分析模式-计量的C++实现——回复ch0877

ch0877翻译了一篇名叫《分析模式-计量》的文章（http://www.csdn.net/Develop/article/14/14449.shtm），很不错。我就试着用C++实现了一下。

以下是源代码：

#ifndef _QUANTITY_H_#define _QUANTITY_H_

#include <crtdbg.h>

template<typename ValueType>class Quantity{public: template<typename UnitType>Weight(const ValueType & amount, const UnitType & unit)  : m_amount(amount), m_unit(new unit_wrap<UnitType, ValueType>(unit)) { } ~Quantity() {  delete m_unit; } Quantity(const Quantity & other)  : m_amount(other.m_amount), m_unit(other.m_unit ? other.m_unit->clone() : 0) { } Quantity & operator=(const Quantity & rhs) {  if(&rhs == this)   return *this;  m_amount = rhs.m_amount;  delete m_unit;  m_unit = rhs.m_unit ? rhs.m_unit->clone() : 0;  return *this; } template<typename UnitType> void convert(const UnitType & new_unit) {  _ASSERT(m_unit);  m_amount = m_unit->to_standard(m_amount);  delete m_unit;  m_unit = new unit_wrap<UnitType, double>(new_unit);  m_amount = m_unit->from_standard(m_amount); } Quantity & operator+=(const Quantity & rhs) {  _ASSERT(m_unit);  _ASSERT(rhs.m_unit);  m_amount = m_unit->from_standard(   m_unit->to_standard(m_amount) + rhs.m_unit->to_standard(rhs.m_amount) );  return *this; } Quantity & operator-=(const Quantity & rhs) {  _ASSERT(m_unit);  _ASSERT(rhs.m_unit);  m_amount = m_unit->from_standard(   m_unit->to_standard(m_amount) - rhs.m_unit->to_standard(rhs.m_amount) );  return *this; } bool operator==(const Quantity & rhs) {  _ASSERT(m_unit);  _ASSERT(rhs.m_unit);  return equal(m_unit->to_standard(m_amount) , rhs.m_unit->to_standard(rhs.m_amount) ); } bool operator>(const Quantity & rhs) {  _ASSERT(m_unit);  _ASSERT(rhs.m_unit);  return (m_unit->to_standard(m_amount) > rhs.m_unit->to_standard(rhs.m_amount) ); }

private: template<typename T>bool equal(const T & l, const T & r) {return (l == r);} template<>bool equal<double>(const double & l, const double & r) {  static double precision = 0.000000001;  return (l - r < precision); }

private: template<typename ValueType> class unit_base { public:  virtual ~unit_base()  {  } public:  virtual unit_base * clone() const = 0;  virtual ValueType to_standard(const ValueType & val) const = 0;  virtual ValueType from_standard(const ValueType & val) const = 0; }; template<typename UnitType, typename ValueType> class unit_wrap : public unit_base<ValueType> { public:  unit_wrap(const UnitType & unit) : m_content(unit)  {  } public:  virtual unit_base<ValueType> * clone() const  {   return new unit_wrap(m_content);  }  virtual ValueType to_standard(const ValueType & val) const  {   return m_content.to_standard(val);  }  virtual ValueType from_standard(const ValueType & val) const  {   return m_content.from_standard(val);  } private:  UnitType m_content; };

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

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

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

#endif /* _QUANTITY_H_ */

以下是示例代码：

typedef Quantity<double> Weight;class UNIT_KG{public: double to_standard(double val) const {return val;} double from_standard(double val) const {return val;}};

class UNIT_G{public: double to_standard(double val) const {return val/1000.0;} double from_standard(double val) const {return val*1000.0;}};

UNIT_KG _kg;UNIT_G _g;

 Weight w1(1.0, _kg); Weight w2(1.0, _g); w1.convert(_g); w1.convert(_kg); w2 = w1; Weight w10(1.0, _kg); Weight w20(500.0, _g); w10 +=  w20; Weight w30(w10 + w20); bool b = (w10 == w20); b = (w30 > w10);