前言：
学习了string的使用，总感觉了解不是很深厚；自己模拟实现string类来帮助自己理解。
这里只是实现了一部分内容（并没有实现完整的string类）。
先来实现string类里面的成员变量：
#include<iostream>namespace HL{class string{ public:private:char* _str;size_t _size;size_t _capacity;const static size_t npos;};const size_t HL::string::npos = -1;}一、string默认成员函数（构造、析构、赋值运算符重载）
1.1、构造函数
1> 默认构造
默认构造函数就是不需要传参的构造函数；这里实现就开辟一个字符的空间存放 '\0'即可（_capacity不包括 '\0' ）。
string(){_str = new char[1];_str[0] = '\0';_size = 0;_capacity = 0;}2> 拷贝构造
拷贝构造，在实现时需要注意：是深拷贝，而不是浅拷贝（值拷贝）。
深拷贝（深拷贝简单来说就是，要开辟一块新的空间，把原空间里的值拷贝到新的空间里）。
string(const string& str){_str = new char[str._capacity + 1];memcpy(_str, str._str, str._size + 1);_size = str._size;_capacity = str._capacity;}3> 其他构造
其他构造函数就有很多了，这里就实现以下这几个：
string (const char* s);
string(const char* s){size_t len = strlen(s);_str = new char[len + 1];memcpy(_str, s, len + 1);_size = len;_capacity = len;}string (const char* s, size_t n);
string(const char* s, size_t n){size_t len = strlen(s);if (n > len){n = len;}_str = new char[n + 1];memcpy(_str, s, n);_str[n] = '\0';_size = n;_capacity = n;}string (size_t n, char c);
string(size_t n, char c){_str = new char[n + 1];for (size_t i = 0; i < n; i++){_str[i] = c;}_str[n] = '\0';_size = n;_capacity = n;}1.2、析构函数
析构函数比较简单，释放开辟的资源即可；
~string(){delete[] _str;_str = nullptr;_size = _capacity = 0;} 1.3、赋值运算符重载 
赋值运算符有3个重载，这里就一一实现：
string& operator= (const string& str );
实现这个有很多种方法，
可以释放原空间，再开辟新的空间，将数据拷贝到新的空间中去
string& operator=(const string& str){delete[] _str;_str = new char[str._capacity];memcpy(_str, str._str, str._size + 1);_size = str._size;_capacity = str._capacity;return *this;}可以调用拷贝构造，构造一个tmp、再将tmp与*this 中的值进行交换（要实现交换函数）
template <typename T>void Swap(T& x, T& y){T tmp = x;x = y;y = tmp;}string& operator= (const string& str){ string tmp(str); Swap(_str, tmp._str); Swap(_size, tmp._size); Swap(_capacity, tmp._capacity); return *this;}这里如果已经实现string类swap成员函数，就可以直接调用。
string& operator= (const char* s );
/*string& operator= (const char* s){size_t len = strlen(s);delete[] _str;_str = new char[len + 1];memcpy(_str, s, len + 1);_size = _capacity = len;return *this;}*/string& operator= (const char* s){string tmp(s);Swap(_str, tmp._str);Swap(_size, tmp._size);Swap(_capacity, tmp._capacity);return *this;}string& operator= (char c );
string& operator= (char c){delete[] _str;_str = new char[2];_str[0] = c;_str[1] = '\0';_size = _capacity = 1;return *this;}二、元素访问与迭代器
2.1、迭代器
迭代器，虽然在string类中使用的不是很多，但在后面的容器中有大用处。
（在string类中就可以简单的理解成指针）。
//迭代器typedef char* iterator;typedef const char* const_iterator; iterator begin(){return _str;}iterator end(){return (_str + _size);}const_iterator begin() const{return _str;}const_iterator end() const{return (_str + _size);}实现了迭代器之后，范围for这个语法糖就可以使用了（底层就是迭代器）。
2.2、下标访问元素
实现下标访问，就是 [ ]运算符重载。
//下标访问 [ ]char& operator[] (size_t pos){assert(pos >= _size);return *(_str + pos);}const char& operator[] (size_t pos) const{assert(pos >= _size);return *(_str + _size);}at函数和 [ ] 运算符重载原理一样，这里就不重复写了。
三、增删查改
在实现增之前，要先实现一个函数，就是调整空间大小的（扩容来用）。
//扩容void reserve(size_t n){if (n > _capacity){char* s = new char[n + 1];memcpy(s, _str, _size);delete[] _str;_str = s;_capacity = n;}}
增删这里就实现这些成员函数。
1、push_back 、append 、operator+=
append重载比较多，这里就实现其中的几个。
