C++深浅拷贝和写时拷贝图文详解

#define _CRT_SECURE_NO_WARNINGS 1
#include <iostream>
#include<assert.h>
using namespace std;

class String
{
	friend ostream& operator<<(ostream &out, const String &s);
public:
	String(const char* str = "")
	{
		m_data = new char[strlen(str) + 1];
		strcpy(m_data, str);
	}
	//String(const String& s)//qian拷贝
	//{
	//	m_data = s.m_data;
	//}
	String(const String& s)//深拷贝
	{
		m_data = new char[strlen(s.m_data) + 1];
		strcpy(m_data, s.m_data);
	}
	String& operator=(const String& s)
	{
		if (this != &s)
		{
			delete[]m_data;
			m_data = new char[strlen(s.m_data) + 1];
			strcpy(m_data, s.m_data);
		}
		return *this;
	}
	~String()
	{
		delete[]m_data;
		m_data = nullptr;
	}
private:
	char* m_data;
};

ostream& operator<<(ostream &out, const String &s)
{
	out << s.m_data;
	return out;
}

void main()
{
	String s1("abc");
	String s2 = s1;
	cout << "s1 = " << s1 << endl;
	cout << "s2 = " << s2 << endl;
}

class String
{
	friend ostream& operator<<(ostream &out, const String &s);
public:
	String(const char* str = "")
	{
		m_data = new char[strlen(str) + 1];
		strcpy(m_data, str);
		m_count++;
	}
	String(const String& s)//浅拷贝
	{
		m_data = s.m_data;
		m_count++;
	}
	String& operator=(const String& s)
	{
		if (this != &s)
		{
			m_data = s.m_data;
			m_count++;
		}
		return *this;
	}
	~String()//浅赋值
	{
		if (--m_count == 0)
		{
			delete[]m_data;
			m_data = nullptr;
		}
	}
private:
	char* m_data;
	static int m_count;//引用计数器
};
int String::m_count = 0;

ostream& operator<<(ostream &out, const String &s)
{
	out << s.m_data;
	return out;
}

void main()
{
	String s1("abc");
	String s2 = s1;
	String s3;
	s3 = s2;
	cout << "s1 = " << s1 << endl;
	cout << "s2 = " << s2 << endl;
	cout << "s3 = " << s3 << endl;
}

class String_rep
{
public:
	String_rep(const char* str = "") :m_count(0)
	{
		m_data = new char[strlen(str) + 1];
		strcpy(m_data, str);
		cout << "creat" << endl;
	}
	String_rep(const String_rep &rep) :m_count(0)
	{
		m_data = rep.m_data;
		increment();
	}
	String_rep & operator=(const String_rep &rep)
	{
		if (this != &rep)
		{
			m_data = rep.m_data;
			increment();
		}
		return *this;
	}
public:
	void increment()
	{m_count++;}
	void decrement()
	{m_count--;} 
private:
	char* m_data;
	int m_count;
};
class String
{
public:
	String(const char* str = "") :pn(new String_rep(str))
	{
		pn->increment();
	}
	~String()
	{
		cout << "Free" << endl;
	}
private:
	String_rep *pn;
};
void main()
{
	String s1("abc");
}

void to_upper()
	{
		String_rep *new_pn = new String_rep(pn->Getdata());//创建新空间
		pn->decrement();//原空间计数器减一
		pn = new_pn;//需要更改的对象的pn指向新空间
		pn->increment();//新空间的计数器加一
		char* p = pn->Getdata();
		while (*p != '\0')
		{
			if (*p >= 'a' && *p <= 'z')
				*p -= 32;
			p++;
		}