variadic-template-fizzbuzz-c++0x.cpp

#include <iostream>
using namespace std;

namespace vtstring {

// 文字列を表す可変引数テンプレート
template <char... a> struct vtstring;
template <char a, char... b>
struct vtstring<a, b...>
{
	static const int size = 1 + vtstring<b...>::size; // サイズ
	
	// C文字列を返す関数 (定数はダメ。初期化リスト未サポートのため？)
	static const char* c_str() {
		static const char ret[] = {a,b...,0}; return ret;
	}
};
// NULL
template <>
struct vtstring<>
{
	static const int size = 0;
	static const char* c_str() { return ""; }
};

// 文字列の再帰的結合
template <class A, class... B>
struct join;
// 再帰
template <class A, class B, class... C>
struct join<A, B, C...>
{
	typedef typename join<typename join<A,B>::result,
			      C...>::result result;
};
// 結合
template <char... a, char... b>
struct join<vtstring<a...>, vtstring<b...>>
{
	typedef vtstring<a..., b...> result;
};

// 数値→文字列変換
template <int n>
struct from_int
{ // 10以上の場合
	typedef typename from_int<n/10>::result tens;
	typedef typename from_int<n%10>::result ones;
	typedef typename join<tens, ones>::result result;
};
// 10未満の場合
#define DEF_FROM_INT(N) \
template <> struct from_int<N> { typedef vtstring<N+'0'> result; }
DEF_FROM_INT(0);
DEF_FROM_INT(1);
DEF_FROM_INT(2);
DEF_FROM_INT(3);
DEF_FROM_INT(4);
DEF_FROM_INT(5);
DEF_FROM_INT(6);
DEF_FROM_INT(7);
DEF_FROM_INT(8);
DEF_FROM_INT(9);

};


// FizzBuzz
template <int i>
struct fizzbuzz {
	template <int n, int mod3, int mod5>
	struct fizzbuzz_ {
		typedef typename vtstring::from_int<n>::result result;
	};
	template <int n, int mod5>
	struct fizzbuzz_<n, 0, mod5> {
		typedef vtstring::vtstring<'F','i','z','z'> result;
	};
	template <int n, int mod3>
	struct fizzbuzz_<n, mod3, 0> {
		typedef vtstring::vtstring<'B','u','z','z'> result;
	};
	template <int n>
	struct fizzbuzz_<n, 0, 0> {
		typedef typename vtstring::vtstring<'F','i','z','z',
						    'B','u','z','z'> result;
	};
	
	typedef typename vtstring::join<
				typename fizzbuzz_<i, i%3, i%5>::result,
				vtstring::vtstring<'\n'>
			>::result result;
};


// i〜maxをループしてTを適用(map)し, Mでfold
template <int i, int max, template<int n> class T,
	  template<class... A> class M>
struct loopN
{
	typedef typename M<
			typename T<i>::result,
			typename loopN<i+1, max, T, M>::result
		>::result result;
};
// 最後のループ
template <int max, template<int n> class T,
	  template<class... A> class M>
struct loopN<max, max, T, M>
{
	typedef typename T<max>::result result;
};


int main()
{
	// Hello World
	typedef vtstring::vtstring<'H','e','l','l','o',','> str;
	typedef vtstring::vtstring<'W','o','r','l','d','!'> str2;
	typedef vtstring::join<str, str2>::result ret;
	cout << ret::c_str() << endl;
	
	// length of vtstring
	cout << "length: " << ret::size << endl;
	
	// Null string
	typedef vtstring::vtstring<> null;
	cout << null::c_str();
	
	// int => vtstring
	cout << vtstring::from_int<100>::result::c_str() << endl;
	
	cout << "---" << endl;
	
	// fizzbuzz
	cout << loopN<1,100,fizzbuzz,vtstring::join>::result::c_str();
	
	return 0;
}