【C++教程从0到1入门编程】第八篇:STL中string类的模拟实现

一、 string类的模拟实现

下面是一个列子

#include <iostream>
namespace y
{class string{public: //string()            //无参构造函数//	:_str(nullptr)//{}//string(char* str)  //有参构造函数//	:_str(str)//{}string():_str(new char[1]){_str[0] = '\0';}string(char* str)   //构造函数在堆上开辟一段strlen+1的空间+1是c_str:_str(new char[strlen(str)+1]){strcpy(_str, str); //strcpy会拷贝\0过去}//string(char* str="")   //构造函数在堆上开辟一段strlen+1的空间+1是c_str//	:_str(new char[strlen(str) + 1])//{//	strcpy(_str, str); //strcpy会拷贝\0过去//}size_t size(){return strlen(_str);}bool empty(){return _str == nullptr;}char& operator[](size_t i)  //用引用返回不仅可以读字符，还可以修改字符{return _str[i];}~string()          //析构函数{if (_str){delete[] _str;_str = nullptr;}}const char* c_str() //返回C的格式字符串{return _str;}private:char* _str;};void TestString1(){string s1("hello");string s2;for (size_t i = 0; i < s1.size(); i++){s1[i] += 1;std::cout << s1[i] << " ";}std::cout << std::endl;for (size_t i = 0; i < s2.size(); i++){s2[i] += 1;std::cout << s2[i] << " ";}std::cout << std::endl;}void TestString2(){string s1("hello");string s2(s1);std::cout << s1.c_str() << std::endl;std::cout << s2.c_str() << std::endl;string s3("world");s1 = s3; //调试点这里，析构也是两次std::cout << s1.c_str() << std::endl;std::cout << s3.c_str() << std::endl;}
}

这段代码中，存在一定的问题，当我们调试时会发现！

默认的拷贝的构造函数出现的问题！

！默认的赋值运算符重载出现的问题。！

        上述string类没有显式定义其拷贝构造函数与赋值运算符重载，此时编译器会合成默认的，当用s1构造s2时，编译器会调用默认的拷贝构造。最终导致的问题是，s1、s2共用同一块内存空间，在释放时同一块空间被释放多次而引起程序崩溃，这种拷贝方式，称为浅拷贝。

此时引出了概念浅拷贝，

浅拷贝：也称位拷贝，编译器只是将对象中的值拷贝过来。如果对象中管理资源，最后就会导致多个对象共享同一份资源，当一个对象销毁时就会将该资源释放掉，而此时另一些对象不知道该资源已经被释放，以为还有效，所以 当继续对资源进项操作时，就会发生发生了访问违规。要解决浅拷贝问题，C++中引入了深拷贝。

