我们结合运算符重载知识实现string 类
在自己实现的String类中可以参考C++中string的方法
例如构造，加法，大小比较,长度，[] 等操作.
当前的MyString 类中,暂时不加入迭代器,我们将在下一节中加入迭代器的代码.

#include <iostream>
using namespace std;

class MyString {

public:

	//构造函数
	MyString(const char * pSource = nullptr) {
		if (pSource != nullptr) {
			pString = new char[strlen(pSource) + 1];
			strcpy(pString, pSource);
		}
		else {
			pString = new char[1];
			pString[0] = '\0';
		}
		cout << "MyString构造函数,对象地址="<<this << endl;
	}

	//拷贝构造
	MyString(const MyString & _rValue) {

		pString = new char[strlen(_rValue.pString) + 1];
		strcpy(pString, _rValue.pString);
		cout << "MyString拷贝构造函数" << endl;
	}

	//赋值函数
	MyString & operator=(const MyString & _rValue) {

		if (this == &_rValue) {
			return *this;
		}
		delete[] this->pString;
		this->pString = nullptr;
	
		char * _tpString = new char[strlen(_rValue.pString) + 1];
		strcpy(_tpString, _rValue.pString);
		
		cout << "MyString赋值函数" << endl;

	}

	//可编辑
	char & operator[](int index) {

		int len = strlen(this->pString);
		if (index<0) { return pString[0]; }
		else if (index>len) { return pString[index]; }
		else { return pString[index]; }

	}

	//不可编辑
	const char operator[](int index) const {

		int len = strlen(this->pString);
		if (index<0) { return pString[0]; }
		else if (index>len) { return pString[index]; }
		else { return pString[index]; }

	}

	bool operator>(const MyString & _rValue) const {
		return strcmp(this->pString, _rValue.pString)>0;
	}
	bool operator<(const MyString & _rValue) const {
		return strcmp(this->pString, _rValue.pString)<0;
	}
	bool operator==(const MyString & _rValue) const {
		return strcmp(this->pString, _rValue.pString)==0;
	}

	int length() const {
		return strlen(this->pString);
	}

	~MyString() {
		if (this->pString != nullptr) {
			delete[] this->pString;
			this->pString = nullptr;
			cout << "MyString析构函数" << this << endl;
		}
	}

	const char * c_str()  const { return this->pString; }

private:
	char * pString ;
	friend MyString operator+  (const MyString & s1, const MyString &s2) ;
	friend ostream & operator<<(ostream & out, const MyString &s) ;

};

MyString operator+(const MyString & s1, const MyString &s2) {
	
	/*
	   方式1 这段代码有 内存泄漏问题  tp 没有释放掉
	   int newLength = strlen(s1.pString) + strlen(s2.pString) + 1;
	   char *tp = new char[newLength + 1];//重新申请空间
	   strcpy(tp, s1.pString);
	   strcat(tp, s2.pString);

	   MyString s(tp);
	   cout << "operator+ = " << &s << endl;
	   return s;
	*/

	/*
	  方式2 对比方式1 效果更高
	*/
	MyString s;
	int newLength = strlen(s1.pString) + strlen(s2.pString) + 1;
	s.pString = new char[newLength + 1];//重新申请空间
	strcpy(s.pString, s1.pString);
	strcat(s.pString, s2.pString);	
	cout << "operator+ = " << &s << endl;
	return s;
}

ostream & operator<<(ostream & out, const MyString &s) {
	    cout << s.pString << endl;
		return out;
}

void test() {

	MyString s1("12345");
	MyString s2("6789ABC");

	cout << s1 << endl;
	cout << s2 << endl;

	MyString s3 = s1 + s2;