is recursive explicit template instantiation possible?
Given a template like
template<int dim> class Point { ... };
this template can be instantiated explicitly like
template class Point<0>;
template class Point<1>;
template class Point<2>;
template class Point<3>;
instead of instantiating every template individually like above, I would like to instantiate them recursively with one call like
template class RecursiveInstantiate<Point, 3>;
where RecursiveInstantiate<T, i>
would instantiate T<i>
, T<i-1>
, ..., T<0>
. Is it somehow possible to create such a class RecursiveInstantiate
? If it is not possible, do you know a way to do it with the preprocessor?
In fact I am interested in generalizing this for classes with multiple template parameters likeNode<int i1,int i2,int i3>
for all combination of i1,i2,i3 in {0,1,2,3}. But I hope to be able to work out this second part by myself.
Any advice, also an explanation why it is impossible what I want to achieve is appreciated.
Update: thank you for your comments so far. I see now more clearly where the problem really is. The line
template class Point<3>;
instantiates the template and e开发者_开发百科xports its symbols to the object file. An instantiation of the form
template class RecursiveInstantiate<Point, 3>;
may instantiate the classes class Point<3>
, class Point<2>
, .... Apparently this only happens locally though. The templates are not exported to the object file. Maybe I will have to look for a solution using the preprocessor.
As I see now that I did not ask my question precisely enough in the beginning, I appreciate your answers and selected ones as correct.
Note: I am trying this on linux with g++/clang as compilers.
You could make a little Instantiator class:
template <unsigned int N> struct Instantiator
{
Point<N> p;
Instantiator<N-1> i;
};
template <> struct Instantiator<0>
{
Point<0> p;
};
Then simply add one explicit instantiation: template struct Instantiator<81>;
You can extend this idea lexicographically to any number of integral parameters.
As @Georg says, let's make it generic:
template <template <unsigned int> class T, unsigned int N> struct Instantiator
{
T<N> t;
Instantiator<T, N-1> i;
};
template <template <unsigned int> class T> struct Instantiator<T, 0>
{
T<0> t;
};
template struct Instantiator<Point, 82>;
You can do that like this:
template<int dim> struct Point {
static const int val = dim;
Point<dim - 1> p;
};
template<> struct Point<0> { ... };
That creates a template specialisation for the template parameter when it is 0
so the recursion stops there, and when you instantiate one like this:
Point<4>
It instantiates from Point<4>
down to Point<0>
. Then you can do
Point<4>::val
to access the value of that particular one.
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