[C++知识库]C++String的实现

#include <iostream>
#include <cstring>
using namespace std;

//写一个string类

class String{
private:
	char *m_str;    //指针成员
public:
	String(const char *str=""):m_str(strcpy(new char[str?strlen(str)+1:1],str?str:"")){
		//m_str = new char[str?strlen(str)+1:1];
		//strcpy(m_str,str?str:"");
		//m_str = strcpy(new char[str?strlen(str)+1:1],str?str:"");
		/*
		if(str == NULL){
			m_str = new char[1];
			strcpy(m_str,"");//m_str[0] = '\0';
		}else{
			m_str = new char[strlen(str)+1];	
			strcpy(m_str,str);
		}
		*/
	}
	String(const String& s):m_str(strcpy(new char[strlen(s.m_str)+1],s.m_str)){
		//m_str = new char[strlen(s.m_str)+1];
		//strcpy(m_str,s.m_str);
	}
	String& operator=(const String& s){
		if(this != &s){
			/*
			delete [] m_str;
			m_str = new char[strlen(s.m_str)+1];
			strcpy(m_str,s.m_str);
			*/
			/*
			char *pt = new char[strlen(s.m_str)+1];
			strcpy(pt,s.m_str);
			delete [] m_str;
			m_str = pt;
			*/
			String stmp(s);//局部对象  临时对象　　析构函数
			swap(stmp.m_str,m_str);
		}
		return *this;
	}
	~String(void){
		if(m_str != NULL){
			delete [] m_str;
			m_str = NULL;
		}
	}
	const char * c_str(){
		return m_str;
	}
	int length()const{
		return strlen(m_str);
	}
	char& operator[](int index){
		return m_str[index];	
	}
	const char& operator[](int index)const{
		return m_str[index];	
	}
};


int main(){
	String s1("Hello world");
	String s2(s1);
	String s3;
	String s4 = s1;  //s4(s1) //拷贝构造函数
	//s3 = s2;//　在进行赋值　　调用拷贝赋值函数
	s3.operator=(s2);
	cout << s1.c_str() << endl;
	cout << s2.c_str() << endl;
	cout << s3.c_str() << endl;
	
	String ss1("Hello");
	String ss2("world");
	String ss3;
	//如果返回的是String& 那么结果为Hello  如果没有引用　结果为world 
	//如果有const则编译报错
	(ss3 = ss2) = ss1;// (ss3.operator=(ss2)).operator=(ss1)
	cout << ss3.c_str() << endl;
	int a,b,c;
	(a = b) = c;
	return 0;	
}