//扩容void reserve(size_t n){if (n > _capacity){char* s = new char[n + 1];memcpy(s, _str, _size);delete[] _str;_str = s;_capacity = n;}}//增void push_back(char c){if (_size >= _capacity){reserve((_capacity == 0) ? 4 : 2 * _capacity);}_str[_size] = c;_size++;_str[_size] = '\0';}void append(const string& str){size_t n = _size + str._size;if (n > _capacity){reserve(n);}for (int i = 0; i < str._size; i++){_str[_size + i] = str._str[i];}_size += str._size;_str[_size] = '\0';}void append(const char* s){size_t len = strlen(s);int n = _size + len;if (n > _capacity){reserve(n);}for (int i = 0; i < len; i++){_str[_size + i] = s[i];}_size = n;_str[_size] = '\0';}void append(size_t n, char c){if (_size + n > _capacity){reserve(_size + n);}for (int i = 0; i < n; i++){_str[_size + i] = c;}_size += n;_str[_size] = '\0';}string& operator+=(const string& str){this->append(str);return *this;}string& operator+=(const char* s){this->append(s);return *this;}string& operator+=(char c){this->push_back(c);return *this;}2、insert、erase
insert重载也比较多，比较冗余；这里也只实现其中的一部分；
erase这里只实现一个
//insert 、erasevoid insert(size_t pos, const string& str){assert(pos >= 0 && pos < _size);size_t len = str._size;size_t n = _size + len;if (n > _capacity){reserve(n);}//挪动数据for (size_t i = n; i >= pos + len; i--){_str[i] = _str[i - len];}memcpy(_str + pos, str._str, str._size);_size += str._size;_str[_size] = '\0';}void insert(size_t pos, const char* s){assert(pos >= 0 && pos < _size);size_t len = strlen(s);size_t n = _size + len;if (n > _capacity){reserve(n);}//挪动数据for (size_t i = n; i >= pos + len; i--){_str[i] = _str[i - len];}memcpy(_str + pos, s, len);_size += len;_str[_size] = '\0';}void insert(size_t pos, size_t n, char c){assert(pos >= 0 && pos < _size);if (_size + n > _capacity){reserve(_size + n);}//挪动数据for (size_t i = _size+n; i >= pos + n; i--){_str[i] = _str[i - n];}for (size_t i = 0; i < n; i++){_str[pos + i] = c;}_size += n;_str[_size] = '\0';}void erase(size_t pos, size_t len = npos){assert(pos >= 0 && pos < _size);if (len == npos){_str[0] = '\0';_size = 0;return;}for (size_t i = pos; (len + i) < _size; i++){_str[i] = _str[i + len];}_size -= len;_str[_size] = '\0';}3、find
//findsize_t find(const string& str, size_t pos = 0){assert(pos >= 0 && pos < _size);char* tmp = strstr(_str + pos, str._str);if (tmp == nullptr){return -1;}return tmp - _str;}size_t find(const char* s, size_t pos = 0){assert(pos >= 0 && pos < _size);char* tmp = strstr(_str + pos, s);if (tmp == nullptr){return -1;}return tmp - _str;}size_t find(char c, size_t pos = 0){assert(pos >= 0 && pos < _size);for (size_t i = pos; i < _size; i++){if (_str[i] == c){return i;}}return -1;}4、swap
swap作为string的成员函数，交换两个string类类型的对象。
//swapvoid swap(string& str){Swap(_str, str._str);Swap(_size, str._size);Swap(_capacity, str._capacity);}这里swap函数内部也可以调用库里面的swap模版（这里我自己写了一个模版Swap）
有了swap函数，上面赋值运算符重载中就可以这样写了：
string& operator= (const string& str){string tmp(str);swap(tmp);return *this;}四、字符串操作函数
1、c_str
c_str()函数返回string类对象中的字符串;
const char* c_str() const{return _str;}char* c_str(){return _str;}2、substr
获得，sting对象中字符串的子串。
//substrstring substr(size_t pos = 0, size_t len = npos) const{assert(pos >= 0 && pos < _size);size_t n = 0;if (len == npos || pos + len > _size){n = _size - pos;}else{n = len;}string ret;for (size_t i = 0; i < n; i++){ret += _str[pos + i];}return ret;}五、其他成员函数
这里max_size是返回容器可以容纳的最大元素的数量，这里就不进行实现了。
reserve在增加元素前已经实现了。（扩容）
//其他成员函数size_t size() const{return _size;}size_t length()const{return _size;}size_t capacity()const{return _capacity;}void clear(){_str[0] = '\0';_size = 0;}bool empty()const{return _size == 0;}void resize(size_t n){if (n > _capacity){reserve(n);}_size = n;_str[_size] = '\0';}void resize(size_t n, char c){if (n > _capacity){reserve(n);}for (size_t i = _size; i < n; i++){_str[i] = c;}_size = n;_str[_size] = '\0';}
六、流插入、流提取
因为成员函数有一个隐藏的this指针，会和istream 和istream 类对象抢占第一个参数的位置，所以我们不能将流插入和流提取写成string类的成员函数。
string中实现了访问元素方成员函数，就可以不将流插入、流提取写成string类的友元函数。
1、流插入运算符重载
