前提知识：上一章介绍了库里面关于string的诸多接口，这章我要根据库里的相关接口，自己实现。

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1.成员变量：

我们需要定义起始位置，已经有效字符个数和容量。

	private:char* _str;size_t _size;size_t _capacity;

2.默认成员函数：

2.1构造函数

默认给的缺省值是空串

		string(const char *str = ""): _size(strlen(str)){_capacity = _size;_str = new char[_capacity + 1];strcpy(_str, str);}

2.2拷贝构造函数 

		//拷贝构造函数string(const string& s){_str = new char (s._capacity + 1);memcpy(_str, s._str, s._size + 1);_size = s._size;_capacity = s._capacity;}

2.3赋值构造

		//赋值构造  先调用拷贝构造string& operator = (string tmp){//	if (this != &s)//	{//		string tmp(s);//		//this->swap(tmp);//		swap(tmp);//	}//	return *this;swap(tmp);return *this;}

2.4析构函数

		//析构函数~string(){delete[] _str;_str = nullptr;_size = 0;_capacity = 0;}

3其他成员函数

3.1交换

		//交换void swap(string s){std::swap(_str,s._str);std::swap(_size, s._size);std::swap(_capacity, s._capacity);}

3.2c_str函数

		//返回字符串首地址const char* c_str(){return _str;}

3.3元素个数

		// 返回元素个数size_t size(){return _size;}

3.4operator[ ]

3.4.1普通版本

		//根据下标寻找元素char& operator[](int pos){assert(pos < _size);return _str[pos];}

3.4.2const 版本

		//根据下标寻找元素 const版本const char& operator[](int pos) const{assert(pos < _size);return _str[pos];}

 

3.5迭代器和范围for

3.5.1普通迭代器版本

		typedef char* iterator;iterator begin(){return _str;}iterator end(){return _str + _size;}

3.5.2const迭代器版本

		const_iterator begin() const{return _str;}const_iterator end() const{return _str + _size;}

3.6扩容

3.6.1reserve

		//扩容 不需要返回值，改变容量和长度就行void reserve(size_t n){if (n > _capacity){char * tmp = new char[n + 1];//有结束标识符 \0memcpy(tmp, _str,_size + 1);delete [] _str;_str = tmp;_capacity = n;}}

3.6.2resize

		void resize(size_t n ,char ch = '\0'){if (n < _capacity){_size = n;_str[_size] = '\0';}else{reserve(n);for (int i = _size; i < n; ++i){_str[i] = ch;}_size = n;_str[_size] = '\0';}}

3.7尾插

3.7.1尾插字符push_back

		void push_back(char ch){//判断是否需要扩容if (_size == _capacity){reserve(_capacity == 0 ? 4 : _capacity * 2);}_str[_size] = ch;_size++;_str[_size] = '\0';//结束标志位}

3.7.2尾插字符串 append

		// 尾插 字符串void append(const char* string){size_t len = strlen(string);if (_size + len > _capacity){reserve(_size + len);}strcpy(_str + _size, string);_size += len;//无须给停止标志位，因为是字符串}

3.7.3连接重载 operator +=

		// += 实现尾插// this指针指向对象 ，对this解引用就是 对象内容string& operator +=(char ch){push_back(ch);return *this;}string& operator +=(const char *string){append(string);return *this;}

3.8插入insert

3.8.1插入字符

		//插入字符void insert(size_t pos,char ch, size_t n){assert(pos <= _size);if (_size + n > _capacity){reserve(_size + n);}//挪动数据size_t end = _size;while (end >= pos && end!= npos){//当一个数字都没有的时候 会发生越界_str[end + n] = _str[end];--end;}//插入数据for (int i = 0; i < n; i++){_str[pos + i] = ch;}_size += n;}

3.8.2插入字符串

		//插入字符串void insert(size_t pos, char* str){assert(pos <= _size);size_t len = strlen(str);if (_size + len > _capacity){reserve(_size + len);}//挪动数据size_t end = _size;while (end >= pos && end != npos){_str[end + len] = _str[end];--end;}//插入数据for (size_t i = 0; i < len; i++){_str[pos + i] = str[i];}_size += len;}

 

3.9擦除erase

		//擦除void erase(size_t pos, size_t n = npos){assert(pos <= _size);if (n == npos || pos + n >= _capacity){_str[pos] = '\0';_size = pos;_str[_size] = '\0';}else{size_t end = pos + n;while (end <= _size){_str[pos++] = _str[end++];}_size -= n;}}

3.10查找find

3.10.1查找字符

		//发现size_t find(char ch,size_t pos = 0){assert(pos < _size);for (size_t i = pos; i < _size; i++){if (_str[i] == ch){return i;}}return npos;}

3.10.2查找字符串

		//发现字符串size_t find(const char* str, size_t pos = 0){assert(pos < _size);const char* ptr = strstr(_str + pos, str);if (ptr){return ptr - _str;}else{return npos;}}

3.11返回子串substr

		// 从某个位置开始 返回长度的字符串string substr(size_t pos = 0, size_t len = npos){assert(pos < _size);size_t n = len;if (len == npos || pos + len > _capacity){n = _size - pos;}string tmp;//临时创建一个对象tmp.reserve(n);for (size_t i = 0; i < n; i++){tmp += _str[i];}return tmp;}

3.12清空clear

		//清空函数void clear(){_str[_size] = '\0';_size = 0;}

3.13比较

3.13.1 operator <()

	    //重载实现 string对象的<bool operator <(const string& s) const{int ret = memcmp(_str, s._str, _size < s._size?_size : s._size);  // 定义一个变量  用来接收 比较两个字符串 长度较小的那个，如果相等则为0，如果大于则为正值，如果小于则为负。return ret == 0 ? _size < s._size : ret < 0;}

3.13.2operator==()

		bool operator == (const string& s) const{return _size == s._size && memcmp(_str, s._str, _size)==0;}

3.13.3 operator<=()

		bool operator <= (const string& s) const{return *this < s || *this == s;}

3.13.4operator>()

		bool operator > (const string& s) const{return !(*this <= s);}

3.13.5operator>=()

		bool operator >= (const string& s) const{return !(*this < s);}

3.13.6operator!=()

		bool operator !=  (const string& s) const{return !(*this == s);}

3.14流插入、提取

3.14.1operator <<

	//流插入ostream& operator << (ostream& out, const string& s){for (auto ch : s){out << ch;}return out;}

3.14.2operator >>

	//流提取istream& operator >> (istream& in, string& s){s.clear();char ch = in.get();//定义一个字符接收//处理前面缓冲区的空格和换行while (ch == ' ' || ch == '\n'){ch = in.get();}char buff[128];int i = 0;while ( ch != '\n'){buff[i++] = ch;if (i == 127){buff[i] = '\0';s += buff;i = 0;}ch = in.get();}if (i != 0){buff[i] = '\0';s += buff;}return in;}