void operator&lt;&lt; (MyVector &amp;msg, const Eigen::Vector3d &amp;vec) { msg.set_x(vec.x()); } </code></pre> <p>which I use as:</p> <pre><code>MyVector msg; Eigen::Vector3d vec(1, 2, 3); msg &lt;&lt; vec; </code></pre> <p>This works fine if the function is defined after MyVector, but I would like to be able to define the function such that it can be included in a translation unit that lacks the MyVector class.</p> <p>I could change the function to this:</p> <pre><code>template&lt;typename Msg&gt; void operator&lt;&lt; (Msg &amp;msg, ... </code></pre> <p>but this is unacceptable because it would apply to other message classes:</p> <pre><code>quaternion_msg &lt;&lt; Eigen::Vector3d(1, 2, 3); // quaternion has xyz but also w! </code></pre> <p>and I want that to cause a build error.</p> <p>Is there some kind of template magic that can do that? If not, is there a better way to provide this operator short of adding MyVector to the header file (or its dependencies)?</p>
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      1. POCan you use an undefined type in a C++ template function if the function is never used?
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    1. COWhy not put the operator << as a member of MyVector?
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      1. POCan you use an undefined type in a C++ template function if the function is never used?
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    2. CO"but I would like to be able to define the function such that it can be included in a translation unit that lacks the MyVector class." Are you trying to use the void operator<< (MyVector &msg, const Eigen::Vector3d &vec) without the MyVector class definition
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      1. POCan you use an undefined type in a C++ template function if the function is never used?
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    3. COyou can create a template specialization for type MyVector and for the generic template stub just add code which would result in a compile error.
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