std::ostream& operator<<(std::ostream& out, const HL::string& str){//for (int i = 0; i < str.size(); i++)//{//out << str[i];//}//return out;for (auto ch : str){out << ch;}return out;}2、流提取运算符重载
1、 s.clear()清理缓冲区（上次cin流提取的剩余）；
2、 创建一个数组，防止多次去开空间（输入到128或者输入结束（‘ ’或者‘\n’）才添加到str中）。
3、下面的代码处理缓冲区的空格。
char ch;
ch = in.get();
while (ch == ' ' || ch == '\n')
{
ch = in.get();
}
4、最后循环里if是遇到空格或者换行结束，将s中输入添加到s中，末尾添加'\0'。
std::istream& operator>>(std::istream& in, HL::string& str){char s[128] = { 0 };char ch;ch = in.get();while (ch == ' ' || ch == '\n'){ch = in.get();}str.clear();int i = 0;while (ch != '\n'){s[i] = ch;i++;if (i == 127){s[i] = '\0';str += s;i = 0;}ch = in.get();}if (i){str += s;}return in;}HL :: string 源码
#pragma once#include<iostream>#include<cassert>template <typename T>void Swap(T& x, T& y){T tmp = x;x = y;y = tmp;}namespace HL{class string{friend std::ostream& operator<<(std::ostream& out, const HL::string& str);friend std::istream& operator>>(std::istream& in, HL::string& str);public://构造函数/*string(){_str = new char[1];_str[0] = '\0';_size = 0;_capacity = 0;}string(char ch){_str = new char[1];_str[0] = ch;_size = 0;_capacity = 0;}*/string(char ch = '\0'){_str = new char[2];_str[0] = ch;_str[1] = '\0';_size = 1;_capacity = 1;}string(const char* s){size_t len = strlen(s);_str = new char[len + 1];memcpy(_str, s, len + 1);_size = len;_capacity = len;}string(const char* s, size_t n){size_t len = strlen(s);if (n > len){n = len;}_str = new char[n + 1];memcpy(_str, s, n);_str[n] = '\0';_size = n;_capacity = n;}string(const string& str){_str = new char[str._capacity + 1];memcpy(_str, str._str, str._size + 1);_size = str._size;_capacity = str._capacity;}string(size_t n, char c){_str = new char[n + 1];for (size_t i = 0; i < n; i++){_str[i] = c;}_str[n] = '\0';_size = n;_capacity = n;}~string(){delete[] _str;_str = nullptr;_size = _capacity = 0;}/*string& operator=(const string& str){delete[] _str;_str = new char[str._capacity];memcpy(_str, str._str, str._size + 1);_size = str._size;_capacity = str._capacity;return *this;}*//*string& operator= (const string& str){string tmp(str);Swap(_str, tmp._str);Swap(_size, tmp._size);Swap(_capacity, tmp._capacity);return *this;}*/string& operator= (const string& str){string tmp(str);swap(tmp);return *this;}/*string& operator= (const char* s){size_t len = strlen(s);delete[] _str;_str = new char[len + 1];memcpy(_str, s, len + 1);_size = _capacity = len;return *this;}*/string& operator= (const char* s){string tmp(s);Swap(_str, tmp._str);Swap(_size, tmp._size);Swap(_capacity, tmp._capacity);return *this;}string& operator= (char c){delete[] _str;_str = new char[2];_str[0] = c;_str[1] = '\0';_size = _capacity = 1;return *this;}//string& operator= (char c)//{//string tmp(c);//Swap(_str, tmp._str);//Swap(_size, tmp._size);//Swap(_capacity, tmp._capacity);//return *this;//}//迭代器typedef char* iterator;typedef const char* const_iterator;iterator begin(){return _str;}iterator end(){return (_str + _size);}const_iterator begin() const{return _str;}const_iterator end() const{return (_str + _size);}//下标访问 [ ]char& operator[] (size_t pos){assert(pos >= 0 && pos < _size);return *(_str + pos);}const char& operator[] (size_t pos) const{assert(pos >= 0 && pos < _size);return *(_str + _size);}//扩容void reserve(size_t n){if (n > _capacity){char* s = new char[n + 1];memcpy(s, _str, _size);delete[] _str;_str = s;_capacity = n;}}//增void push_back(char c){if (_size >= _capacity){reserve((_capacity == 0) ? 