4. string类的模拟实现

首先，我们先补充一下关于编码的知识：

int main()
{char buff1[] = "abcd";char buff2[] = "比特";cout << sizeof(buff1) << endl;cout << sizeof(buff2) << endl;cout << buff1 << endl;cout << buff2 << endl;return 0;
}

---

如果严格按照标准的话，我们应该要实现成 basic_string ，但是这样难度太大，要考虑各种编码的拷贝，所以我们就实现的稍微简单一些，不要实现成模板了。

4.1 构造 + 析构

//string::string()
//{
//	_str = new char[1]{ '\0' };
//	_size = 0;
//	_capacity = 0;
//}string::string(const char* str):_size(strlen(str))
{_str = new char[_size + 1];_capacity = _size;strcpy(_str, str);
}//s2(s1)
string::string(const string& s)
{_str = new char[s._capacity + 1];strcpy(_str, s._str);_size = s._size;_capacity = s._capacity;
}string::~string()
{delete[] _str;_str = nullptr;_size = _capacity = 0;
}

4.2 c_str

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

4.3 下标遍历

size_t string::size() const
{return _size;
}char& string::operator[](size_t pos)
{assert(pos < _size);return _str[pos];
}

const char& string::operator[](size_t pos) const
{assert(pos < _size);return _str[pos];
}

4.4 迭代器

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

string::const_iterator string::begin() const
{return _str;
}string::const_iterator string::end() const
{return _str + _size;
}

4.5 插入

void string::reserve(size_t n)
{if (n > _capacity){char* tmp = new char[n + 1];strcpy(tmp, _str);delete[] _str;_str = tmp;_capacity = n;}
}void string::push_back(char ch)
{if (_size == _capacity){size_t newcapacity = 0 == _capacity ? 4 : _capacity * 2;reserve(newcapacity);}_str[_size] = ch;_str[_size + 1] = '\0';++_size;
}//"hello"  "xxxxxxxxxxxxx"
void string::append(const char* str)
{size_t len = strlen(str);if (_size + len > _capacity){reserve(_size + len);}strcpy(_str + _size, str);_size += len;
}string& string::operator+=(char ch)
{push_back(ch);return *this;
}
string& string::operator+=(const char* str)
{append(str);return *this;
}

---

void string::insert(size_t pos, char ch)
{assert(pos <= _size);if (_size == _capacity){size_t newcapacity = 0 == _capacity ? 4 : _capacity * 2;reserve(newcapacity);}/*int end = _size;while (end >= (int)pos){_str[end + 1] = _str[end];--end;}*/size_t end = _size + 1;while (end > pos){_str[end] = _str[end - 1];--end;}_str[pos] = ch;++_size;
}

---

void string::insert(size_t pos, const char* str)
{assert(pos <= _size);size_t len = strlen(str);if (_size + len > _capacity){reserve(_size + len);}/*int end = _size;while (end >= (int)pos){_str[end + len] = _str[end];--end;}*/size_t end = _size + len;while (end > pos + len - 1){_str[end] = _str[end - len];--end;}memcpy(_str + pos, str, len);_size += len;
}

4.6 删除

const size_t string::npos = -1;void string::erase(size_t pos, size_t len)
{assert(pos < _size);//len 大于等于后面字符个数时，有多少删多少if (len >= _size - pos){_str[pos] = '\0';_size = pos;}else{strcpy(_str + pos, _str + pos + len);_size -= len;}
}

4.7 查找

size_t string::find(char ch, size_t pos)
{for (size_t i = pos; i < _size; i++){if (_str[i] == ch){return i;}}return npos;
}size_t string::find(const char* str, size_t pos)
{char* p = strstr(_str + pos, str);return p - _str;
}

4.8 赋值

//s1 = s3
//s1 = s1
string& string::operator=(const string& s)
{if (this != &s){char* tmp = new char[s._capacity + 1];strcpy(tmp, s._str);delete[] _str;_str = tmp;_size = s._size;_capacity = s._capacity;}return *this;
}

4.9 交换

//s1.swap(s3)
void string::swap(string& s)
{std::swap(_str, s._str);std::swap(_size, s._size);std::swap(_capacity, s._capacity);
}

4.10 提取子串

string string::substr(size_t pos, size_t len)
{//len大于后面剩余字符，有多少取多少if (len > _size - pos){string sub(_str + pos);return sub;}else{string sub;sub.reserve(len);for (size_t i = 0; i < len; i++){sub += _str[pos + i];}return sub;}
}

