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POUsing C++11 std::enable_if to enable member function if vector is specific length
 

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  1. POUsing C++11 std::enable_if to enable member function if vector is specific length
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    <p>I am writing a simple vector class and I would like to have some member functions that are only available in vectors of certain lengths (cross product for a 3 element vector for example). I stumbled across std::enable_if and it looks like it may be able to do what I want, but I don't seem to be able to get it working correctly.</p> <pre><code>#include &lt;iostream&gt; #include &lt;type_traits&gt; template&lt;typename T, unsigned int L&gt; class Vector { private: T data[L]; public: Vector&lt;T,L&gt;(void) { for(unsigned int i = 0; i &lt; L; i++) { data[i] = 0; } } T operator()(const unsigned int i) const { return data[i]; } T&amp; operator()(const unsigned int i) { return data[i]; } Vector&lt;typename std::enable_if&lt;L==3, T&gt;::type, L&gt; cross(const Vector&lt;T,L&gt;&amp; vec2) const { Vector&lt;T,L&gt; result; result(0) = (*this)(1) * vec2(2) - (*this)(2) * vec2(1); result(1) = (*this)(2) * vec2(0) - (*this)(0) * vec2(2); result(2) = (*this)(0) * vec2(1) - (*this)(1) * vec2(0); return result; } }; int main(void) { Vector&lt;double,3&gt; v1; Vector&lt;double,3&gt; v2; Vector&lt;double,3&gt; v3; //Vector&lt;double,4&gt; v4; v1(0) = 1; v1(1) = 2; v1(2) = 3; v2(0) = 4; v2(1) = 5; v2(2) = 6; v3 = v1.cross(v2); std::cout &lt;&lt; v3(0) &lt;&lt; std::endl; std::cout &lt;&lt; v3(1) &lt;&lt; std::endl; std::cout &lt;&lt; v3(2) &lt;&lt; std::endl; return 0; } </code></pre> <p>The code above compiles and runs correctly, however if I uncomment the declaration of <code>Vector&lt;double,4&gt; v4</code> I get the following error at compilation:</p> <pre><code>vec.cpp: In instantiation of ‘class Vector&lt;double, 4u&gt;’: vec.cpp:46:22: required from here vec.cpp:29:59: error: no type named ‘type’ in ‘struct std::enable_if&lt;false, double&gt;’ </code></pre> <p>Is someone able to point out where I am going wrong?</p>
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    1. COpossible duplicate of [Can I use boost::enable\_if on a member function?](http://stackoverflow.com/questions/4880922/can-i-use-boostenable-if-on-a-member-function)
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      1. POUsing C++11 std::enable_if to enable member function if vector is specific length
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    2. COYou probably want `typename std::enable_if<L==3, Vector<T, L>>::type` rather than `Vector<typename std::enable_if<L==3, T>::type, L>`.
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    3. CO(You probably want your const `operator()` to return `T const&` too. Current code lets clients do something like `Vector<double, 3> v; Vector<double, 3> const& const_v = v; const_v(2) = 42.0; // Note that the write of 42 happens to a temporary. Not what the user expected`)
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