CGAL 5.4 - STL Extensions for CGAL
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#include <CGAL/Default.h>
Default
is a tag class.
It can be used to state that one wants to use the default argument of a template parameter of a class template.
This can be useful in several cases: (a) when one needs a non-default value for another template parameter coming next (since C++ only supports defaults at the end of lists), (b) when the default is actually a complex expression, e.g. refering to previous template parameters (in this case, it shortens compiler error messages and mangled symbol names), (c) when defining the default involves circular dependencies of type instantiations (there, it breaks the cycle in a nice way).
Using the mechanism is easy: just plug Default
as template argument in the place where you would like to use the default. You should refer to the documentation of the template class you are using in order to know whether this functionality is offered.
Also beware that the type of the instantiated template class will not be the same when instantiating it using Default
instead of the type of the default argument, even though their interfaces will otherwise be the same. This may have consequences in some cases.
In order to help the template class writer, Default
provides a convenient way to extract the desired type for a template parameter which may be defaulted using Default
. It is enough to fetch the type as Default::Get<Parameter, Type>::type
, as in the example program below.
Example
File STL_Extension/Default.cpp
Classes | |
struct | Get |
A nested template providing a typedef type which equals Type if Parameter is Default , and Parameter otherwise. More... | |
struct | Lazy_get |
A nested template providing a typedef type which equals MetaFct::type if Parameter is Default , and Parameter otherwise. More... | |