4.11 比较大小

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

4.12 流插入 && 流提取

void string::clear()
{_str[0] = '\0';_size = 0;
}//一个字符一个字符放入str里，会有很多次扩容，可以优化
//istream& operator>> (istream& is, string& str)
//{
//	str.clear();//	//流提取（>>）提取不了空格和换行，istream里的函数get()可以
//	char ch = is.get();
//	while (ch != ' ' && ch != '\n')
//	{
//		str += ch;
//		ch = is.get();
//	}//	return is;
//}istream& operator>> (istream& is, string& str)
{str.clear();char buff[128];int i = 0;char ch = is.get();while (ch != ' ' && ch != '\n'){buff[i++] = ch;// 0 - 126if (i == 127){buff[i] = '\0';str += buff;i = 0;}ch = is.get();}if (i != 0){buff[i] = '\0';str += buff;}return is;
}ostream& operator<< (ostream& os, const string& str)
{for (size_t i = 0; i < str.size(); i++){os << str[i];}return os;
}

5. 现代版写法的String类

我们之前写的拷贝构造和赋值运算符重载是传统写法，其实还有现代写法：

//现代写法（让别人干活，交换）
//s2(s1)
string::string(const string& s)
{string tmp(s._str);/*std::swap(tmp._str, _str);std::swap(tmp._size, _size);std::swap(tmp._capacity, _capacity);*///这个是我们写的string类里的交换函数swap(tmp);
}

---

/*string& string::operator=(const string& s)
{if (this != &s){string tmp(s._str);swap(tmp);}return *this;
}*///s1 = s3
string& string::operator=(string tmp)
{swap(tmp);return *this;
}

5.1 完整代码

//string.h#include <iostream>
#include <assert.h>
using namespace std;namespace bit
{class string{public:typedef char* iterator;typedef const char* const_iterator;iterator begin();iterator end();const_iterator begin() const;const_iterator end() const;//string();string(const char* str = "");string(const string& s);//string& operator=(const string& s);string& operator=(string tmp);~string();const char* c_str() const;size_t size() const;char& operator[](size_t pos);const char& operator[](size_t pos) const;void reserve(size_t n);void push_back(char ch);void append(const char* str);string& operator+=(char ch);string& operator+=(const char* str);void insert(size_t pos, char ch);void insert(size_t pos, const char* str);void erase(size_t pos = 0, size_t len = npos);size_t find(char ch, size_t pos = 0);size_t find(const char* str, size_t pos =  0);void swap(string& s);string substr(size_t pos = 0, size_t len = npos);bool operator<(const string& s) const;bool operator>(const string& s) const;bool operator<=(const string& s) const;bool operator>=(const string& s) const;bool operator==(const string& s) const;bool operator!=(const string& s) const;void clear();private://char _buff[16];char* _str = nullptr;size_t _size = 0;size_t _capacity = 0;const static size_t npos;//特例，const静态成员变量只有整型可以这样声明定义（了解即可，不建议这样写）//const static size_t npos = -1;//不支持//const static double N = 2.2;};istream& operator>> (istream& is, string& str);ostream& operator<< (ostream& os, const string& str);
}

