STL中的string你了解吗

class MyString
{
private:
	char *_str;//字符串指针
	size_t _size;//字符串大小
	size_t _capacity;//总共能存多少有效字符，不包括'\0'
	static size_t npos;//迭代器相关
}
size_t MyString:: npos = -1;

//构造函数
MyString(const char* str = "")//缺省参数
{
	_size = strlen(str);//初始化
	_capacity = _size;
	_str = new char[_capacity + 1];//'\0'的空间+1
	strcpy(_str, str);
}
//析构函数
~MyString()
{
	delete[] _str;//释放内存
	_str = nullptr;//将指针置空
	_size = 0;//清理工作
	_capacity = 0;
}
//拷贝构造函数
MyString(const MyString& str)
{
	_size = str._size;
	_capacity = str._capacity;
	_str = new char[_capacity + 1];
	strcpy(_str, str._str);
}
//赋值运算符重载
MyString& operator=(const MyString& str)
{
	if (_str != str._str)
	{
		delete[] _str;
		_size = str._size;
		_capacity = str._capacity;
		_str = new char[_capacity + 1];
		strcpy(_str, str._str);
	}
	return *this;
}
 

char& operator[](size_t index)
{
	assert(index < _size&&index >= 0);
	return _str[index];
}
const char& operator[](size_t index)const
{
	assert(index < _size&&index >= 0);
	return _str[index];
}

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

void test_string()
{
	MyString ms;
	ms = "123";
	MyString::iterator it = ms.begin();
	while (it != ms.end())
	{
		cout << *it << endl;
		it++;
	}
}

const iterator cbegin()const
{
	return _str;
}
const iterator cend()const
{
	return _str + _size;
}

//返回字符串大小
size_t size()const
{
	return _size;
}
size_t capacity()const
{
	return _capacity;
}
//判断是否为空字符串
bool empty()const
{
	return _size == 0;
}
//更改容量
void reserve(size_t n = 0)
{
	if (n > _capacity)
	{
		char* tmp = new char[n + 1];
		strcpy(tmp, _str);
		delete[] _str;
		_str = tmp;
		_capacity = n;
	}
}
//更改大小
//resize分三种情况
void resize(size_t n = 0,char ch = '\0')
{
	if (n >= 0 && n <= _size)
	{
		_size = n;
		_str[_size] = '\0';
	}
	else if (n > _size && n <= _capacity)
	{
		for (size_t i = _size; i < n; i++)
			_str[i] = ch;
		_size = n;
		_str[_size] = '\0';
	}
	else if (n > _capacity)
	{
		reserve(n);
		for (size_t i = _size; i < n; i++)
		{
			_str[i] = ch;
		}
		_size = n;
		_str[_size] = '\0';
	}
	else
		assert(0);
}

MyString& operator+=(const char* str)
{
	int len = strlen(str);
	if (len + _size > _capacity)//判断是否超出容量
	{
		reserve(len + _size);
	}
	strcpy(_str + _size, str);
	_size += len;
	return *this;
}
MyString& operator+=(char ch)
{
	if (_size == _capacity)//扩容
    {
	    size_t newcapacity = (_capacity) == 0 ? 2 : _capacity * 2;
	    reserve(newcapacity);
    }
    _str[_size] = ch;
    _size++;
    _str[_size] = '\0';//尾插过后会把'\0给覆盖了，重新在最后一个位置补一个'\0'
	return *this;
}

//需要在类外面重载
//输出流
ostream& operator<<(ostream& out, const MyString& str)
{
	for (size_t i = 0; i < str.size(); i++)
	{
		out << str[i];
	}
	return out;
}
//输入流
istream& operator>>(istream& in, MyString& str)
{
	while (1)
	{
		char ch = in.get();
		if (ch != ' '&&ch != '\n')//cin遇到空格和'\n'会结束
		{
			str += ch;
		}
		else
			break;
	}
	return in;
}
 

//getline是遇到'\n'才结束
istream& getline(istream& in, MyString& s)
{
	while (1)
	{
		char ch;
		ch = in.get();//从缓存去读入所有输入字符
		if (ch != '\n')
		{
			s += ch;
		}
		else
			break;
	}
	return in;
}

void push_back(char ch)//插入一个字符，尾插
{
	if (_size == _capacity)
	{
		size_t newcapacity = (_capacity) == 0 ? 2 : _capacity * 2;
		reserve(newcapacity);
	}
	_str[_size] = ch;
	_size++;
	_str[_size] = '\0';//尾插过后会把'\0给覆盖了，重新在最后一个位置补一个'\0'
}

MyString& insert(size_t pos, const char ch)
{
	assert(pos <= _size && pos >= 0);
	if (_size == _capacity)
	{
		size_t newcapacity = _capacity == 0 ? 2 : 2 * _capacity;
		reserve(newcapacity);
	}
	int end = _size;
	while (end >= (int)pos)//为什么要强转，如果是头插，end最终=-1，
	{					   //-1和无符号比较会向无符号转变成一个32位的最大值，成为死循环
		_str[end + 1] = _str[end];
		end--;
	}
	_str[pos] = ch;
	_size++;
	return *this;
}
MyString& insert(size_t pos, const char* str)
{
	assert(pos <= _size && pos >= 0);
	size_t len = strlen(str);
	if (_size+ len > _capacity)
	{				
		reserve(_capacity + len);
	}
	int end = _size;
	while (end >= (int)pos)//往后挪
	{
		_str[end + len] = _str[end];
		end--;
	}
	for (size_t i = 0; i < len; i++)
	{
		_str[pos + i] = str[i];
	}
 
	_size += len;
	return *this;
}

//npos = -1(无符号整型最大值)
MyString& erase(size_t pos, size_t len = npos)
{
	assert(pos >= 0 && pos < _size);
	if (pos + len >= _size || len == npos)
	{
		_size = pos;
		_str[_size] = '\0';
	}
	else
	{
		for (size_t i = 0; i < _size - len - pos; i++)
		{
			_str[i + pos] = _str[i + pos + len];
		}
		_size -= len;
	}
	_str[_size] = '\0';
	return *this;
}