string类的实现

#include <iostream>
using namespace std;

class String
{
public:
	//普通构造函数
	String(const char *str = NULL);
	//拷贝构造函数
	String(const String &other);
	//赋值构造函数
	String & operator = (const String &other);
	//析构函数
	~String(void);
	//打印数据
	void Display();

private:
	void IsSuccessMemory()
	{
		if(NULL == m_data)
		{
			cout<<"failure! Error,Menory allocation failure!";
			exit(0);
		}
	}
	//用于保存字符串
	char *m_data;
};


String::String(const char* str)
{
	
	if(NULL == str)
	{
		m_data = new char[ 1 ];
		IsSuccessMemory();
		*m_data = '\0';	
	}
	else
	{
		int length = strlen(str);
		m_data = new char [length + sizeof(char)];
		IsSuccessMemory();
		strcpy(m_data, str);
	}
	
}

String::String(const String &other)
{
	int length = strlen(other.m_data);
	m_data = new char [ length + sizeof(char)];
	IsSuccessMemory();
	strcpy(m_data, other.m_data);
}

String & String::operator = (const String &other)
{
	if(this == &other)
	{
		return *this;
	}

	delete [] m_data;
        m_data = NULL;
	int length = strlen(other.m_data);
	m_data = new char [length + sizeof(char)];
	IsSuccessMemory();
	strcpy(m_data,other.m_data);
	//返回本对象的引用
	return *this;
}


String::~String()
{
	delete [] m_data;
}

void String::Display()
{
	cout<<m_data<<endl;
}

void main()
{
	String str("Hello");//普通构造函数
	String s2 = str;//拷贝构造函数
	s2.Display();
	String s3;
	s3 = str;//赋值函数
	s3.Display();
}

class String {
public:
    String(const char* str = NULL);
    String(const String& other);
    String& operator= (const String& other);
    ~String();

private:
    char* m_data;
}

String:String(const char* str) {
    if (str == NULL) {
        m_data = new (std::nothrow) char[sizeof(char)];
        if (m_data == NULL) {
            printf("failed to alloc memory of m_data.");
            exit(0);
        }
        *m_data = '\0';
    } else {
        m_data = new (std::nothrow) char[strlen(str) + sizeof(char)];
        if (m_data == NULL) {
            printf("failed to alloc memory of m_data.");
            exit(0);
        }
        strcpy(m_data, str);
    }
}

String::String(const String& other) {
    m_data = new (std::nothrow) char[strlen(other.m_data) + sizeof(char)];
    if (m_data == NULL) {
        printf("failed to alloc memory of m_data.");
        exit(0);
    }
    strcpy(m_data, other.m_data);
}

String& String::operator=(const String& other) {
    if (this == &other) {
        return *this;
    }
    delete []m_data;
    m_data = NULL;
    m_data = new (std::nothrow) char[strlen(other.m_data) + sizeof(char)];
    if (m_data == NULL) {
        printf("failed to alloc memory of m_data.");
        exit(0);
    }
    strcpy(m_data, other.m_data);
    return *this;
}

String:~String() {
    if (m_data == NULL) {
        delete []m_data;
        m_data = NULL;
    }
}