//string.cpp#include "string.h"namespace bit
{const size_t string::npos = -1;//string::string()//{//	_str = new char[1]{ '\0' };//	_size = 0;//	_capacity = 0;//}string::iterator string::begin(){return _str;}string::iterator string::end(){return _str + _size;}string::const_iterator string::begin() const{return _str;}string::const_iterator string::end() const{return _str + _size;}string::string(const char* str):_size(strlen(str)){_str = new char[_size + 1];_capacity = _size;strcpy(_str, str);}//传统写法（实在人）//s2(s1)/*string::string(const string& s){_str = new char[s._capacity + 1];strcpy(_str, s._str);_size = s._size;_capacity = s._capacity;}*///现代写法（让别人干活，交换）//s2(s1)string::string(const string& s){string tmp(s._str);/*std::swap(tmp._str, _str);std::swap(tmp._size, _size);std::swap(tmp._capacity, _capacity);*///这个是我们写的string类里的交换函数swap(tmp);}//s1 = s3//s1 = s1/*string& string::operator=(const string& s){if (this != &s){char* tmp = new char[s._capacity + 1];strcpy(tmp, s._str);delete[] _str;_str = tmp;_size = s._size;_capacity = s._capacity;}return *this;}*//*string& string::operator=(const string& s){if (this != &s){string tmp(s._str);swap(tmp);}return *this;}*///s1 = s3string& string::operator=(string tmp){swap(tmp);return *this;}string::~string(){delete[] _str;_str = nullptr;_size = _capacity = 0;}const char* string::c_str() const{return _str;}size_t string::size() const{return _size;}char& string::operator[](size_t pos){assert(pos < _size);return _str[pos];}const char& string::operator[](size_t pos) const{assert(pos < _size);return _str[pos];}void string::reserve(size_t n){if (n > _capacity){char* tmp = new char[n + 1];strcpy(tmp, _str);delete[] _str;_str = tmp;_capacity = n;}}void string::push_back(char ch){/*if (_size == _capacity){size_t newcapacity = 0 == _capacity ? 4 : _capacity * 2;reserve(newcapacity);}_str[_size] = ch;_str[_size + 1] = '\0';++_size;*/insert(_size, ch);}//"hello"  "xxxxxxxxxxxxx"void string::append(const char* str){/*size_t len = strlen(str);if (_size + len > _capacity){reserve(_size + len);}strcpy(_str + _size, str);_size += len;*/insert(_size, str);}string& string::operator+=(char ch){push_back(ch);return *this;}string& string::operator+=(const char* str){append(str);return *this;}void string::insert(size_t pos, char ch){assert(pos <= _size);if (_size == _capacity){size_t newcapacity = 0 == _capacity ? 4 : _capacity * 2;reserve(newcapacity);}/*int end = _size;while (end >= (int)pos){_str[end + 1] = _str[end];--end;}*/size_t end = _size + 1;while (end > pos){_str[end] = _str[end - 1];--end;}_str[pos] = ch;++_size;}void string::insert(size_t pos, const char* str){assert(pos <= _size);size_t len = strlen(str);if (_size + len > _capacity){reserve(_size + len);}/*int end = _size;while (end >= (int)pos){_str[end + len] = _str[end];--end;}*/size_t end = _size + len;while (end > pos + len - 1){_str[end] = _str[end - len];--end;}memcpy(_str + pos, str, len);_size += len;}void string::erase(size_t pos, size_t len){assert(pos < _size);//len 大于等于后面字符个数时，有多少删多少if (len >= _size - pos){_str[pos] = '\0';_size = pos;}else{strcpy(_str + pos, _str + pos + len);_size -= len;}}size_t string::find(char ch, size_t pos){for (size_t i = pos; i < _size; i++){if (_str[i] == ch){return i;}}return npos;}size_t string::find(const char* str, size_t pos){char* p = strstr(_str + pos, str);return p - _str;}//s1.swap(s3)void string::swap(string& s){std::swap(_str, s._str);std::swap(_size, s._size);std::swap(_capacity, s._capacity);}string string::substr(size_t pos, size_t len){//len大于后面剩余字符，有多少取多少if (len > _size - pos){string sub(_str + pos);return sub;}else{string sub;sub.reserve(len);for (size_t i = 0; i < len; i++){sub += _str[pos + i];}return sub;}}bool string::operator<(const string& s) const{return strcmp(_str, s._str) < 0;}bool string::operator>(const string& s) const{return !(*this <= s);}bool string::operator<=(const string& s) const{return *this < s || *this == s;}bool string::operator>=(const string& s) const{return !(*this < s);}bool string::operator==(const string& s) const{return strcmp(_str, s._str) == 0;}bool string::operator!=(const string& s) const{return !(*this == s);}void string::clear(){_str[0] = '\0';_size = 0;}//一个字符一个字符放入str里，会有很多次扩容，可以优化//istream& operator>> (istream& is, string& str)//{//	str.clear();//	//流提取（>>）提取不了空格和换行，istream里的函数get()可以//	char ch = is.get();//	while (ch != ' ' && ch != '\n')//	{//		str += ch;//		ch = is.get();//	}//	return is;//}istream& operator>> (istream& is, string& str){str.clear();char buff[128];int i = 0;char ch = is.get();while (ch != ' ' && ch != '\n'){buff[i++] = ch;// 0 - 126if (i == 127){buff[i] = '\0';str += buff;i = 0;}ch = is.get();}if (i != 0){buff[i] = '\0';str += buff;}return is;}ostream& operator<< (ostream& os, const string& str){for (size_t i = 0; i < str.size(); i++){os << str[i];}return os;}
}

