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string()

//注意事项：
1.初始化列表随声明的顺序进行初始化
2.const char* str = nullptr 是错误写法，编译器会报错
3.const char* str = ‘\0’类型不匹配
4.capacity+1 的原因是 _capacity是有效字符的数量加一是为了给‘\0’留空间

//构造函数
string(const char* str = "")
{_size = strlen(str);_capacity = _size;_str = new char(_capacity + 1);memcpy(_str, str, _size + 1);
}

string()

//这里重点注意深浅拷贝
浅拷贝：也称位拷贝，编译器只是将对象中的值拷贝过来.(如果自己没写拷贝函数，编译器会自动生成浅拷贝的拷贝函数)
浅拷贝的两大缺陷：
1.如果对象中管理资源，最后就会导致多个对象共
享同一份资源.
2.当一个对象销毁时就会将该资源释放掉，而此时另一些对象不知道该资源已经被释放，以为
还有效，所以当继续对资源进项操作时，就会发生发生了访问违规。

深拷贝：如果一个类中涉及到资源的管理，其拷贝构造函数、赋值运算符重载以及析构函数必须要显式给出。一般情况都是按照深拷贝方式提供。

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

3.~string()

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

4. push_back()

//尾插字符
void push_back(char c)
{if (_size == _capacity){int newcapacity = _capacity == 0 ? 4 : 	_capacity * 2;reserve(newcapacity);_str[_size++] = c;_str[_size] = '\0';}
}

5.append()

//尾插字符串
void append(const char* str)
{size_t len = strlen(str);if (_size + len > _capacity){reserve(_size+len);}/*strcpy(_str+_size, str);*/memcpy(_str+_size,str,len+1);_size = _size + len;
}

6.insert()

//在pos添加字符或字符串
string& insert(size_t pos, size_t n,char c)
{assert(pos <= _size);if (_size+n > _capacity){reserve(_size+n);}size_t end = _size;while (end >= pos && end != npos){_str[end + n] = _str[end];end--;}for (int i = 0; i < n; i++){_str[pos+i] = c;}_size += n;return *this;}string& insert(size_t pos, const char* str)
{assert(pos <= _size);int len = strlen(str);if (_size + len > _capacity){reserve(_size + len);}size_t end = _size;while (end >= pos && end != npos){_str[end + len] = _str[end];end--;}for (int i = 0; i < len; i++){_str[pos + i] = str[i];}_size += len;return *this;
}

7.erase()

//消除从pos位置开始n个字符
string& erase(size_t pos, size_t len = npos)
{assert(pos < _size);if (len == npos || pos + len >= _size){_str[pos] = '\0';_size = pos;}while (pos <= _size-len){_str[pos] = _str[pos + len];pos++;}_size -= len;return *this;
}

8.find()

 返回c在string中第一次出现的位置
size_t find(char c, size_t pos = 0) const
{for (size_t i = pos; i < _size; i++){if (_str[i] == c){return i;}}} 返回子串s在string中第一次出现的位置size_t find(const char* s, size_t pos = 0) const
{char* str = _str + pos;while (*str){const char* scopy = s;const char* strcopy = str;while (*scopy && *scopy == *strcopy){scopy++;strcopy++;	}if (*scopy == '\0'){return str - _str;}else{str++;}}
return npos;
}

9.clear()

//清理字符
void clear()
{_size = 0;_str[0] ='\0';
}

10.size()

//返回有效字符大小
//这里const 是为了能让const对象和非const对象都能够调用
size_t size() const
{return _size;
}

11.c_str()

//返回字符串
//这里const 是为了能让const对象和非const对象都能够调用
const char* c_str() const
{return _str;
}

12.capacity()

//返回有效容量大小
//这里const 是为了能让const对象和非const对象都能够调用
size_t capacity() const
{return _capacity;
}

13. operator<

bool operator<(const string& s)const
{int ret = memcmp(_str, s._str, _size < s._size ? _size : s._size);return ret == 0 ? _size < s._size : ret < 0;	
}

14.operator==

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

15.operator<=

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

16.operator>

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

17.operator>=

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

18.operator!=

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

19.operator[]

//可读可改
char& operator[](size_t index)
{assert(index < _size);return _str[index];
}
//只读不改
const char& operator[](size_t index)const;
{assert(index < _size);return _str[index];
}

20.operator=

string& operator=(string s)//这里调用拷贝构造，不改变原对象
{if (this != &s){std::swap(_size, s._size);std::swap(_capacity, s._capacity);std::swap(_str, s._str);}return *this;
}string& operator=(const string& s)
{if (&s != this){string tmp(s);std::swap(_size, tmp._size);std::swap(_capacity, tmp._capacity);std::swap(_str, tmp._str);}
return *this;
}

21.operator +=

//添加字符
string& operator+=(char ch)
{push_back(ch);return *this;
}
//添加字符串
string& operator+=(const char* str)
{append(str);return *this;
}

22.operator<<

//输出输入流只能用引用接受和返回
ostream& operator<<(ostream& out, const string& s)
{for (auto ch : s){out << ch;}return out;
}

23.operator>>

ostream& operator>>(ostream& in, string& s)
{s.clear();char ch = in.get();while (ch == ' ' || ch == '&'){ch = in.get();}char buff[128];int i = 0;while (ch != ' ' && ch != '\n'){if (i == 127){buff[i++] = '\0';i = 0;s += buff;}s += ch;ch =in.get();}if (i != 0){buff[i] = '\0';s += buff;}return in;
}

24.resize()

//设置有效字符大小，如果比原先小则保持不变，如果比原先大则在尾部添加。
void resize(size_t n, char c = '\0')
{if (n < _size){_size = n;_str[_size] = '\0';}else{reserve(n);for (size_t i = _size; i < n; i++){_str[i] = c;}_size = n;_str[_size] = '\0';}
}

25.reserve()

//设置容量大小，只增不减
void reserve(size_t n)
{if (n > _capacity){char*  tmp = new char[n + 1];memcpy(tmp, _str, _size + 1);_capacity = n;delete[] _str;_str = tmp;	}
}

26.substr()

string substr(size_t pos = 0, size_t len = npos)
{assert(pos < _size);int n = 0;if (len == npos || pos + len >= _size){n = _size - pos;}string tmp;tmp.reserve(n);for (size_t i = pos; i < pos+n; i++){tmp+=_str[i];//为什么不加\0 }return tmp;
}