# Algebraic Hierarchy

### Intent

To hide multiple closely related algebraic abstractions (numbers) behind a single generic abstraction and provide a generic interface to it.

### Also Known As

• State (Gamma et al.)

### Motivation

In pure object-oriented languages like Smalltalk, variables are run-time bindings to objects that act like labels. Binding a variable to an object is like sticking a label on it. Assignment in these languages is analogous to peeling a label off one object and putting it on another. On the other hand, in C and C++, variables are synonyms for addresses or offsets instead of being labels for objects. Assignment does not mean re-labelling, it means overwriting old contents with new one. Algebraic Hierarchy idiom uses delegated polymorphism to simulate weak variable to object binding in C++. Algebraic Hierarchy uses Envelope Letter idiom in its implementation. The motivation behind this idiom is to be able write code like the one below.

```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
```

### Solution and Sample Code

Complete code showing implementation of Algebraic Hierarchy idiom is shown below.

```#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
{
}

Number Number::realAdd (RealNumber const &n) const
{
}

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
{
}

Number Complex::realAdd (RealNumber const &n) const
{
return Number::makeComplex (this->rpart + n.val,
this->ipart);
}

Number Complex::complexAdd (Complex const &n) const
{
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
{
}

Number RealNumber::realAdd (RealNumber const &n) const
{
return Number::makeReal (this->val + n.val);
}

Number RealNumber::complexAdd (Complex const &n) const
{
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;
}
```