X的放大与缩小（运算符重载）

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1.题目概述

X字母可以放大和缩小，变为n行X（n=1,3,5,7,9,…,21）。

现假设一个n行（n>0，奇数）X图案，遥控器可以控制X图案的放大与缩小。遥控器有5个按键，1）show，显示当前X图案；2）show++, 显示当前X图案，再放大图案，n+2；3）++show，先放大图案，n+2，再显示图案；4）show–，显示当前X图案，再缩小图案，n-2；5）–show，先缩小图案，n-2，再显示图案。假设X图案的放大和缩小在1-21之间。n=1时，缩小不起作用，n=21时，放大不起作用。

2.代码

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/*例如，3行x图案如下：
        xxx
         x
        xxx      */

#include <iostream>
using namespace std;

class CXGraph
{
	char** x;
	int N;
public:
	CXGraph(){}
	CXGraph(int n)
	{
		N = n;
		x = new char*[n];
		for (int i = 0; i < n; ++i)
		{
			x[i] = new char[n];
		}

		int temp = n;
		for (int i = 0; i < ((n+1)/2); i++)
		{
			for (int j = 0; j < i; j++)
			{
				x[i][j] = ' ';
				x[n - i - 1][j] = ' ';
			}
			for (int j = i; j < temp; j++)
			{
				x[i][j] = 'X';
				x[n - i - 1][j] = 'X';
			}
			temp --;
		}
	}

	CXGraph(const CXGraph& tmp)
	{
		N = tmp.N;
		x = new char* [N];
		for (int i = 0; i < N; ++i)
		{
			x[i] = new char[N];
		}
		

		for (int i = 0; i < N; i++)
		{
			for (int j = 0; j < N; j++)
			{
				x[i][j] = tmp.x[i][j];
			}
		}
			
	}
	~CXGraph()
	{
		for (int i = 0; i < N; i++)
		{
			delete []x[i];
		}
		delete[]x;
	}

    //这里规范来说，应该全部加const，但是当时编译时出了问题，发现把const去掉就好了
    //结尾贴出改进方法--作为类成员函数重载
	friend CXGraph operator++(CXGraph& a);//前缀运算++a
	friend CXGraph operator++(CXGraph& a, int b);//后缀运算a++
	friend CXGraph operator--(CXGraph& a);
	friend CXGraph operator--(CXGraph& a, int b);
	friend ostream& operator<<(ostream& os, const CXGraph& x);
	CXGraph& operator=(const CXGraph& a);
};

CXGraph operator++(CXGraph& a)
{
	if (a.N == 21) return a;
	for (int i = 0; i < a.N; ++i)
		delete[] a.x[i];
	delete[] a.x;
	a.N += 2;
	a.x = new char* [a.N];
	for (int i = 0; i < a.N; ++i)
		a.x[i] = new char[a.N];
	int temp = a.N;
	for (int i = 0; i < ((a.N + 1) / 2); i++)
	{
		for (int j = 0; j < i; j++)
		{
			a.x[i][j] = ' ';
			a.x[a.N - i - 1][j] = ' ';
		}
		for (int j = i; j < temp; j++)
		{
			a.x[i][j] = 'X';
			a.x[a.N - i - 1][j] = 'X';
		}
		temp--;
	}
	return a;
}

CXGraph operator++(CXGraph& a, int b)
{
	if (a.N == 21) return a;
	CXGraph t(a);
	for (int i = 0; i < a.N; ++i)
	{
		delete[] a.x[i];
	}
	delete[] a.x;

	a.N += 2;
	a.x = new char* [a.N];
	for (int i = 0; i < a.N; ++i)
	{
		a.x[i] = new char[a.N];
	}

	int temp = a.N;
	for (int i = 0; i < ((a.N + 1) / 2); i++)
	{
		for (int j = 0; j < i; j++)
		{
			a.x[i][j] = ' ';
			a.x[a.N - i - 1][j] = ' ';
		}
		for (int j = i; j < temp; j++)
		{
			a.x[i][j] = 'X';
			a.x[a.N - i - 1][j] = 'X';
		}
		temp--;
	}
	return t;
}

