分析模式-计量的C++实现——完美版本

以下是http://www.csdn.net/develop/Article/14/14487.shtm的一个改进版本，本来是作为评论附在上一篇文章后面的，但是作为评论发表的话，会丢失很多文字，很奇怪的说。本文是我和singleracer讨论的结果，个人认为几乎接近完美，希望能对大家有帮助，大家可以自己加入其它的单位体系和单位。

计量的模板类：

#ifndef _QUANTITY_H_
#define _QUANTITY_H_

#ifdef _DEBUG
#define ASSERT_SAME_TYPE( T1, T2) do {T1 v=T2();v; }while(false)
#else
#define ASSERT_SAME_TYPE( T1, T2)
#endif

#include "singleton.h"

template<class UnitType>
struct UnitTraits {
 typedef UnitType::Validator Validator;
};

template<class ValueType>
class ValueTraits {
public:
 static bool equal(const ValueType & l, const ValueType & r)
 {return (l == r);}
};

template<>
class ValueTraits<double> {
public:
 static bool equal(const double & l, const double & r)
 {
  static double precision = 0.000000001;
  return (l - r < precision);
 }
};

template<>
class ValueTraits<float> {
public:
 static bool equal(const float & l, const float & r)
 {
  static float precision = 0.000000001f;
  return (l - r < precision);
 }
};

template<typename ValueType, class UnitValidator, class _Traits = ValueTraits<ValueType> >
class Quantity
{
public:
 typedef UnitValidator Validator;

 template<typename UnitType>Quantity(const ValueType & amount, const UnitType & )
  : m_amount(amount), m_unit(Singleton< unit_wrap<UnitType, ValueType> >::Instance())
 {
  ASSERT_SAME_TYPE(UnitTraits<UnitType>::Validator, Validator);
 }
 ~Quantity()
 {
 }
 Quantity(const Quantity & other)
  : m_amount(other.m_amount), m_unit(other.m_unit)
 {
 }
 Quantity & operator=(const Quantity & rhs)
 {
  if(&rhs != this)
  {
   m_amount = rhs.m_amount;
   m_unit = rhs.m_unit;
  }
  return *this;
 }
 template<typename UnitType> void convert(const UnitType & )
 {
  ASSERT_SAME_TYPE(UnitTraits<UnitType>::Validator, Validator);
  unit_base<ValueType> * unit = Singleton< unit_wrap<UnitType, ValueType> >::Instance();
  if(unit != m_unit)
  {
   m_amount = m_unit->to_standard(m_amount);
   m_unit = unit;
   m_amount = m_unit->from_standard(m_amount);
  }

 }
 Quantity & operator+=(const Quantity & rhs)
 {
  m_amount += unitize( rhs );
  return *this;
 }
 Quantity & operator-=(const Quantity & rhs)
 {
  m_amount -= unitize( rhs );
  return *this;
 }
 bool operator==(const Quantity & rhs)
 {
  return _Traits::equal( m_amount, unitize(rhs));
 }
 bool operator>(const Quantity & rhs)
 {
  return m_amount > unitize( rhs );
 }
 bool operator>=(const Quantity & rhs)
 {
  return (m_amount > unitize( rhs ) ? true : (equal( m_amount, unitize(rhs))) );
 }
 bool operator<(const Quantity & rhs)
 {
  return m_amount < unitize( rhs );
 }
 bool operator<=(const Quantity & rhs)
 {
  return (m_amount < unitize( rhs ) ? true : (equal( m_amount, unitize(rhs))) );
 }

private:
 // 对于单位相同的运算不需要转换，也就没有丢失精度的问题
 ValueType unitize( const Quantity & rhs )
 {
  if( m_unit == rhs.m_unit )
   return rhs.m_amount;
  else
   return m_unit->from_standard(rhs.m_unit->to_standard(rhs.m_amount));
 }

private:
 template<typename ValueType>
 class unit_base
 {
 public:
  virtual ValueType to_standard(const ValueType & val) const = 0;
  virtual ValueType from_standard(const ValueType & val) const = 0;
 };
 // 把unit_wrap设计为singleton，这样比较单位是否相同只需作简单的指针比较
 template<typename UnitType, typename ValueType>
 class unit_wrap : public unit_base<ValueType>
 {
protected:
 unit_wrap() {}
 ~unit_wrap() {}
 public:
  virtual ValueType to_standard(const ValueType & val) const
  {
   return UnitType::to_standard(val);
  }
  virtual ValueType from_standard(const ValueType & val) const
  {
   return UnitType::from_standard(val);
  }
 };

private:
 ValueType    m_amount;
 unit_base<ValueType> * m_unit;
};

#define QUANTITY Quantity<ValueType,Validator,Traits>
template<typename ValueType,typename Validator,typename Traits>
QUANTITY inline _cdecl operator+(const QUANTITY & a,
           const QUANTITY & b)
{return (QUANTITY(a)+=b);}

template<typename ValueType,typename Validator,typename Traits>
QUANTITY inline _cdecl operator-(const QUANTITY & a, const QUANTITY & b)
{return (QUANTITY(a)-=b);}

