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C++元程序设计

metaprogramming templates c++

Yippie-Ki-Yay · 技术社区 · 16 年前

我有以下问题:

假设我有一些基本的柜台 class Counter . 假设我们还有一些 类,可以计数。 我们来列举一些 class CountedA 和 class CountedB .

现在,每节课 计算(例如 CountedA 和 CountedB ) 具有以下静态声明的部件: 一 enum 一个 int 部分,就像 计数数据 .

例如,它的声明可以如下所示:

enum CountedType { A, B };

template <CountedType Type, int N>
class Counted { };

// Now we can declare 'CountedA' and 'CountedB'
typedef Counted<A, 25> CountedA;
typedef Counted<B, 7> CountedB;

现在,柜台的声明:

// C++0x variadic or simply bunch of 'typename XX' definitions for C++03
template <typename T0, typename T1, typename ...>
class Counter
{
   // I don't know how to implement this
   // for now!
   int GetTotalN() { ... }

   // Retrieve the corresponding type
   // so that GetTypeAt<0> returns
   // enum from 'T0'
   template <int Pos>
   CountedType GetTypeAt() { ... }
};

我希望能够写一些像:

class RealCounter : public Counter<CountedA, CountedB> { };

使用方法如下:

RealCounter counter;
int n = counter.GetTotalN();
CountedType type = counter.GetTypeAt<0>();

现在,我确信这是可以做到的。但实现它的最佳方法是什么? (别问我为什么需要这么疯狂的东西。)

做 boost::mpl 为这个案子做点什么?

谢谢您。

小更新:

在这个特定的例子中, GetTotalN() 应该返回 25 + 7 .

例如,如果我们添加, typedef Counted<C, 2> CountedC ,然后是结果

RealCounter : public Counter<CountedA, CountedB, CountedC>

应该成为 25 + 7 + 2 .

2 回复 | 直到 14 年前

aschepler 16 年前

enum CountedType { A, B };

template <CountedType Type, int N>
struct Counted {};

struct DummyCounted {};

template <int Pos, typename T>
struct IndexedType {};

template <unsigned int Terms>
struct PartialSum
{
  template <typename CounterT>
  static int getSum(const CounterT& ctr)
  { return PartialSum<Terms-1>::getSum(ctr) + ctr.template GetNAt<Terms>(); }
};

template <> struct PartialSum<0U>
{
  template <typename CounterT>
  static int getSum(const CounterT& ctr)
  { return ctr.template GetNAt<0>(); }
};

template <typename T0, typename T1=DummyCounted,
  typename T2=DummyCounted, typename T3=DummyCounted,
  typename T4=DummyCounted, typename T5=DummyCounted,
  typename T6=DummyCounted, typename T7=DummyCounted,
  typename T8=DummyCounted, typename T9=DummyCounted>
class Counter :
  public IndexedType<0, T0>, public IndexedType<1, T1>,
  public IndexedType<2, T2>, public IndexedType<3, T3>,
  public IndexedType<4, T4>, public IndexedType<5, T5>,
  public IndexedType<6, T6>, public IndexedType<7, T7>,
  public IndexedType<8, T8>, public IndexedType<9, T9>
{
public:
  static int GetTotalN() {
    return PartialSum<9>().getSum( Counter() );
  }

  template <int Pos>
  static CountedType GetTypeAt() { return _getTypeAt<Pos>( Counter() ); }

  template <int Pos>
  static int GetNAt() { return _getNAt<Pos>( Counter() ); }

private:
  template <int Pos, CountedType Type, int N>
  static CountedType _getTypeAt(const IndexedType<Pos, Counted<Type,N> >&)
  { return Type; }

  template <int Pos, CountedType Type, int N>
  static int _getNAt(const IndexedType<Pos, Counted<Type,N> >&)
  { return N; }

  template <int Pos>
  static int _getNAt(const IndexedType<Pos, DummyCounted>&)
  { return 0; }

};

#include "Counter.hpp"
#include <iostream>

typedef Counted<A, 25> CountedA;
typedef Counted<B, 7> CountedB;

class RealCounter : public Counter<CountedA, CountedB> {};

int main()
{
  RealCounter counter;
  int n = counter.GetTotalN();
  CountedType type = counter.GetTypeAt<0>();

  std::cout << "n is " << n
            << "\ntype check is " << (type == A) << std::endl;
  return 0;
}

n is 32
type check is 1

main constexpr

Community Mohan Dere 9 年前

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