string底层实现，参考了网上的一些代码，注释都写在代码里了，比较清楚。供大家参考交流

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#pragma once
#includeclass MyString
{public:
	typedef char* iterator;//定义一下迭代器类型，显得正式一点
	typedef const char* const_iterator;

private:
	char* _str;
	size_t _size;//大小
	size_t _capacity;//容量

public:
	static char* MyStrcpy(char* dst, const char* src);

	MyString(const char* str = "");
	MyString(const MyString& str);
	~MyString();

	iterator begin();
	const_iterator cbegin() const;

	iterator end();
	const_iterator cend() const;

	size_t size() const;
	size_t capacity() const;
	char* c_str() const;
	
	MyString& insert(size_t pos, char c);
	MyString& insert(size_t pos, const char* str);

	void push_back(const char c);
	MyString& append(const char* str);
	MyString& erase(size_t pos, size_t len=-1);
	void reserve(size_t n = 0);
	void resize(size_t n, char c = '\0');
	size_t find(char c, size_t pos = 0) const;
	size_t find(const char* str, size_t pos = 0) const;

	MyString& operator+=(const char c);
	MyString& operator+=(const char* str);
	char& operator[](size_t pos);
};

#include "MyString.h"

//字符串复制函数，静态方法
char* MyString::MyStrcpy(char* dst, const char* src)
{if (dst == nullptr || src == nullptr || dst == src)
		return dst;
	while (*src != '\0')
	{*dst = *src;
		dst++;
		src++;
	}
	*dst = '\0';
	return dst;
}
//构造函数
MyString::MyString(const char* str) :_size(strlen(str))
{_capacity = _size;
	_str = new char[_capacity + 1];//多一个是用来存'\0'的，之前的strlen()不会把'\0'算进去
	MyStrcpy(_str, str);
}
//拷贝构造函数
MyString::MyString(const MyString& str)
{char* temp = new char[str._capacity + 1];
	MyStrcpy(temp, str._str);
	_str = temp;
	_size = str._size;
	_capacity = str._capacity;
}
//析构函数
MyString::~MyString()
{if (_str)
	{delete[] _str;
		_size =0;
		_capacity = 0;
	}
}

MyString::iterator MyString::begin()
{return _str;
}

MyString::const_iterator MyString::cbegin() const
{return _str;
}

MyString::iterator MyString::end()
{return _str + _size;
}

MyString::const_iterator MyString::cend() const
{return _str + _size;
}


size_t MyString::size() const
{return _size;
}

size_t MyString::capacity() const
{return _capacity;
}

char* MyString::c_str() const
{return _str;
}

//扩容函数
void MyString::reserve(size_t n)
{//n大于现有容量的时候，会申请扩容，然后把之前的内容复制过去。否则什么都不做
	if (n >_capacity)
	{char* temp = new char[n+1];
		MyStrcpy(temp, _str);
		delete[] _str;
		_str = temp;
		_capacity = n;
	}
}


//插入字符
MyString& MyString::insert(size_t pos, char c)
{//无法插入，直接返回
	if (pos >_size)
		return *this;
	//如果此时存的字符串已经装满了，需要先两倍扩容一下
	if (_size == _capacity)
	{size_t newcap = _capacity == 0 ? 4 : 2 * _capacity;
		reserve(newcap);
	}

	//找到要插入的地方插入字符
	//因为_capacity 若不够的话，此时已经扩容了，所以_str[it]是不会越界的
	size_t it = _size + 1;
	while (it != pos)
	{_str[it] = _str[it - 1];
		it--;
	}
	_str[pos] = c;
	_size++;

	return *this;
}

//插入字符串
MyString& MyString::insert(size_t pos, const char* str)
{//无法插入，直接返回
	if (pos >_size) 
		return *this;
	
	//验证是否要扩容
	size_t len = strlen(str);
	reserve(_size + len);

	//找到并且插入字符串
	size_t it = _size + len;
	while (it - pos >len - 1)
	{_str[it] = _str[it - len];
		it--;
	}
	for (size_t i = 0; i< len; i++)
	{_str[pos] = str[i];
		pos++;
	}
	_size = _size + len;
	return *this;
}

void MyString::push_back(const char c)
{insert(_size, c);
}

MyString& MyString::append(const char* str)
{insert(_size, str);
	return *this;
}

MyString& MyString::erase(size_t pos, size_t len)
{//-1是默认值，代表全部删除
	if (len == -1 || len >= _size - pos)
	{_str[pos] = '\0';
		_size = pos;
	}
	else
	{for (size_t i = pos + len; i<= _size; i++)
		{	_str[i - len] = _str[i];
		}
		_size = _size - len;
	}
	return *this;
}

void MyString::resize(size_t n, char c)
{reserve(n);
	if (n >= _size)
	{memset(_str + _size, c, n - _size);
		_size = n;
		_str[_size] = '\0';
	}
	else
	{_str[n] = '\0';
		_size = n;
	}
}

size_t MyString::find(char c, size_t pos) const
{while (pos< _size)
	{if (_str[pos] == c)
			return pos;
		pos++;
	}
	return -1;
}

size_t MyString::find(const char* str, size_t pos) const
{const char* p = strstr(_str + pos, str);
	if (p == nullptr)
		return -1;
	return p - _str;
}

MyString& MyString::operator+=(const char c)
{insert(_size, c);
	return *this;
}

MyString& MyString::operator+=(const char* str)
{insert(_size, str);
	return *this;
}

char& MyString::operator[](size_t pos)
{return _str[pos];
}

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