TITLE: Template metaprograms, or abusing your compiler

[ This material taken from "Using C++ Template Metaprograms", by Todd
  Veldhuizen, C++ Report, May 95, Vol 7, No 4, pp 36-43. ]

  This shows a quirky, interesting little technique for those who have
  tight performance requirements requiring such optimizations as 
  loop unrolling. The obtuse use of templates and associated loss
  of clarity does not make this a useful technique for general 
  programming requirements. It relies on knowing key information at
  compile time. An example will make this clearer...


	template <int N> 
	class Factorial 
	{
	  public:
		enum { value = N * Factorial<N-1>::value };
	};

	class Factorial<1>
	{
	  public:
		enum { value = 1 };
	};

  The article cited above contains examples for bubblesort and explains
  how to implement compile-time control flow structures (if/else, switch,
  loop) and compile-time functions. In addition to the simple example
  below, the article shows a compile-time sine function and alludes to
  a compile-time FFT!

  Consider the example compile-time function below to count the number
  of set bits in the lower nibble of an integer

	template<int N>
	class countBits
	{
		enum {
			bit3 = (N & 0x08) ? 1 : 0,
			bit2 = (N & 0x04) ? 1 : 0,
			bit1 = (N & 0x02) ? 1 : 0,
			bit0 = (N & 0x01) ? 1 : 0
		};
	  public:
		enum { nbits = bit0+bit1+bit2+bit3 };
	};

	...
	int i = countBits<13>::nbits;
	...