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134 lines
6.0 KiB
134 lines
6.0 KiB
namespace Eigen {
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/** \page TopicTemplateKeyword The template and typename keywords in C++
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There are two uses for the \c template and \c typename keywords in C++. One of them is fairly well known
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amongst programmers: to define templates. The other use is more obscure: to specify that an expression refers
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to a template function or a type. This regularly trips up programmers that use the %Eigen library, often
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leading to error messages from the compiler that are difficult to understand, such as "expected expression" or
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"no match for operator<".
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\eigenAutoToc
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\section TopicTemplateKeywordToDefineTemplates Using the template and typename keywords to define templates
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The \c template and \c typename keywords are routinely used to define templates. This is not the topic of this
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page as we assume that the reader is aware of this (otherwise consult a C++ book). The following example
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should illustrate this use of the \c template keyword.
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\code
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template <typename T>
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bool isPositive(T x)
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{
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return x > 0;
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}
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\endcode
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We could just as well have written <tt>template <class T></tt>; the keywords \c typename and \c class have the
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same meaning in this context.
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\section TopicTemplateKeywordExample An example showing the second use of the template keyword
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Let us illustrate the second use of the \c template keyword with an example. Suppose we want to write a
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function which copies all entries in the upper triangular part of a matrix into another matrix, while keeping
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the lower triangular part unchanged. A straightforward implementation would be as follows:
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<table class="example">
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<tr><th>Example:</th><th>Output:</th></tr>
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<tr><td>
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\include TemplateKeyword_simple.cpp
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</td>
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<td>
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\verbinclude TemplateKeyword_simple.out
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</td></tr></table>
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That works fine, but it is not very flexible. First, it only works with dynamic-size matrices of
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single-precision floats; the function \c copyUpperTriangularPart() does not accept static-size matrices or
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matrices with double-precision numbers. Second, if you use an expression such as
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<tt>mat.topLeftCorner(3,3)</tt> as the parameter \c src, then this is copied into a temporary variable of type
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MatrixXf; this copy can be avoided.
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As explained in \ref TopicFunctionTakingEigenTypes, both issues can be resolved by making
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\c copyUpperTriangularPart() accept any object of type MatrixBase. This leads to the following code:
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<table class="example">
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<tr><th>Example:</th><th>Output:</th></tr>
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<tr><td>
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\include TemplateKeyword_flexible.cpp
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</td>
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<td>
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\verbinclude TemplateKeyword_flexible.out
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</td></tr></table>
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The one line in the body of the function \c copyUpperTriangularPart() shows the second, more obscure use of
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the \c template keyword in C++. Even though it may look strange, the \c template keywords are necessary
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according to the standard. Without it, the compiler may reject the code with an error message like "no match
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for operator<".
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\section TopicTemplateKeywordExplanation Explanation
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The reason that the \c template keyword is necessary in the last example has to do with the rules for how
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templates are supposed to be compiled in C++. The compiler has to check the code for correct syntax at the
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point where the template is defined, without knowing the actual value of the template arguments (\c Derived1
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and \c Derived2 in the example). That means that the compiler cannot know that <tt>dst.triangularView</tt> is
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a member template and that the following < symbol is part of the delimiter for the template
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parameter. Another possibility would be that <tt>dst.triangularView</tt> is a member variable with the <
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symbol refering to the <tt>operator<()</tt> function. In fact, the compiler should choose the second
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possibility, according to the standard. If <tt>dst.triangularView</tt> is a member template (as in our case),
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the programmer should specify this explicitly with the \c template keyword and write <tt>dst.template
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triangularView</tt>.
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The precise rules are rather complicated, but ignoring some subtleties we can summarize them as follows:
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- A <em>dependent name</em> is name that depends (directly or indirectly) on a template parameter. In the
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example, \c dst is a dependent name because it is of type <tt>MatrixBase<Derived1></tt> which depends
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on the template parameter \c Derived1.
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- If the code contains either one of the constructs <tt>xxx.yyy</tt> or <tt>xxx->yyy</tt> and \c xxx is a
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dependent name and \c yyy refers to a member template, then the \c template keyword must be used before
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\c yyy, leading to <tt>xxx.template yyy</tt> or <tt>xxx->template yyy</tt>.
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- If the code contains the construct <tt>xxx::yyy</tt> and \c xxx is a dependent name and \c yyy refers to a
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member typedef, then the \c typename keyword must be used before the whole construct, leading to
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<tt>typename xxx::yyy</tt>.
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As an example where the \c typename keyword is required, consider the following code in \ref TutorialSparse
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for iterating over the non-zero entries of a sparse matrix type:
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\code
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SparseMatrixType mat(rows,cols);
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for (int k=0; k<mat.outerSize(); ++k)
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for (SparseMatrixType::InnerIterator it(mat,k); it; ++it)
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{
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/* ... */
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}
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\endcode
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If \c SparseMatrixType depends on a template parameter, then the \c typename keyword is required:
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\code
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template <typename T>
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void iterateOverSparseMatrix(const SparseMatrix<T>& mat;
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{
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for (int k=0; k<m1.outerSize(); ++k)
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for (typename SparseMatrix<T>::InnerIterator it(mat,k); it; ++it)
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{
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/* ... */
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}
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}
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\endcode
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\section TopicTemplateKeywordResources Resources for further reading
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For more information and a fuller explanation of this topic, the reader may consult the following sources:
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- The book "C++ Template Metaprogramming" by David Abrahams and Aleksey Gurtovoy contains a very good
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explanation in Appendix B ("The typename and template Keywords") which formed the basis for this page.
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- http://pages.cs.wisc.edu/~driscoll/typename.html
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- http://www.parashift.com/c++-faq-lite/templates.html#faq-35.18
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- http://www.comeaucomputing.com/techtalk/templates/#templateprefix
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- http://www.comeaucomputing.com/techtalk/templates/#typename
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*/
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}
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