Template-ing a 'for' loop in C++?

Question

I have a C++ snippet below with a run-time for loop,

for(int i = 0; i < I; i++)
  for (int j = 0; j < J; j++)
    A( row(i,j), column(i,j)         


        

Answer 1

This is the way to do it directly:

template 
struct inner
{
  static void value()
  {
    A(row::value, column::value) = f::value;
    inner::value();
  }
};

template  struct inner { static void value() {} };

template 
struct outer
{
  static void value()
  {
    inner::value();
    outer::value();
  }
};

template <> struct outer { static void value() {} };

void test()
{
  outer<0>::value();
}

You can pass A through as a parameter to each of the values if necessary.

Here's a way with variadic templates that doesn't require hard coded I and J:

#include 

template 
struct Inner;

template 
struct Outer;

template 
struct Inner>
{
  static void value() { (A(column::value, row::value), ...); }
};


template 
struct Outer, Columns>
{
  static void value() { (Inner::value(), ...); }
};

template 
void expand()
{
  Outer, std::make_index_sequence>::value();
}

void test()
{
  expand<3, 5>();
}

(snippet with generated assembly: https://godbolt.org/g/DlgmEl)