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      1 namespace Eigen {
      2 
      3 /** \page TopicTemplateKeyword The template and typename keywords in C++
      4 
      5 There are two uses for the \c template and \c typename keywords in C++. One of them is fairly well known
      6 amongst programmers: to define templates. The other use is more obscure: to specify that an expression refers
      7 to a template function or a type. This regularly trips up programmers that use the %Eigen library, often
      8 leading to error messages from the compiler that are difficult to understand.
      9 
     10 \eigenAutoToc
     11 
     12 
     13 \section TopicTemplateKeywordToDefineTemplates Using the template and typename keywords to define templates
     14 
     15 The \c template and \c typename keywords are routinely used to define templates. This is not the topic of this
     16 page as we assume that the reader is aware of this (otherwise consult a C++ book). The following example
     17 should illustrate this use of the \c template keyword.
     18 
     19 \code
     20 template <typename T>
     21 bool isPositive(T x)
     22 {
     23     return x > 0;
     24 }
     25 \endcode
     26 
     27 We could just as well have written <tt>template &lt;class T&gt;</tt>; the keywords \c typename and \c class have the
     28 same meaning in this context.
     29 
     30 
     31 \section TopicTemplateKeywordExample An example showing the second use of the template keyword
     32 
     33 Let us illustrate the second use of the \c template keyword with an example. Suppose we want to write a
     34 function which copies all entries in the upper triangular part of a matrix into another matrix, while keeping
     35 the lower triangular part unchanged. A straightforward implementation would be as follows:
     36 
     37 <table class="example">
     38 <tr><th>Example:</th><th>Output:</th></tr>
     39 <tr><td>
     40 \include TemplateKeyword_simple.cpp
     41 </td>
     42 <td>
     43 \verbinclude TemplateKeyword_simple.out
     44 </td></tr></table>
     45 
     46 That works fine, but it is not very flexible. First, it only works with dynamic-size matrices of
     47 single-precision floats; the function \c copyUpperTriangularPart() does not accept static-size matrices or
     48 matrices with double-precision numbers. Second, if you use an expression such as
     49 <tt>mat.topLeftCorner(3,3)</tt> as the parameter \c src, then this is copied into a temporary variable of type
     50 MatrixXf; this copy can be avoided.
     51 
     52 As explained in \ref TopicFunctionTakingEigenTypes, both issues can be resolved by making 
     53 \c copyUpperTriangularPart() accept any object of type MatrixBase. This leads to the following code:
     54 
     55 <table class="example">
     56 <tr><th>Example:</th><th>Output:</th></tr>
     57 <tr><td>
     58 \include TemplateKeyword_flexible.cpp
     59 </td>
     60 <td>
     61 \verbinclude TemplateKeyword_flexible.out
     62 </td></tr></table>
     63 
     64 The one line in the body of the function \c copyUpperTriangularPart() shows the second, more obscure use of
     65 the \c template keyword in C++.  Even though it may look strange, the \c template keywords are necessary
     66 according to the standard. Without it, the compiler may reject the code with an error message like "no match
     67 for operator<".
     68 
     69 
     70 \section TopicTemplateKeywordExplanation Explanation
     71 
     72 The reason that the \c template keyword is necessary in the last example has to do with the rules for how
     73 templates are supposed to be compiled in C++. The compiler has to check the code for correct syntax at the
     74 point where the template is defined, without knowing the actual value of the template arguments (\c Derived1
     75 and \c Derived2 in the example). That means that the compiler cannot know that <tt>dst.triangularPart</tt> is
     76 a member template and that the following &lt; symbol is part of the delimiter for the template
     77 parameter. Another possibility would be that <tt>dst.triangularPart</tt> is a member variable with the &lt;
     78 symbol refering to the <tt>operator&lt;()</tt> function. In fact, the compiler should choose the second
     79 possibility, according to the standard. If <tt>dst.triangularPart</tt> is a member template (as in our case),
     80 the programmer should specify this explicitly with the \c template keyword and write <tt>dst.template
     81 triangularPart</tt>.
     82 
     83 The precise rules are rather complicated, but ignoring some subtleties we can summarize them as follows:
     84 - A <em>dependent name</em> is name that depends (directly or indirectly) on a template parameter. In the
     85   example, \c dst is a dependent name because it is of type <tt>MatrixBase&lt;Derived1&gt;</tt> which depends
     86   on the template parameter \c Derived1.
     87 - If the code contains either one of the contructions <tt>xxx.yyy</tt> or <tt>xxx-&gt;yyy</tt> and \c xxx is a
     88   dependent name and \c yyy refers to a member template, then the \c template keyword must be used before 
     89   \c yyy, leading to <tt>xxx.template yyy</tt> or <tt>xxx-&gt;template yyy</tt>.
     90 - If the code contains the contruction <tt>xxx::yyy</tt> and \c xxx is a dependent name and \c yyy refers to a
     91   member typedef, then the \c typename keyword must be used before the whole construction, leading to
     92   <tt>typename xxx::yyy</tt>.
     93 
     94 As an example where the \c typename keyword is required, consider the following code in \ref TutorialSparse
     95 for iterating over the non-zero entries of a sparse matrix type:
     96 
     97 \code
     98 SparseMatrixType mat(rows,cols);
     99 for (int k=0; k<mat.outerSize(); ++k)
    100   for (SparseMatrixType::InnerIterator it(mat,k); it; ++it)
    101   {
    102     /* ... */
    103   }
    104 \endcode
    105 
    106 If \c SparseMatrixType depends on a template parameter, then the \c typename keyword is required:
    107 
    108 \code
    109 template <typename T>
    110 void iterateOverSparseMatrix(const SparseMatrix<T>& mat;
    111 {
    112   for (int k=0; k<m1.outerSize(); ++k)
    113     for (typename SparseMatrix<T>::InnerIterator it(mat,k); it; ++it)
    114     {
    115       /* ... */
    116     }
    117 }
    118 \endcode
    119 
    120 
    121 \section TopicTemplateKeywordResources Resources for further reading
    122 
    123 For more information and a fuller explanation of this topic, the reader may consult the following sources:
    124 - The book "C++ Template Metaprogramming" by David Abrahams and Aleksey Gurtovoy contains a very good
    125   explanation in Appendix B ("The typename and template Keywords") which formed the basis for this page.
    126 - http://pages.cs.wisc.edu/~driscoll/typename.html
    127 - http://www.parashift.com/c++-faq-lite/templates.html#faq-35.18
    128 - http://www.comeaucomputing.com/techtalk/templates/#templateprefix
    129 - http://www.comeaucomputing.com/techtalk/templates/#typename
    130 
    131 */
    132 }
    133