4 : 2 * _capacity);}_str[_size] = c;_size++;_str[_size] = '\0';}void append(const string& str){size_t n = _size + str._size;if (n > _capacity){reserve(n);}for (int i = 0; i < str._size; i++){_str[_size + i] = str._str[i];}_size += str._size;_str[_size] = '\0';}void append(const char* s){size_t len = strlen(s);size_t n = _size + len;if (n > _capacity){reserve(n);}for (int i = 0; i < len; i++){_str[_size + i] = s[i];}_size = n;_str[_size] = '\0';}void append(size_t n, char c){if (_size + n > _capacity){reserve(_size + n);}for (int i = 0; i < n; i++){_str[_size + i] = c;}_size += n;_str[_size] = '\0';}string& operator+=(const string& str){this->append(str);return *this;}string& operator+=(const char* s){this->append(s);return *this;}string& operator+=(char c){this->push_back(c);return *this;}//insert 、erasevoid insert(size_t pos, const string& str){assert(pos >= 0 && pos < _size);size_t len = str._size;size_t n = _size + len;if (n > _capacity){reserve(n);}//挪动数据for (size_t i = n; i >= pos + len; i--){_str[i] = _str[i - len];}memcpy(_str + pos, str._str, str._size);_size += str._size;_str[_size] = '\0';}void insert(size_t pos, const char* s){assert(pos >= 0 && pos < _size);size_t len = strlen(s);size_t n = _size + len;if (n > _capacity){reserve(n);}//挪动数据for (size_t i = n; i >= pos + len; i--){_str[i] = _str[i - len];}memcpy(_str + pos, s, len);_size += len;_str[_size] = '\0';}void insert(size_t pos, size_t n, char c){assert(pos >= 0 && pos < _size);if (_size + n > _capacity){reserve(_size + n);}//挪动数据for (size_t i = _size+n; i >= pos + n; i--){_str[i] = _str[i - n];}for (size_t i = 0; i < n; i++){_str[pos + i] = c;}_size += n;_str[_size] = '\0';}void erase(size_t pos, size_t len = npos){assert(pos >= 0 && pos < _size);if (len == npos){_str[0] = '\0';_size = 0;return;}for (size_t i = pos; (len + i) < _size; i++){_str[i] = _str[i + len];}_size -= len;_str[_size] = '\0';}//findsize_t find(const string& str, size_t pos = 0){assert(pos >= 0 && pos < _size);char* tmp = strstr(_str + pos, str._str);if (tmp == nullptr){return -1;}return tmp - _str;}size_t find(const char* s, size_t pos = 0){assert(pos >= 0 && pos < _size);char* tmp = strstr(_str + pos, s);if (tmp == nullptr){return -1;}return tmp - _str;}size_t find(char c, size_t pos = 0){assert(pos >= 0 && pos < _size);for (size_t i = pos; i < _size; i++){if (_str[i] == c){return i;}}return -1;}//swapvoid swap(string& str){Swap(_str, str._str);Swap(_size, str._size);Swap(_capacity, str._capacity);}//c_strconst char* c_str() const{return _str;}char* c_str(){return _str;}//substrstring substr(size_t pos = 0, size_t len = npos) const{assert(pos >= 0 && pos < _size);size_t n = 0;if (len == npos || pos + len > _size){n = _size - pos;}else{n = len;}string ret;for (size_t i = 0; i < n; i++){ret += _str[pos + i];}return ret;}//其他成员函数size_t size() const{return _size;}size_t length()const{return _size;}size_t capacity()const{return _capacity;}void clear(){_str[0] = '\0';_size = 0;}bool empty()const{return _size == 0;}void resize(size_t n){if (n > _capacity){reserve(n);}_size = n;_str[_size] = '\0';}void resize(size_t n, char c){if (n > _capacity){reserve(n);}for (size_t i = _size; i < n; i++){_str[i] = c;}_size = n;_str[_size] = '\0';}private:char* _str;size_t _size;size_t _capacity;const static size_t npos;};const size_t HL::string::npos = -1;std::ostream& operator<<(std::ostream& out, const HL::string& str){//for (int i = 0; i < str.size(); i++)//{//out << str[i];//}//return out;for (auto ch : str){out << ch;}return out;}std::istream& operator>>(std::istream& in, HL::string& str){char s[128] = { 0 };char ch;ch = in.get();while (ch == ' ' || ch == '\n'){ch = in.get();}str.clear();int i = 0;while (ch != '\n'){s[i] = ch;i++;if (i == 127){s[i] = '\0';str += s;i = 0;}ch = in.get();}if (i){str += s;}return in;}};