#endif /* _QUANTITY_H_ */

重量：
#ifndef _WEIGHT_H_
#define _WEIGHT_H_

#include "../Quantity.h"

class WeightValidator {};
typedef Quantity<double, WeightValidator> Weight;

/*
template<typename ValueType, typename UnitType>
Weight inline _cdecl operator*(const ValueType& value, const UnitType& unit )
{return Weight(value, unit);}
*/

class Kilogram
{
public:
 typedef WeightValidator Validator;
 static double to_standard(double val)
 {return val;}
 static double from_standard(double val)
 {return val;}
};

class Gramme
{
public:
 typedef WeightValidator Validator;
 static double to_standard(double val)
 {return val/1000.0;}
 static double from_standard(double val)
 {return val*1000.0;}
};

Kilogram _kg;
Gramme _g;

#endif /* _WEIGHT_H_ */

长度：
#ifndef _LENGTH_H_
#define _LENGTH_H_

#include "../Quantity.h"

class LengthValidator {};
typedef Quantity<double, LengthValidator> Length;

/*
template<typename ValueType, typename UnitType>
Length inline _cdecl operator*(const ValueType& value, const UnitType& unit )
{return Length(value, unit);}
*/

class Meter
{
public:
 typedef LengthValidator Validator;
 static double to_standard(double val)
 {return val;}
 static double from_standard(double val)
 {return val;}
};

class Kilometer
{
public:
 typedef LengthValidator Validator;
 static double to_standard(double val)
 {return val*1000.0;}
 static double from_standard(double val)
 {return val/1000.0;}
};

Meter _m;
Kilometer _km;

#endif /* _LENGTH_H_ */

测试代码：
// Quantity.cpp : Defines the entry point for the consol
93da
e application.
//

#include "stdafx.h"
#include "weight.h"
#include "length.h"

void testWeight()
{
 Weight w1(1.0, _kg); //构造函数
 Weight w2(1.0, _g); //构造函数

 Weight w3 = w1; //拷贝构造函数
 w3 = w2;  //赋值操作

 Weight w4(1.0, _kg);
 w4 += w1;  //相同单位相加
 w4 += w2;  //不同单位相加
 Weight w5 = w4 + w1;  //相同单位相加
 w5 = w4 + w2;  //不同单位相加

 Weight w6(2.0, _kg);
 w6 -= w1;  //相同单位相减
 w6 -= w2;  //不同单位相减
 Weight w7 = w5 - w1;  //相同单位相减
 w7 = w5 - w2;  //不同单位相减

 // 比较
 Weight a(1.0, _kg);
 Weight b(2.0, _kg);
 Weight c(2000.0, _g);
 Weight d(3000.0, _g);
 bool f = (a < b);
 f = (a < c);
 f = (b==c);
 f = (b < d);

 //单位换算
 Weight w10(1.0, _kg);
 w10.convert(_g);
 w10.convert(_kg);
}

void testLength()
{
 Length l1(1.0, _km); //构造函数
 Length l2(1.0, _m); //构造函数

 Length l3 = l1; //拷贝构造函数
 l3 = l2;  //赋值操作

 Length l4(1.0, _km);
 l4 += l1;  //相同单位相加
 l4 += l2;  //不同单位相加
 Length l5 = l4 + l1;  //相同单位相加
 l5 = l4 + l2;  //不同单位相加

 Length l6(2.0, _km);
 l6 -= l1;  //相同单位相减
 l6 -= l2;  //不同单位相减
 Length l7 = l5 - l1;  //相同单位相减
 l7 = l5 - l2;  //不同单位相减

 // 比较
 Length a(1.0, _km);
 Length b(2.0, _km);
 Length c(2000.0, _m);
 Length d(3000.0, _m);
 bool f = (a < b);
 f = (a < c);
 f = (b==c);
 f = (b < d);

 //单位换算
 Length l10(1.0, _km);
 l10.convert(_m);
 l10.convert(_km);
}

void testError()
{
#if(0)
 Weight w1; // 没有缺省构造函数
 Length l1; // 没有缺省构造函数
#endif
#if(0)
 Weight w2(1.0, _km); //错误的单位
 Length l2(1.0, _g);  //错误的单位
#endif
#if(0)
 Weight w3(1.0, _kg);
 Length l3(1.0, _m);
 w3.convert(_km); //错误的单位
 l3.convert(_g);  //错误的单位
#endif
#if(0)
 Weight w4(1.0, _kg);
 Length l4(1.0, _m);
 w4 += l4; //错误的类型
 w4 -= l4; //错误的类型
#endif
#if(0)
 Weight w5(1.0, _kg);
 Length l5(1.0, _m);
 bool f = (w5 == l5);
 f = (w5 > l5);
 f = (w5 < l5);
#endif
}

int main(int argc, char* argv[])
{
 testWeight();
 testLength();
 testError();
 return 0;
}