//Test.cpp#include "string.h"namespace bit
{void test_string1(){string s1("hello world");cout << s1.c_str() << endl;for (size_t i = 0; i < s1.size(); i++){s1[i]++;}for (size_t i = 0; i < s1.size(); i++){cout << s1[i] << " ";}cout << endl;//封装：统一屏蔽了底层实现细节，提供了一种简单通用的访问容器的方式string::iterator it1 = s1.begin();while (it1 != s1.end()){cout << *it1 << " ";++it1;}cout << endl;for (auto e : s1){cout << e << " ";}cout << endl;string s2;cout << s2.c_str() << endl;const string s3("xxxxxxx");string::const_iterator it3 = s3.begin();while (it3 != s3.end()){//*it3 = 'y';//errcout << *it3 << " ";++it3;}cout << endl;for (size_t i = 0; i < s3.size(); i++){//s3[i]++;cout << s3[i] << " ";}cout << endl;}void test_string2(){string s1("hello world");cout << s1.c_str() << endl;s1.push_back('x');cout << s1.c_str() << endl;s1.append("yyyyy");cout << s1.c_str() << endl;s1 += 'z';s1 += "mmmmmm";cout << s1.c_str() << endl;}void test_string3(){string s1("hello world");cout << s1.c_str() << endl;s1.insert(6, 'x');cout << s1.c_str() << endl;s1.insert(0, 'x');cout << s1.c_str() << endl;string s2("hello world");cout << s2.c_str() << endl;s2.insert(6, "yyy");cout << s2.c_str() << endl;s2.insert(0, "yyy");cout << s2.c_str() << endl;string s3("hello world");cout << s3.c_str() << endl;//s3.erase(6, 10);s3.erase(6);cout << s3.c_str() << endl;string s4("hello world");cout << s4.c_str() << endl;s4.erase(6, 3);cout << s4.c_str() << endl;}void test_string4(){string s1("hello world");cout << s1.find('o') << endl;cout << s1.find("wor") << endl;}void test_string5(){string s1("hello world");string s2(s1);s1[0] = 'x';cout << s1.c_str() << endl;cout << s2.c_str() << endl;string s3("yyyy");s1 = s3;cout << s1.c_str() << endl;cout << s3.c_str() << endl;string s4("zzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzz");s1 = s4;cout << s1.c_str() << endl;cout << s4.c_str() << endl;s1 = s1;cout << s1.c_str() << endl;cout << s3.c_str() << endl;std::swap(s1, s3);cout << s1.c_str() << endl;cout << s3.c_str() << endl;s1.swap(s3);cout << s1.c_str() << endl;cout << s3.c_str() << endl;}void test_string6(){string url("https://gitee.com/ailiangshilove/cpp-class/blob/master/%E8%AF%BE%E4%BB%B6%E4%BB%A3%E7%A0%81/C++%E8%AF%BE%E4%BB%B6V6/string%E7%9A%84%E6%8E%A5%E5%8F%A3%E6%B5%8B%E8%AF%95%E5%8F%8A%E4%BD%BF%E7%94%A8/TestString.cpp");size_t pos1 = url.find(':');string url1 = url.substr(0, pos1 - 0);cout << url1.c_str() << endl;size_t pos2 = url.find('/', pos1 + 3);string url2 = url.substr(pos1 + 3, pos2 - (pos1 + 3));cout << url2.c_str() << endl;string url3 = url.substr(pos2 + 1);cout << url3.c_str() << endl;}void test_string7(){//string s1("hello world");//cout << s1 << endl;string s1;cin >> s1;cout << s1 << endl;}void test_string8(){string s1("hello world");string s2(s1);cout << s1 << endl;cout << s2 << endl;string s3("xxxxxxxxxxxxxxxxxxxxxxxxx");s1 = s3;cout << s1 << endl;cout << s3 << endl;}
}int main()
{bit::test_string8();return 0;
}

6. 写时拷贝（了解）

写时拷贝就是一种拖延症，是在浅拷贝的基础之上增加了引用计数的方式来实现的。

引用计数：用来记录资源使用者的个数。在构造时，将资源的计数给成1，每增加一个对象使用该资源，就给计数增加1，当某个对象被销毁时，先给该计数减1，然后再检查是否需要释放资源，如果计数为1，说明该对象时资源的最后一个使用者，将该资源释放；否则就不能释放，因为还有其他对象在使用该资源。

举个例子：

void copy_on_write()
{//如果计数不等于1，再去做深拷贝
}void string::push_back(char ch)
{//写时拷贝copy_on_write();/*if (_size == _capacity){size_t newcapacity = 0 == _capacity ? 4 : _capacity * 2;reserve(newcapacity);}_str[_size] = ch;_str[_size + 1] = '\0';++_size;*/insert(_size, ch);
}

可以用下面的代码来看是否是用了写时拷贝：

void test_string9()
{std::string s1("hello world");std::string s2(s1);cout << (void*)s1.c_str() << endl;cout << (void*)s2.c_str() << endl;
}

Windows的VS下没有用写时拷贝，而Linux的g++就使用了写时拷贝。