那么深拷贝呢？

二、string类的模拟实现

头文件代码：

#include<iostream>
#include<assert.h>
namespace yyw
{class string{public:typedef char* iterator;public:string(const char* str = "")        //构造函数{_size = strlen(str);_capacity = _size;_str = new char[_capacity + 1];strcpy(_str, str);}//string s2(s1)string(const string& s):_str(nullptr), _size(0), _capacity(0){string tmp(s._str);this->swap(tmp);}//s1=s3string& operator=(const string& s) //(string s){//this->swap(s);string tmp(s._str);this->swap(tmp);return *this;}void swap(string& s){std::swap(_str, s._str);std::swap(_size, s._size);std::swap(_capacity, s._capacity);}~string()                         //析构函数{if (_str){delete[] _str;_size = _capacity = 0;_str = nullptr;}}//string(const string& s)   //拷贝构造void push_back(char ch)           //增加字符{if (_size == _capacity)      //增加空间{size_t newcapacity = _capacity == 0 ? 6 : _capacity * 2;char* tmp = new char[newcapacity + 1];strcpy(tmp, _str);delete[] _str;_str = tmp;_capacity = newcapacity;}_str[_size] = ch;_size++;_str[_size] = '\0';    //_size的位置设置为\0}void append(char* str)     //追加字符串{size_t len = strlen(str);if (_size + len > _capacity)   //注意不能按2倍去增容{size_t newcapacity = _size + len;char* tmp = new char[newcapacity + 1];strcpy(tmp, _str);delete[] _str;_str = tmp;_capacity = newcapacity;}strcpy(_str + _size, str);_size += len;//_str[_size + len] = '\0'; strcpy已经把\0拷贝过去了}//s1+='ch' s1就是thisstring& operator+=(char ch){this->push_back(ch);return *this;}//s1+="ch" s1就是thisstring& operator+=(char* ch){this->append(ch);return *this;}string& insert(size_t pos, char ch)       //在pos位置插入字符{assert(pos <= _size);if (_size == _capacity){size_t newcapacity = _capacity == 0 ? 4 : _capacity * 2;char* tmp = new char[newcapacity + 1];strcpy(tmp, _str);delete[] _str;_str = tmp;//delete[] _str;  //注意这里不能写反了_capacity = newcapacity;}size_t end = _size;while (end >= (int)pos){_str[end + 1] = _str[end];end--;}_str[pos] = ch;_size++;return *this;}string& insert(size_t pos, char* str)     //在pos位置插入字符串{assert(pos < _size);size_t len = strlen(str);if (_size + len > _capacity){size_t newcapacity = _size + len;char* tmp = new char[newcapacity + 1];strcpy(tmp, _str);delete[] _str;_str = tmp;_capacity = newcapacity;}size_t end = _size;while (end >= (int)pos){_str[end + len] = _str[end];   //这里是挪len个不是1个end--;}//strncpy也可以//strncpy(_str + pos, str, len);//strcpy会把\0拷贝过去，不可以//写个循环从pos依次往后放for (size_t i = 0; i < len; i++){_str[pos] = str[i];pos++;}_size += len;//返回自己return *this;}string& erase(size_t pos, size_t len = npos){assert(pos < _size);if (len >= _size - pos){_str[pos] = '\0';_size = pos;}else{size_t i = pos + len;while (i <= _size){_str[i - len] = _str[i];i++;}_size = _size - len;}return *this;}size_t find(char ch, size_t pos)     //在pos位置查找字符{for (size_t i = pos; i < _size; i++){if (_str[i] == ch){return i;}}return npos;}size_t find(char* str, size_t pos)   //在pos位置查找字符串{char* p = strstr(_str, str);if (p == NULL){return npos;}else{return (p - str);}}void resize(size_t newsize, char ch = '\0')  //填充字符ch{if (newsize < _size)   //第三种情况{_str[newsize] = '\0';_size = newsize;}else{if (newsize > _capacity)   //增加容量{size_t newcapacity = newsize;char* tmp = new char[newcapacity];strcpy(tmp, _str);delete[]_str;_str = tmp;_capacity = newcapacity;}for (size_t i = _size; i < newsize; i++)  //把字符ch往_size后面填{_str[i] = ch;}_size = newsize;_str[_size] = '\0';}}iterator begin()                //iterator迭代器的原理{return _str;}iterator end(){return (_str + _size);}const char* c_str(){return _str;}char& operator[](size_t i)   //重载[]可以遍历输出字符串，加&是既可以读，也可以写{assert(i < _size);return _str[i];}const char& operator[](size_t i) const{assert(i < _size);return _str[i];}//s1<s s1就是thisbool operator<(const string& s){int ret = strcmp(_str, s._str);return ret < 0;}bool operator<=(const string& s){return *this < s || *this == s;}bool operator>(const string& s){return !(*this <= s);}bool operator>=(const string& s){return !(*this < s);}bool operator==(const string& s){int ret = strcmp(_str, s._str);return ret == 0;}bool operator!=(const string& s){return !(*this == s);}bool empty(){return _size == 0;}size_t size()                   //求字符串的大小{return _size;}size_t capacity()              //求字符串的容量{return _capacity;}private:char* _str;size_t _size;              //已经有多少个有效字符个数size_t _capacity;          //能存多少个有效字符个数 \0不是有效字符，\0是标识结束的字符static size_t npos;       //insert用的位置};size_t string::npos = -1;std::ostream& operator<<(std::ostream& _out, string& s) 	//重载输出运算符<<{for (size_t i = 0; i < s.size(); i++){std::cout << s[i];}return _out;}std::istream& operator>>(std::istream& _in, string& s)     //重载输入运算符<<{//for (size_t i = 0; i < s.size(); i++)  错误写法//{//	std::cin >> s[i];//}while (1){char ch;ch = _in.get();if (ch == ' ' || ch == '\n'){break;}else{s += ch;}}return _in;}//std::istream& operator>>(std::istream& _in, string &s)     //重载输入运算符<<//{//	//for (size_t i = 0; i < s.size(); i++)  错误写法//	//{//	//	std::cin >> s[i];//	//}//	while (1)//	{//		char ch;//		ch = _in.get();//		if ( ch == '\n')//		{//			break;//		}//		else//		{//			s += ch;//		}//	}//	return _in;//}void TestString1(){string s1;string s2("bit");for (size_t i = 0; i < s1.size(); i++){std::cout << s1[i] << " ";}std::cout << std::endl;for (size_t i = 0; i < s2.size(); i++){std::cout << s2[i] << " ";}std::cout << std::endl;}void TestString2(){string s1;string s2("bit");std::cout << s1 << std::endl;std::cout << s2 << std::endl;s1.push_back('b');std::cout << s1 << std::endl;s2.push_back(' ');std::cout << s2 << std::endl;s1 += 'a';std::cout << s1 << std::endl;s2.insert(1, 'a');std::cout << s2 << std::endl;std::cout << s2.size() << std::endl;std::cout << s2.capacity() << std::endl;}void TestString3(){string s1;std::cin >> s1;std::cout << s1 << std::endl;}
}

测试代码：

#define _CRT_SECURE_NO_WARNINGS   1
#include"string.h"
int main()
{yyw::TestString1();yyw::string s3("hello");yyw::string::iterator it = s3.begin();while (it != s3.end()){std::cout << *it << " ";it++;}std::cout << std::endl;for (auto e : s3){std::cout << e << " ";}std::cout << std::endl;yyw::TestString2();yyw::TestString3();return 0;
}