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<div class="title">The template and typename keywords in C++ </div> </div>
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<div class="textblock"><p>There are two uses for the <code>template</code> and <code>typename</code> keywords in C++. One of them is fairly well known amongst programmers: to define templates. The other use is more obscure: to specify that an expression refers to a template function or a type. This regularly trips up programmers that use the Eigen library, often leading to error messages from the compiler that are difficult to understand.</p>
<h1><a class="anchor" id="TopicTemplateKeywordToDefineTemplates"></a>
Using the template and typename keywords to define templates</h1>
<p>The <code>template</code> and <code>typename</code> keywords are routinely used to define templates. This is not the topic of this page as we assume that the reader is aware of this (otherwise consult a C++ book). The following example should illustrate this use of the <code>template</code> keyword.</p>
<div class="fragment"><div class="line"><span class="keyword">template</span> <<span class="keyword">typename</span> T></div>
<div class="line"><span class="keywordtype">bool</span> isPositive(T x)</div>
<div class="line">{</div>
<div class="line"> <span class="keywordflow">return</span> x > 0;</div>
<div class="line">}</div>
</div><!-- fragment --><p>We could just as well have written <code>template <class T></code>; the keywords <code>typename</code> and <code>class</code> have the same meaning in this context.</p>
<h1><a class="anchor" id="TopicTemplateKeywordExample"></a>
An example showing the second use of the template keyword</h1>
<p>Let us illustrate the second use of the <code>template</code> keyword with an example. Suppose we want to write a function which copies all entries in the upper triangular part of a matrix into another matrix, while keeping the lower triangular part unchanged. A straightforward implementation would be as follows:</p>
<table class="example">
<tr>
<th>Example:</th><th>Output: </th></tr>
<tr>
<td><div class="fragment"><div class="line"><span class="preprocessor">#include <Eigen/Dense></span></div>
<div class="line"><span class="preprocessor">#include <iostream></span></div>
<div class="line"></div>
<div class="line"><span class="keyword">using namespace </span>Eigen;</div>
<div class="line"></div>
<div class="line"><span class="keywordtype">void</span> copyUpperTriangularPart(<a class="code" href="classEigen_1_1Matrix.html">MatrixXf</a>& dst, <span class="keyword">const</span> <a class="code" href="classEigen_1_1Matrix.html">MatrixXf</a>& src)</div>
<div class="line">{</div>
<div class="line"> dst.triangularView<<a class="code" href="group__enums.html#ggab59c1bec446b10af208f977a871d910bae70afef0d3ff7aca74e17e85ff6c9f2e">Upper</a>>() = src.triangularView<<a class="code" href="group__enums.html#ggab59c1bec446b10af208f977a871d910bae70afef0d3ff7aca74e17e85ff6c9f2e">Upper</a>>();</div>
<div class="line">}</div>
<div class="line"></div>
<div class="line"><span class="keywordtype">int</span> main()</div>
<div class="line">{</div>
<div class="line"> <a class="code" href="classEigen_1_1Matrix.html">MatrixXf</a> m1 = <a class="code" href="classEigen_1_1DenseBase.html#a2278addf9a3c977d40322571a0df8ac9">MatrixXf::Ones</a>(4,4);</div>
<div class="line"> <a class="code" href="classEigen_1_1Matrix.html">MatrixXf</a> m2 = <a class="code" href="classEigen_1_1DenseBase.html#a8e759dafdd9ecc446d397b7f5435f60a">MatrixXf::Random</a>(4,4);</div>
<div class="line"> std::cout << <span class="stringliteral">"m2 before copy:"</span> << std::endl;</div>
<div class="line"> std::cout << m2 << std::endl << std::endl;</div>
<div class="line"> copyUpperTriangularPart(m2, m1);</div>
<div class="line"> std::cout << <span class="stringliteral">"m2 after copy:"</span> << std::endl;</div>
<div class="line"> std::cout << m2 << std::endl << std::endl;</div>
<div class="line">}</div>
</div><!-- fragment --> </td><td><pre class="fragment">m2 before copy:
0.68 0.823 -0.444 -0.27
-0.211 -0.605 0.108 0.0268
0.566 -0.33 -0.0452 0.904
0.597 0.536 0.258 0.832
m2 after copy:
1 1 1 1
-0.211 1 1 1
0.566 -0.33 1 1
0.597 0.536 0.258 1
</pre> </td></tr>
</table>