CXGraph operator--(CXGraph& a)
{
	if (a.N == 1) return a;
	for (int i = 0; i < a.N; ++i)
	{
		delete[] a.x[i];
	}
	delete[] a.x;

	a.N -= 2;
	a.x = new char* [a.N];
	for (int i = 0; i < a.N; ++i)
		a.x[i] = new char[a.N];
	int temp = a.N;
	for (int i = 0; i < ((a.N + 1) / 2); i++)
	{
		for (int j = 0; j < i; j++)
		{
			a.x[i][j] = ' ';
			a.x[a.N - i - 1][j] = ' ';
		}
		for (int j = i; j < temp; j++)
		{
			a.x[i][j] = 'X';
			a.x[a.N - i - 1][j] = 'X';
		}
		temp--;
	}
	return a;
}

CXGraph operator--(CXGraph& a, int b)
{
	if (a.N == 1) return a;
	CXGraph t(a.N);	
	for (int i = 0; i < a.N; ++i)
	{
		delete[] a.x[i];
	}
	delete[] a.x;

	a.N -= 2;
	int temp = a.N;
	a.x = new char* [a.N];
	for (int i = 0; i < a.N; ++i)
		a.x[i] = new char[a.N];

	for (int i = 0; i < ((a.N + 1) / 2); i++)
	{
		for (int j = 0; j < i; j++)
		{
			a.x[i][j] = ' ';
			a.x[a.N - i - 1][j] = ' ';
		}
		for (int j = i; j < temp; j++)
		{
			a.x[i][j] = 'X';
			a.x[a.N - i - 1][j] = 'X';
		}
		temp--;
	}

	return t;
}

ostream& operator<<(ostream& os, const CXGraph& x)
{
	int t = x.N;
	for (int i = 0; i < (x.N+1)/2; i++)
	{
		for (int j = 0; j < t; j++)
		{
			cout << x.x[i][j] ;
		}
		cout << endl;
		t--;
	}

	t += 2;
	for (int i = (x.N + 1) / 2; i < x.N; i++)
	{
		for (int j = 0; j < t; j++)
		{
			cout << x.x[i][j];
		}
		cout << endl;
		t++;
	}

	return os;
}

CXGraph& CXGraph::operator=(const CXGraph& a)
{
	for (int i = 0; i < N; ++i) {
		delete[] x[i];
	}
	delete[] x;

	N = a.N;
	x = new char* [N];
	for (int i = 0; i < N; ++i) {
		x[i] = new char[N];
		for (int j = 0; j < N; ++j) {
			x[i][j] = a.x[i][j];
		}
	}

	return *this;
}

int main()
{
	int t, n;
	string command;
	cin >> n;
	CXGraph xGraph(n);
	cin >> t;
	while (t--)
	{
		cin >> command;
		if (command == "show++")
		{
			cout << xGraph++ << endl;
		}
		else if (command == "++show")
		{
			cout << ++xGraph << endl;
		}
		else if (command == "show--")
		{
			cout << xGraph-- << endl;
		}
		else if (command == "--show")
		{
			cout << --xGraph << endl;
		}
		else if (command == "show")
		{
			cout << xGraph << endl;
		}
	}
	return 0;
}

3.要点

1.运算符重载的两种形式

  • 类成员函数（有this指针）ps：二元运算符以成员函数形式重载，左操作数必须为类对象，目标对象作为左操作数

  • 友元函数（无this指针）

  • 一般单目运算符最好重载为类的成员函数，双目运算符则重载为类的友元函数

  • 一些双目运算符不能重载为友元函数：=、()、[]、->

  • 若需要修改对象的状态，最好选择成员函数重载

2.改进

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CXGraph& operator++()//++a
{
	if (N == 21) return *this;

	// 释放原有内存
	for (int i = 0; i < N; ++i)
		delete[] x[i];
	delete[] x;

	N += 2;
	x = new char* [N];
	for (int i = 0; i < N; ++i)
		x[i] = new char[N];

	int temp = N;
	for (int i = 0; i < ((N + 1) / 2); i++) {
		for (int j = 0; j < i; j++) {
			x[i][j] = ' ';
			x[N - i - 1][j] = ' ';
		}
		for (int j = i; j < temp; j++) {
			x[i][j] = 'X';
			x[N - i - 1][j] = 'X';
		}
		temp--;
	}

	return *this;
}

！！！CXGraph CXGraph::operator++(int)【a++】返回的是局部变量，而返回局部变量（临时对象）的引用属于悬空引用，不正确。所以与前置加加不同，这里不返回引用。

3.关于<<和>>的运算符重载

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friend ostream& operator<<(ostream& os,const CXGraph& t);
friend istream& operator>>(istream& is,CXGraph& t);