doc+example for CGAL::Default.

STL_Extension/doc_tex/STL_Extension/main.tex

@@ -28,6 +28,8 @@ arguments and for composition, functors for projection and creation and
 adaptor classes around iterators and circulators. See also circulators in
 Chapter~\ref{chapterCirculators}.  A class storing polymorphic objects
 is also provided, as well as a class to manage the uncertainty of some values.
+Finally, a class which helps specifying that the default types in template
+parameter lists are desired is also provided.
 
 \section{Doubly-Connected List Managing Items in Place}
 
@@ -133,3 +135,11 @@ but it is important that providers of predicates that are meant to be
 filtered by \ccc{Filtered_predicate}, know about it.
 
 It can also be used in other contexts as well, as it is a general tool.
+
+\section{Default types in template arguments lists}
+
+In \CC, it is possible to specify defaults at the end of a template argument
+list.  Specifying that one wishes to use the default is simply done by omitting
+it.  This is however possible only at the end of the list.  \ccc{CGAL::Default}
+provides a simple mechanism that performs something equivalent anywhere in the
+sequence.

STL_Extension/doc_tex/STL_Extension_ref/Default.tex

@@ -0,0 +1,45 @@
+\begin{ccRefClass}{Default}
+
+\ccDefinition
+\ccRefName\ is a tag class to be used in a template parameter list, in order to
+indicate that the default value is to be considered.
+
+This is shorter and clearer than actually writing the default type, which may
+sometimes be complicated, and might even change in the future. The \CC\ language
+only provides a possibility when no following argument needs to be non-defaulted.
+
+It can also be useful to shorten compiler error messages and mangled symbol names,
+and also to help define defaults in rare cases that involve circular
+dependencies, by breaking the cycle in a simple way.
+
+Note that there is no magic : this only works if the template class where
+\ccRefName\ is plugged-in takes special care for it, so you should refer to the
+manual of such classes to know whether this possibility is offered.
+
+Beware that, of course, the resulting template type parameterized by \ccRefName\ will
+not formally be the same type as the one parameterized by the actual documented,
+expanded, default.  This may have consequences.
+
+\ccIsModel
+\ccc{DefaultConstructible, CopyConstructible}
+
+\begin{ccAdvanced}
+
+\ccTypes
+
+In order to help the template class writer, \ccRefName\ provides a
+convenient way to extract the desired value for an argument which may be
+defaulted using \ccRefName.
+
+\ccNestedType{template <typename Argument, typename Value> struct Get;}
+{This nested template offers a further nested type \ccc{type} which is either
+equal to \ccc{Argument} if \ccc{Argument} is not \ccRefName\, or
+\ccc{Value} otherwise.}
+
+\end{ccAdvanced}
+
+\ccHeading{Example code}
+
+\ccIncludeExampleCode{STL_Extension/Default.cpp}
+
+\end{ccRefClass} 

STL_Extension/doc_tex/STL_Extension_ref/main.tex

@@ -32,6 +32,7 @@
 \input{STL_Extension_ref/stl_extension.tex}
 \input{STL_Extension_ref/Uncertain.tex}
 \input{STL_Extension_ref/array.tex}
+\input{STL_Extension_ref/Default.tex}
 
 %%\cgalColumnLayout
 

STL_Extension/examples/STL_Extension/Default.cpp

@@ -0,0 +1,18 @@
+#include <CGAL/Default.h>
+
+struct A {};
+
+template < typename A1_ = A, // we could also write CGAL::Default here instead
+           typename A2 = int >
+struct B
+{
+    typedef typename CGAL::Default::Get<A1_, A>::type  A1;
+    A1 a1;
+};
+
+int main ()
+{
+    B<CGAL::Default, double> b;
+
+    A a = b.a1; //  It is really of type A.
+}