1namespace Eigen { 2 3/** \page TopicTemplateKeyword The template and typename keywords in C++ 4 5There are two uses for the \c template and \c typename keywords in C++. One of them is fairly well known 6amongst programmers: to define templates. The other use is more obscure: to specify that an expression refers 7to a template function or a type. This regularly trips up programmers that use the %Eigen library, often 8leading to error messages from the compiler that are difficult to understand, such as "expected expression" or 9"no match for operator<". 10 11\eigenAutoToc 12 13 14\section TopicTemplateKeywordToDefineTemplates Using the template and typename keywords to define templates 15 16The \c template and \c typename keywords are routinely used to define templates. This is not the topic of this 17page as we assume that the reader is aware of this (otherwise consult a C++ book). The following example 18should illustrate this use of the \c template keyword. 19 20\code 21template <typename T> 22bool isPositive(T x) 23{ 24 return x > 0; 25} 26\endcode 27 28We could just as well have written <tt>template <class T></tt>; the keywords \c typename and \c class have the 29same meaning in this context. 30 31 32\section TopicTemplateKeywordExample An example showing the second use of the template keyword 33 34Let us illustrate the second use of the \c template keyword with an example. Suppose we want to write a 35function which copies all entries in the upper triangular part of a matrix into another matrix, while keeping 36the lower triangular part unchanged. A straightforward implementation would be as follows: 37 38<table class="example"> 39<tr><th>Example:</th><th>Output:</th></tr> 40<tr><td> 41\include TemplateKeyword_simple.cpp 42</td> 43<td> 44\verbinclude TemplateKeyword_simple.out 45</td></tr></table> 46 47That works fine, but it is not very flexible. First, it only works with dynamic-size matrices of 48single-precision floats; the function \c copyUpperTriangularPart() does not accept static-size matrices or 49matrices with double-precision numbers. Second, if you use an expression such as 50<tt>mat.topLeftCorner(3,3)</tt> as the parameter \c src, then this is copied into a temporary variable of type 51MatrixXf; this copy can be avoided. 52 53As explained in \ref TopicFunctionTakingEigenTypes, both issues can be resolved by making 54\c copyUpperTriangularPart() accept any object of type MatrixBase. This leads to the following code: 55 56<table class="example"> 57<tr><th>Example:</th><th>Output:</th></tr> 58<tr><td> 59\include TemplateKeyword_flexible.cpp 60</td> 61<td> 62\verbinclude TemplateKeyword_flexible.out 63</td></tr></table> 64 65The one line in the body of the function \c copyUpperTriangularPart() shows the second, more obscure use of 66the \c template keyword in C++. Even though it may look strange, the \c template keywords are necessary 67according to the standard. Without it, the compiler may reject the code with an error message like "no match 68for operator<". 69 70 71\section TopicTemplateKeywordExplanation Explanation 72 73The reason that the \c template keyword is necessary in the last example has to do with the rules for how 74templates are supposed to be compiled in C++. The compiler has to check the code for correct syntax at the 75point where the template is defined, without knowing the actual value of the template arguments (\c Derived1 76and \c Derived2 in the example). That means that the compiler cannot know that <tt>dst.triangularView</tt> is 77a member template and that the following < symbol is part of the delimiter for the template 78parameter. Another possibility would be that <tt>dst.triangularView</tt> is a member variable with the < 79symbol referring to the <tt>operator<()</tt> function. In fact, the compiler should choose the second 80possibility, according to the standard. If <tt>dst.triangularView</tt> is a member template (as in our case), 81the programmer should specify this explicitly with the \c template keyword and write <tt>dst.template 82triangularView</tt>. 83 84The precise rules are rather complicated, but ignoring some subtleties we can summarize them as follows: 85- A <em>dependent name</em> is name that depends (directly or indirectly) on a template parameter. In the 86 example, \c dst is a dependent name because it is of type <tt>MatrixBase<Derived1></tt> which depends 87 on the template parameter \c Derived1. 88- If the code contains either one of the constructs <tt>xxx.yyy</tt> or <tt>xxx->yyy</tt> and \c xxx is a 89 dependent name and \c yyy refers to a member template, then the \c template keyword must be used before 90 \c yyy, leading to <tt>xxx.template yyy</tt> or <tt>xxx->template yyy</tt>. 91- If the code contains the construct <tt>xxx::yyy</tt> and \c xxx is a dependent name and \c yyy refers to a 92 member typedef, then the \c typename keyword must be used before the whole construct, leading to 93 <tt>typename xxx::yyy</tt>. 94 95As an example where the \c typename keyword is required, consider the following code in \ref TutorialSparse 96for iterating over the non-zero entries of a sparse matrix type: 97 98\code 99SparseMatrixType mat(rows,cols); 100for (int k=0; k<mat.outerSize(); ++k) 101 for (SparseMatrixType::InnerIterator it(mat,k); it; ++it) 102 { 103 /* ... */ 104 } 105\endcode 106 107If \c SparseMatrixType depends on a template parameter, then the \c typename keyword is required: 108 109\code 110template <typename T> 111void iterateOverSparseMatrix(const SparseMatrix<T>& mat; 112{ 113 for (int k=0; k<m1.outerSize(); ++k) 114 for (typename SparseMatrix<T>::InnerIterator it(mat,k); it; ++it) 115 { 116 /* ... */ 117 } 118} 119\endcode 120 121 122\section TopicTemplateKeywordResources Resources for further reading 123 124For more information and a fuller explanation of this topic, the reader may consult the following sources: 125- The book "C++ Template Metaprogramming" by David Abrahams and Aleksey Gurtovoy contains a very good 126 explanation in Appendix B ("The typename and template Keywords") which formed the basis for this page. 127- http://pages.cs.wisc.edu/~driscoll/typename.html 128- http://www.parashift.com/c++-faq-lite/templates.html#faq-35.18 129- http://www.comeaucomputing.com/techtalk/templates/#templateprefix 130- http://www.comeaucomputing.com/techtalk/templates/#typename 131 132*/ 133} 134