更多 C++ 习语/代数层次结构
将多个紧密相关的代数抽象（数字）隐藏在一个通用的抽象后面，并提供一个通用的接口。
- 状态（Gamma 等人）
在像 Smalltalk 这样的纯面向对象语言中，变量是对充当标签的对象的运行时绑定。将变量绑定到对象就像在对象上贴上标签。这些语言中的赋值类似于从一个对象上剥下标签，然后将它贴到另一个对象上。另一方面，在 C 和 C++ 中，变量是地址或偏移量的同义词，而不是对象的标签。赋值并不意味着重新标记，它意味着用新内容覆盖旧内容。代数层次结构习语使用委托多态来模拟 C++ 中的弱变量到对象绑定。代数层次结构在其实现中使用了信封信函习语。这个习语背后的动机是能够编写像这样的代码
Number n1 = Complex (1, 2); // Label n1 for a complex number.
Number n2 = Real (10); // Label n2 for a real number.
Number n3 = n1 + n2; // Result of addition is labelled n3.
Number n2 = n3; // Re-labelling.
显示代数层次结构习语实现的完整代码
#include <iostream>
struct BaseConstructor { BaseConstructor(int=0) {} };
class RealNumber;
class Complex;
class Number;
class Number
{
friend class RealNumber;
friend class Complex;
public:
Number & operator = (const Number &n);
Number (const Number &n);
virtual ~Number();
virtual Number operator + (Number const &n) const;
void swap (Number &n) throw ();
static Number makeReal (double r);
static Number makeComplex (double rpart, double ipart);
protected:
Number ();
Number (BaseConstructor);
private:
void redefine (Number *n);
virtual Number complexAdd (Complex const &n) const;
virtual Number realAdd (RealNumber const &n) const;
Number *rep;
short referenceCount;
};
class Complex : public Number
{
friend class RealNumber;
friend class Number;
Complex (double d, double e);
Complex (const Complex &c);
virtual ~Complex ();
virtual Number operator + (Number const &n) const;
virtual Number realAdd (RealNumber const &n) const;
virtual Number complexAdd (Complex const &n) const;
double rpart, ipart;
};
class RealNumber : public Number
{
friend class Complex;
friend class Number;
RealNumber (double r);
RealNumber (const RealNumber &r);
virtual ~RealNumber ();
virtual Number operator + (Number const &n) const;
virtual Number realAdd (RealNumber const &n) const;
virtual Number complexAdd (Complex const &n) const;
double val;
};
/// Used only by the letters.
Number::Number (BaseConstructor)
: rep (0),
referenceCount (1)
{}
/// Used by static factory functions.
Number::Number ()
: rep (0),
referenceCount (0)
{}
/// Used by user and static factory functions.
Number::Number (const Number &n)
: rep (n.rep),
referenceCount (0)
{
std::cout << "Constructing a Number using Number::Number" << std::endl;
if (n.rep)
n.rep->referenceCount++;
}
Number Number::makeReal (double r)
{
Number n;
n.redefine (new RealNumber (r));
return n;
}
Number Number::makeComplex (double rpart, double ipart)
{
Number n;
n.redefine (new Complex (rpart, ipart));
return n;
}
Number::~Number()
{
if (rep && --rep->referenceCount == 0)
delete rep;
}
Number & Number::operator = (const Number &n)
{
std::cout << "Assigning a Number using Number::operator=" << std::endl;
Number temp (n);
this->swap (temp);
return *this;
}
void Number::swap (Number &n) throw ()
{
std::swap (this->rep, n.rep);
}
Number Number::operator + (Number const &n) const
{
return rep->operator + (n);
}
Number Number::complexAdd (Complex const &n) const
{
return rep->complexAdd (n);
}
Number Number::realAdd (RealNumber const &n) const
{
return rep->realAdd (n);
}
void Number::redefine (Number *n)
{
if (rep && --rep->referenceCount == 0)
delete rep;
rep = n;
}
Complex::Complex (double d, double e)
: Number (BaseConstructor()),
rpart (d),
ipart (e)
{
std::cout << "Constructing a Complex" << std::endl;
}
Complex::Complex (const Complex &c)
: Number (BaseConstructor()),
rpart (c.rpart),
ipart (c.ipart)
{
std::cout << "Constructing a Complex using Complex::Complex" << std::endl;
}
Complex::~Complex()
{
std::cout << "Inside Complex::~Complex()" << std::endl;
}
Number Complex::operator + (Number const &n) const
{
return n.complexAdd (*this);
}
Number Complex::realAdd (RealNumber const &n) const
{
std::cout << "Complex::realAdd" << std::endl;
return Number::makeComplex (this->rpart + n.val,
this->ipart);
}
Number Complex::complexAdd (Complex const &n) const
{
std::cout << "Complex::complexAdd" << std::endl;
return Number::makeComplex (this->rpart + n.rpart,
this->ipart + n.ipart);
}
RealNumber::RealNumber (double r)
: Number (BaseConstructor()),
val (r)
{
std::cout << "Constructing a RealNumber" << std::endl;
}
RealNumber::RealNumber (const RealNumber &r)
: Number (BaseConstructor()),
val (r.val)
{
std::cout << "Constructing a RealNumber using RealNumber::RealNumber" << std::endl;
}
RealNumber::~RealNumber()
{
std::cout << "Inside RealNumber::~RealNumber()" << std::endl;
}
Number RealNumber::operator + (Number const &n) const
{
return n.realAdd (*this);
}
Number RealNumber::realAdd (RealNumber const &n) const
{
std::cout << "RealNumber::realAdd" << std::endl;
return Number::makeReal (this->val + n.val);
}
Number RealNumber::complexAdd (Complex const &n) const
{
std::cout << "RealNumber::complexAdd" << std::endl;
return Number::makeComplex (this->val + n.rpart, n.ipart);
}
namespace std
{
template <>
void swap (Number & n1, Number & n2)
{
n1.swap (n2);
}
}
int main (void)
{
Number n1 = Number::makeComplex (1, 2);
Number n2 = Number::makeReal (10);
Number n3 = n1 + n2;
std::cout << "Finished" << std::endl;
return 0;
}
- Coplien，James（1992 年）。高级 C++ 编程风格和习语。 Addison-Wesley。