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string底层是一个是字符数组

为了跟库里的string区别，所以定义一个命名空间将类string包含

一、构造

1.构造函数

注意：将char*传给const char*是范围缩小，因此只能1：1构造一个

strlen遇到nullptr解引用会报错，因此缺省参数只能设置为" "

string(const char* str = ""):_size(strlen(str))
{if (_size == 0){_capacity = 3;}else{_capacity = _size;}_str = new char[_capacity + 1];strcpy(_str, str);
}

2.拷贝构造

深拷贝：重新开辟一个空间

浅拷贝（值拷贝）：拷贝一个指针，指向同一片空间

string(const string& s):_size(s._size),_capacity(s._capacity)
{_str = new char[_capacity + 1];strcpy(_str, s._str);
}

3.析构

内置类型，调用默认成员函数，自定义类型调用默认构造

~string()
{if (_str != NULL){delete[] _str;_str = NULL;}_size = 0;_capacity = 0;
}

4.赋值

赋值运算符也是默认成员函数，如果不写会进行浅拷贝/值拷贝

string& operator=(const string& s)
{_size = s._size;_capacity = s._capacity;delete[] _str;_str = new char[_capacity+1];strcpy(_str, s._str);return *this;
}

二、迭代器

使用typedef 将用iterator代替char*

1.begin

iterator begin()
{return _str;
}

2.end

iterator end()
{return _str + _size;
}

三、修改

1.push_back

void push_back(char c)
{if (_size + 1 > _capacity){reserve(_size + 1);char* tmp = new char[_capacity + 1];strcpy(tmp, _str);delete[] _str;_str = tmp;}_str[_size] = c;_size++;_str[_size] = '\0';
}

2.+=

复用尾插

string& operator+=(char c)
{push_back(c);return *this;
}

3.append

通过reserve类似扩容的操作，扩大了字符串长度的空间，并且在原字符串‘\0’的位置上拷贝str字符串

void append(const char* str)
{size_t len = strlen(str);if (len + _size > _capacity){reserve(len + _size);char* tmp = new char[_capacity + 1];strcpy(tmp, _str);delete[] _str;_str = tmp;}strcpy(_str + _size, str);_size += len;
}

4.clear

void clear()
{_size = 0;_str[0] = '\0';
}

5.swap

void swap(T& a, T& b)
{T tmp = a;a = b;b = tmp;
}
void swap(string& s)
{bit::swap<char*>(_str, s._str);bit::swap<size_t>(_size, s._size);bit::swap<size_t>(_capacity, s._capacity);
}

6.c_str

const char* c_str()const
{const char* tmp = _str;return tmp;
}

四、容量

1.size

	size_t size()const{return _size;}

2.capacity

size_t capacity()const
{return _capacity;
}

3.empty

bool empty()const
{if (_size == 0){return true;}else{return false;}
}

4.resize

分三种情况

n<_size  删除数据

_size<n<_capacity 剩余空间初始化

n>_capacity 扩容+初始化

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

5.reserve

为防止new失败，所以使用临时变量tmp指向new出来的空间，若new成功，释放旧空间，并将—_str指向新的空间

void reserve(size_t n)
{if (n > _capacity){_capacity = n;char* tmp = new char[_capacity + 1];strcpy(tmp, _str);delete[]_str;_str = tmp;}
}

五、访问

1.[ ]

char& operator[](size_t index)
{return _str[index];
}
const char& operator[](size_t index)const
{return _str[index];
}

2."<,<=,>,>=,==,!="

通过strcmp，比较字符串从头开始字符的ASCII值，再通过复用来实现剩下的

const只能调用const,非const既可以调用非const，也可以调用const

bool operator<(const string& s)
{return (*this >= s);
}
bool operator<=(const string& s)
{return !(*this > s);
}
bool operator>(const string& s)
{return (strcmp(_str, s._str) > 0);
}
bool operator>=(const string& s)
{return (*this == s || *this > s);
}
bool operator==(const string& s)
{return strcmp(_str, s._str) == 0;
}
bool operator!=(const string& s)
{return !(*this==s);
}

3.find

size_t find(char c, size_t pos = 0)const
{while (pos < _size){if (_str[pos] == c){return pos;}pos++;}return npos;
}
size_t find(const char* s, size_t pos = 0)const
{const char* p = strstr(_str, s);if (p != nullptr){return _str - p;}return npos;
}

4.insert

注意size_t类型变量没有负数

string& insert(size_t pos, char c)
{if (_size + 1 > _capacity){reserve(_size + 1);char* tmp = new char[_capacity + 1];strcpy(tmp, _str);delete[]_str;_str = tmp;}size_t end = _size;while (end >= pos){_str[end + 1] = _str[end];;end--;}_str[pos] = c;_size++;_str[_size] = '\0';return *this;
}
string& insert(size_t pos, const char* str)
{size_t len = strlen(str);if (_size + len > _capacity){reserve(_size + len);char* tmp = new char[_capacity + 1];strcpy(tmp, _str);delete[]_str;_str = tmp;}size_t end = _size;while (end >= pos){_str[end + len] = _str[end];end--;}strncpy(_str + pos, str, len);_size += len;return *this;
}

5.erase

pos位置开始删除len个数据

string& erase(size_t pos, size_t len)
{if (len == npos && len + pos > _size){_str[pos] = '\0';}else{while (pos+len<_size){_str[pos] = _str[pos + len];pos++;}_size -= len;_str[_size] = '\0';}return *this;
}

6.流插入<<

ostream& operator<<(ostream& _cout, const bit::string& s)
{for (int i = 0; i < s._size; i++){_cout << s[i] << "";}_cout << endl;return _cout;
}

7.流提取>>

输入多个值，C++规定 空格/换行是值与值之间的区分

istream& operator>>(istream& _cin, bit::string& s)
{s.clear();char ch = _cin.get();char buf[128];int index = 0;while (ch != ' ' && ch != '\n'){buf[index++] = ch;if (index == 127){buf[index] = '\0';s += buf;index = 0;}ch = _cin.get();}if (index != 0){s += buf;}return _cin;
}