<p>That works fine, but it is not very flexible. First, it only works with dynamic-size matrices of single-precision floats; the function <code>copyUpperTriangularPart()</code> does not accept static-size matrices or matrices with double-precision numbers. Second, if you use an expression such as <code>mat.topLeftCorner(3,3)</code> as the parameter <code>src</code>, then this is copied into a temporary variable of type MatrixXf; this copy can be avoided.</p>
<p>As explained in <a class="el" href="TopicFunctionTakingEigenTypes.html">Writing Functions Taking Eigen Types as Parameters</a>, both issues can be resolved by making <code>copyUpperTriangularPart()</code> accept any object of type <a class="el" href="classEigen_1_1MatrixBase.html" title="Base class for all dense matrices, vectors, and expressions. ">MatrixBase</a>. This leads to the following code:</p>
<table class="example">
<tr>
<th>Example:</th><th>Output: </th></tr>
<tr>
<td><div class="fragment"><div class="line"><span class="preprocessor">#include <Eigen/Dense></span></div>
<div class="line"><span class="preprocessor">#include <iostream></span></div>
<div class="line"></div>
<div class="line"><span class="keyword">using namespace </span>Eigen;</div>
<div class="line"></div>
<div class="line"><span class="keyword">template</span> <<span class="keyword">typename</span> Derived1, <span class="keyword">typename</span> Derived2></div>
<div class="line"><span class="keywordtype">void</span> copyUpperTriangularPart(<a class="code" href="classEigen_1_1MatrixBase.html">MatrixBase<Derived1></a>& dst, <span class="keyword">const</span> <a class="code" href="classEigen_1_1MatrixBase.html">MatrixBase<Derived2></a>& src)</div>
<div class="line">{</div>
<div class="line"> <span class="comment">/* Note the 'template' keywords in the following line! */</span></div>
<div class="line"> dst.template triangularView<Upper>() = src.template triangularView<Upper>();</div>
<div class="line">}</div>
<div class="line"></div>
<div class="line"><span class="keywordtype">int</span> main()</div>
<div class="line">{</div>
<div class="line"> <a class="code" href="classEigen_1_1Matrix.html">MatrixXi</a> m1 = <a class="code" href="classEigen_1_1DenseBase.html#a2278addf9a3c977d40322571a0df8ac9">MatrixXi::Ones</a>(5,5);</div>
<div class="line"> <a class="code" href="classEigen_1_1Matrix.html">MatrixXi</a> m2 = <a class="code" href="classEigen_1_1DenseBase.html#a8e759dafdd9ecc446d397b7f5435f60a">MatrixXi::Random</a>(4,4);</div>
<div class="line"> std::cout << <span class="stringliteral">"m2 before copy:"</span> << std::endl;</div>
<div class="line"> std::cout << m2 << std::endl << std::endl;</div>
<div class="line"> copyUpperTriangularPart(m2, m1.<a class="code" href="classEigen_1_1DenseBase.html#a6f5fc5fe9d3fb70e62d4a9b1795704a8">topLeftCorner</a>(4,4));</div>
<div class="line"> std::cout << <span class="stringliteral">"m2 after copy:"</span> << std::endl;</div>
<div class="line"> std::cout << m2 << std::endl << std::endl;</div>
<div class="line">}</div>
</div><!-- fragment --> </td><td><pre class="fragment">m2 before copy:
7 9 -5 -3
-2 -6 1 0
6 -3 0 9
6 6 3 9
m2 after copy:
1 1 1 1
-2 1 1 1
6 -3 1 1
6 6 3 1
</pre> </td></tr>
</table>
<p>The one line in the body of the function <code>copyUpperTriangularPart()</code> shows the second, more obscure use of the <code>template</code> keyword in C++. Even though it may look strange, the <code>template</code> keywords are necessary according to the standard. Without it, the compiler may reject the code with an error message like "no match
for operator<".</p>
<h1><a class="anchor" id="TopicTemplateKeywordExplanation"></a>
Explanation</h1>
<p>The reason that the <code>template</code> keyword is necessary in the last example has to do with the rules for how templates are supposed to be compiled in C++. The compiler has to check the code for correct syntax at the point where the template is defined, without knowing the actual value of the template arguments (<code>Derived1</code> and <code>Derived2</code> in the example). That means that the compiler cannot know that <code>dst.triangularPart</code> is a member template and that the following < symbol is part of the delimiter for the template parameter. Another possibility would be that <code>dst.triangularPart</code> is a member variable with the < symbol refering to the <code>operator<()</code> function. In fact, the compiler should choose the second possibility, according to the standard. If <code>dst.triangularPart</code> is a member template (as in our case), the programmer should specify this explicitly with the <code>template</code> keyword and write <code>dst.template triangularPart</code>.</p>
