Re: Template Metaprogramming

Luc Danton <>
Fri, 22 Oct 2010 02:28:54 +0200
On 22/10/2010 02:17, Luc Danton wrote:

On 22/10/2010 02:06, Bushido Hacks wrote:

On Oct 21, 6:04 pm, Luc Danton<> wrote:

 On 21/10/2010 19:05, Bushido Hacks wrote:

 I learned about Template Metaprogramming (TMP) when I was browsing
 Wikipedia a few days ago. According to the article it was an
 accidental discovery, but to me it is more like an open


 to do things with my code that I though would only cause trouble.

 The Turing-completeness of TMP makes it a definite


 I've written up some ideas and posted them on Pastebin. I

would like

 to know if these are feasible.

 Looking at all your code so far, I fail to see how this qualify as


 Rather, it looks like it's an attempt at generic programming.

 Are you familiar with the canonical TMP example?


 template<int N>
 struct factorial {
     static const int value = N * factorial<N - 1>::value;


 struct factorial<0> {
     static const int value = 0;


     std::cout<< factorial<5>::value<< '\n';


I find that to be a terrible example because it doesn't use a class
structure, nor displays the usage of TMP with Constructors, Copy
constructors, Destructors, and Operators as well as private items.

It can be rewritten as

template<int N>
class factorial {
   static const int value = N * factorial<N - 1>::value;

if it suits your fancy. Similarly there are no constructors or members
and the like because there never is an instance of factorial in the
program. What good are constructors and private data members if I never
use an instance? You'd need C++0x with constexpr functions, constexpr
constructors and user-defined literals to (possibly) make use of objects
with TMP.

Actually I went ahead and here is a version that:
- uses the class keyword
- has a constructor
- has a copy constructor
- has a virtual destructor
- has an operator
- has private items
Those are all declared private and not defined to better enforce
encapsulation, too.

template<int N>
class factorial {
   static const int value = factorial<N - 1>::value;


   virtual ~factorial(); // decombobulator

   factorial(factorial const&); // copy constructor

   // copy assignment op
   factorial& operator=(factorial const&);

   int member;
   double another_member;
   float yet_another_member;

The specialization for N = 0 is left as an exercise to the reader.

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