How do I refactor this loop using template metaprogramming? - c++

Question

I am new to template meta-programming but I'm trying to refactor some matrix manipulation code for a speed boost. In particular, right now my function looks like this:
template<int SIZE> void do_something(matrix A) {
for (int i = 0; i < SIZE; ++i) {
// do something on column i of A
}
}
I saw some techniques that use templates to rewrite this as
#define SIZE whatever
template<int COL> void process_column(matrix A) {
// do something on column COL of A
process_column<COL + 1>(A);
}
template<> void process_column<SIZE - 1>(matrix A) {
return;
}
void do_something(matrix A) {
process_column<0>(A);
}
When I did that to my function and set compiler flags to inline appropriately, I saw a pretty decent (~10%) speed boost. But the problem is that SIZE is #defined not a template parameter and I will definitely be using different sizes in my program. So I want something like
template<int COL, int SIZE> void process_column(matrix A) {
// do something on column COL of A
process_column<COL + 1, SIZE>(A);
}
/* HOW DO I DECLARE THE SPECIFIC INSTANCE????
The compiler rightfully complained when I tried this: */
template<int SIZE> void process_column<SIZE - 1, SIZE>(matrix A) {
return;
}
template<int SIZE> void do_something(matrix A) {
process_column<0, SIZE>(A);
}
How do I declare the specific instance to get the loop to terminate? Thanks in advance!

Answer 1

You cannot partially specialize a template function
but you can for template class.
Following may help you:
namespace detail {
template<int COL, int SIZE>
struct process_column
{
static void call(matrix& A) {
// do something on column COL of A
process_column<COL + 1, SIZE>::call(A);
}
};
template<int SIZE>
struct process_column<SIZE, SIZE> // Stop the recursion
{
static void call(matrix& A) { return; }
};
} // namespace detail
template<int SIZE> void do_something(matrix& A) {
detail::process_column<0, SIZE>::call(A);
}
An alternative with C++11:
#if 1 // Not in C++11, but present in C++1y
#include <cstdint>
template <std::size_t ...> struct index_sequence {};
template <std::size_t I, std::size_t ...Is>
struct make_index_sequence : make_index_sequence<I - 1, I - 1, Is...> {};
template <std::size_t ... Is>
struct make_index_sequence<0, Is...> : index_sequence<Is...> {};
#endif
namespace details {
template <template <std::size_t> class T, std::size_t ... Is, typename ... Args>
void for_each_column_apply(const index_sequence<Is...>&, Args&&...args)
{
int dummy[] = {(T<Is>()(std::forward<Args>(args)...), 0)...};
static_cast<void>(dummy); // remove warning for unused variable
}
} // namespace details
template <template <std::size_t> class T, std::size_t N, typename ... Args>
void for_each_column_apply(Args&&... args)
{
details::for_each_column_apply<T>(index_sequence<N>(), std::forward<Args>(args)...);
}
Usage:
class Matrix {};
template <std::size_t COL>
struct MyFunctor
{
void operator() (Matrix&m /* other needed args*/) const
{
// Do the job for Nth column
}
};
int main() {
constexpr SIZE = 42;
Matrix m;
for_each_column_apply<MyFunctor, SIZE>(m /* other args needed by MyFunctor*/);
return 0;
}