c++string的基本實現

class String
{
public:
	typedef char* Iterator;
	friend ostream& operator<<(ostream&_cout,String&s);

	String(const char* str = "")
	{
		//構造string類對象時，如果傳遞nullptr時，認爲非法，加上斷言
		if (nullptr == str)
		{
			assert(false);
			return;
		}
		_size = strlen(str);  //確定長度
		_capacity = _size;//確定容量
		_str = new char[_capacity+1];//申請空間
		strcpy(_str, str);//拷貝值
	}
	String(const String&s)
		:_str(new char[s._capacity+1]) //申請新的空間
		, _size(s._size)
		, _capacity(s._capacity)
	{
		strcpy(_str, s._str);//值拷貝
	}
	String& operator=(const String& s)
	{
		if (this != &s) //不等於自身
		{
			char* pStr = new char[s._capacity+1];//創造另一個變量
			strcpy(pStr, s._str);
			this->_size = s._size;
			this->_capacity = s._capacity;
			delete[] this->_str;
			this->_str = pStr;
		}
		return *this;
	}
	~String()
	{
		if (_str)
		{
			delete[] _str;
			_str = nullptr;
			_capacity = 0;
			_size = 0;
		}
	}

	//Iterator
	Iterator Begin()
	{
		return _str;
	}
	Iterator End()
	{
		return _str + _size;
	}
	//modify
	void PushBack(char c)
	{
		if (_size == _capacity)//檢查擴容
		{
			Reserve(_capacity * 2);
		}
		_str[_size++] = c;
		_str[_size] = '\0';
	}
	void Append(size_t n, char c)
	{
		for (size_t i = 0; i < n; ++i)
		{
			PushBack(c);
		}
	}
	String& operator+=(char c)
	{
		//if (_size == _capacity)//檢查擴容
		//{
		//	Reserve(_capacity * 2);
		//}
		//this->_str[_size++] = c;
		//this->_size = '\0'
		PushBack(c);
		return *this;
	}
	void Clear()
	{
		_size = 0;
		_str[_size] = '\0';
	}
	void Swap(String& s)
	{
		swap(_str, s._str);
		swap(_size, s._size);
		swap(_capacity, s._capacity);
	}
	const char* C_str() const
	{
		return _str;
	}

	//capacity;
	size_t Size() const
	{
		return _size;
	}

	size_t Capacity() const
	{
		return _capacity;
	}
	bool Empty() const
	{
		return 0 == _size;
	}
	void Resize(size_t newSize, char c = char())
	{
		if (newSize > _size)
		{
			//如果newSize大於底層空間大小，則需要重新開闢空間
			if (newSize > _capacity)
			{
				Reserve(newSize);
			}
			memset(_str+_size,c,newSize-_size);
		}
		_size = newSize;
		_str[newSize] = '\0';
	}
	void Reserve(size_t newCapacity)
	{
		//如果新容量大於舊容量，則開闢空間
		if (newCapacity > _capacity)
		{
			char* str = new char[newCapacity + 1];
			strcpy(str, _str);

			//釋放舊空間，使用新空間
			delete[] _str;
			_str = str;
			_capacity = newCapacity;
		}
	}

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

ostream& operator<<(ostream&_cout, String& s)
{
	cout << s._str;
	return _cout;
}