おまけ

より良いコードとして紹介されてたコードを標準ライブラリの範囲内で書き直してみた。全く同じプログラムじゃないけど、どうせサンプルなんだし細かい動作はどうでもいいでしょ。

// rectangle.h
#pragma once
#include <iosfwd>

class rectangle
{
    struct impl;
    impl* pimpl;
public:
    rectangle();
    rectangle(const rectangle&);
    rectangle(int top, int left, int bottom, int right);
    ~rectangle();

    int top() const;
    int left() const;
    int bottom() const;
    int right() const;

    double area() const;
    double diagonal_len() const;

    friend std::ostream& operator<<(std::ostream&, const rectangle&);
    friend std::istream& operator>>(std::istream&, rectangle&);
};

// rectangle.cpp
#include "rectangle.h"

#include <iostream>
#include <cmath>

struct rectangle::impl
{
    int top;
    int left;

    int bottom;
    int right;

    impl(int top, int left, int bottom, int right)
        : top(top), left(left), bottom(bottom), right(right)
    {}

    impl(const impl& other)
        : top(other.top), left(other.left), bottom(other.bottom), right(other.right)
    {}
};

rectangle::rectangle()
    : pimpl(new rectangle::impl(0, 0, 0, 0))
{}

rectangle::rectangle(const rectangle& other)
    : pimpl(new rectangle::impl(*other.pimpl))
{}

rectangle::rectangle(int top, int left, int bottom, int right)
    : pimpl(new rectangle::impl(top, left, bottom, right))
{}

rectangle::~rectangle() { delete pimpl; }

int rectangle::top() const { return pimpl->top; }
int rectangle::left() const { return pimpl->left; }
int rectangle::bottom() const { return pimpl->bottom; }
int rectangle::right() const { return pimpl->right; }

namespace
{
    inline int abs(int n) { return n < 0 ? -n : n; }
    inline int square(int n) { return n * n; }
}

double rectangle::area() const
{
    return abs(pimpl->top - pimpl->bottom) * abs(pimpl->left - pimpl->right);
}

double rectangle::diagonal_len() const
{
    return std::sqrt(static_cast<double>(square(pimpl->top - pimpl->bottom) + square(pimpl->left - pimpl->right)));
}

std::ostream& operator<<(std::ostream& os, const rectangle& rect)
{
    os << "(" << rect.pimpl->top << "," << rect.pimpl->left << ")-("
        << rect.pimpl->bottom << "," << rect.pimpl->right << ")の長方形";
    return os;
}

std::istream& operator>>(std::istream& is, rectangle& rect)
{
    is >> rect.pimpl->top >> rect.pimpl->left >> rect.pimpl->bottom >> rect.pimpl->right;
    return is;
}

// main.cpp
#include <iostream>
#include "rectangle.h"

void print_rectangle(std::ostream& os, const rectangle& rect)
{
    os << rect << '\n'
        << "面積＝" << rect.area() << '\n'
        << "対角線の長さ＝" << rect.diagonal_len() << std::endl;
}

int main()
{
    rectangle rect;
    std::cout << "座標を入力してください(top left bottom right): ";
    std::cin >> rect;
    print_rectangle(std::cout, rect);

    std::cout << "新しい長方形" << std::endl;

    int newbottom, newright;
    std::cout << "新しい長方形の右下の座標を入力してください(bottom right):";
    std::cin >> newbottom >> newright;

    rectangle other(rect.top(), rect.left(), newbottom, newright);
    print_rectangle(std::cout, other);
}

C++ なんて本当に久しぶりに触ったな・・・