<p>The precise rules are rather complicated, but ignoring some subtleties we can summarize them as follows:</p>
<ul>
<li>A <em>dependent name</em> is name that depends (directly or indirectly) on a template parameter. In the example, <code>dst</code> is a dependent name because it is of type <code><a class="el" href="classEigen_1_1MatrixBase.html" title="Base class for all dense matrices, vectors, and expressions. ">MatrixBase</a><Derived1></code> which depends on the template parameter <code>Derived1</code>.</li>
<li>If the code contains either one of the contructions <code>xxx.yyy</code> or <code>xxx->yyy</code> and <code>xxx</code> is a dependent name and <code>yyy</code> refers to a member template, then the <code>template</code> keyword must be used before <code>yyy</code>, leading to <code>xxx.template yyy</code> or <code>xxx->template yyy</code>.</li>
<li>If the code contains the contruction <code>xxx::yyy</code> and <code>xxx</code> is a dependent name and <code>yyy</code> refers to a member typedef, then the <code>typename</code> keyword must be used before the whole construction, leading to <code>typename xxx::yyy</code>.</li>
</ul>
<p>As an example where the <code>typename</code> keyword is required, consider the following code in <a class="el" href="group__TutorialSparse.html">Sparse matrix manipulations</a> for iterating over the non-zero entries of a sparse matrix type:</p>
<div class="fragment"><div class="line">SparseMatrixType mat(rows,cols);</div>
<div class="line"><span class="keywordflow">for</span> (<span class="keywordtype">int</span> k=0; k<mat.outerSize(); ++k)</div>
<div class="line"> <span class="keywordflow">for</span> (SparseMatrixType::InnerIterator it(mat,k); it; ++it)</div>
<div class="line"> {</div>
<div class="line"> <span class="comment">/* ... */</span></div>
<div class="line"> }</div>
</div><!-- fragment --><p>If <code>SparseMatrixType</code> depends on a template parameter, then the <code>typename</code> keyword is required:</p>
<div class="fragment"><div class="line"><span class="keyword">template</span> <<span class="keyword">typename</span> T></div>
<div class="line"><span class="keywordtype">void</span> iterateOverSparseMatrix(<span class="keyword">const</span> SparseMatrix<T>& mat;</div>
<div class="line">{</div>
<div class="line"> <span class="keywordflow">for</span> (<span class="keywordtype">int</span> k=0; k<m1.<a class="code" href="classEigen_1_1DenseBase.html#af080d3d3f82e0d4391f19af22a5eedb8">outerSize</a>(); ++k)</div>
<div class="line"> <span class="keywordflow">for</span> (<span class="keyword">typename</span> SparseMatrix<T>::InnerIterator it(mat,k); it; ++it)</div>
<div class="line"> {</div>
<div class="line"> <span class="comment">/* ... */</span></div>
<div class="line"> }</div>
<div class="line">}</div>
</div><!-- fragment --><h1><a class="anchor" id="TopicTemplateKeywordResources"></a>
Resources for further reading</h1>
<p>For more information and a fuller explanation of this topic, the reader may consult the following sources:</p>
<ul>
<li>The book "C++ Template Metaprogramming" by David Abrahams and Aleksey Gurtovoy contains a very good explanation in Appendix B ("The typename and template Keywords") which formed the basis for this page.</li>
<li><a href="http://pages.cs.wisc.edu/~driscoll/typename.html">http://pages.cs.wisc.edu/~driscoll/typename.html</a></li>
<li><a href="http://www.parashift.com/c++-faq-lite/templates.html#faq-35.18">http://www.parashift.com/c++-faq-lite/templates.html#faq-35.18</a></li>
<li><a href="http://www.comeaucomputing.com/techtalk/templates/#templateprefix">http://www.comeaucomputing.com/techtalk/templates/#templateprefix</a></li>
<li><a href="http://www.comeaucomputing.com/techtalk/templates/#typename">http://www.comeaucomputing.com/techtalk/templates/#typename</a> </li>
</ul>
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