【C++】—— string模拟实现

前言：

学习了string的使用，总感觉了解不是很深厚；自己模拟实现string类来帮助自己理解。

这里只是实现了一部分内容（并没有实现完整的string类）。

先来实现string类里面的成员变量：

#include<iostream>
namespace HL
{class string{public:private:char* _str;size_t _size;size_t _capacity;const static size_t npos;};const size_t HL::string::npos = -1;
}

一、string默认成员函数（构造、析构、赋值运算符重载）

1.1、构造函数

1> 默认构造

默认构造函数就是不需要传参的构造函数；这里实现就开辟一个字符的空间存放 '\0'即可（_capacity不包括 '\0' ）。

string(){_str = new char[1];_str[0] = '\0';_size = 0;_capacity = 0;}

2> 拷贝构造

拷贝构造，在实现时需要注意：是深拷贝，而不是浅拷贝（值拷贝）。

深拷贝（深拷贝简单来说就是，要开辟一块新的空间，把原空间里的值拷贝到新的空间里）。

		string(const string& str){_str = new char[str._capacity + 1];memcpy(_str, str._str, str._size + 1);_size = str._size;_capacity = str._capacity;}

3> 其他构造

其他构造函数就有很多了，这里就实现以下这几个：

string (const char* s);

		string(const char* s){size_t len = strlen(s);_str = new char[len + 1];memcpy(_str, s, len + 1);_size = len;_capacity = len;}

string (const char* s, size_t n);

		string(const char* s, size_t n){size_t len = strlen(s);if (n > len){n = len;}_str = new char[n + 1];memcpy(_str, s, n);_str[n] = '\0';_size = n;_capacity = n;}

string (size_t n, char c);

		string(size_t n, char c){_str = new char[n + 1];for (size_t i = 0; i < n; i++){_str[i] = c;}_str[n] = '\0';_size = n;_capacity = n;}

1.2、析构函数

析构函数比较简单，释放开辟的资源即可；

		~string(){delete[] _str;_str = nullptr;_size = _capacity = 0;}

1.3、赋值运算符重载

赋值运算符有3个重载，这里就一一实现：

string& operator= (const string& str );

实现这个有很多种方法，

可以释放原空间，再开辟新的空间，将数据拷贝到新的空间中去

		string& operator=(const string& str){delete[] _str;_str = new char[str._capacity];memcpy(_str, str._str, str._size + 1);_size = str._size;_capacity = str._capacity;return *this;}

可以调用拷贝构造，构造一个tmp、再将tmp与*this 中的值进行交换（要实现交换函数）

template <typename T>
void Swap(T& x, T& y)
{T tmp = x;x = y;y = tmp;
}
string& operator= (const string& str)
{string tmp(str);Swap(_str, tmp._str);Swap(_size, tmp._size);Swap(_capacity, tmp._capacity);return *this;
}

这里如果已经实现string类swap成员函数，就可以直接调用。

string& operator= (const char* s );

		/*string& operator= (const char* s){size_t len = strlen(s);delete[] _str;_str = new char[len + 1];memcpy(_str, s, len + 1);_size = _capacity = len;return *this;}*/string& operator= (const char* s){string tmp(s);Swap(_str, tmp._str);Swap(_size, tmp._size);Swap(_capacity, tmp._capacity);return *this;}

string& operator= (char c );

		string& operator= (char c){delete[] _str;_str = new char[2];_str[0] = c;_str[1] = '\0';_size = _capacity = 1;return *this;}

二、元素访问与迭代器

2.1、迭代器

迭代器，虽然在string类中使用的不是很多，但在后面的容器中有大用处。

（在string类中就可以简单的理解成指针）。

		//迭代器typedef char* iterator;typedef const char* const_iterator;iterator begin(){return _str;}iterator end(){return (_str + _size);}const_iterator begin() const{return _str;}const_iterator end() const{return (_str + _size);}

实现了迭代器之后，范围for这个语法糖就可以使用了（底层就是迭代器）。

2.2、下标访问元素

实现下标访问，就是 [ ]运算符重载。

		//下标访问 [ ]char& operator[] (size_t pos){assert(pos >= _size);return *(_str + pos);}const char& operator[] (size_t pos) const{assert(pos >= _size);return *(_str + _size);}

at函数和 [ ] 运算符重载原理一样，这里就不重复写了。

三、增删查改

在实现增之前，要先实现一个函数，就是调整空间大小的（扩容来用）。

		//扩容void reserve(size_t n){if (n > _capacity){char* s = new char[n + 1];memcpy(s, _str, _size);delete[] _str;_str = s;_capacity = n;}}

增删这里就实现这些成员函数。

1、push_back 、append 、operator+=

append重载比较多，这里就实现其中的几个。

		//扩容void reserve(size_t n){if (n > _capacity){char* s = new char[n + 1];memcpy(s, _str, _size);delete[] _str;_str = s;_capacity = n;}}//增void push_back(char c){if (_size >= _capacity){reserve((_capacity == 0) ? 4 : 2 * _capacity);}_str[_size] = c;_size++;_str[_size] = '\0';}void append(const string& str){size_t n = _size + str._size;if (n > _capacity){reserve(n);}for (int i = 0; i < str._size; i++){_str[_size + i] = str._str[i];}_size += str._size;_str[_size] = '\0';}void append(const char* s){size_t len = strlen(s);int n = _size + len;if (n > _capacity){reserve(n);}for (int i = 0; i < len; i++){_str[_size + i] = s[i];}_size = n;_str[_size] = '\0';}void append(size_t n, char c){if (_size + n > _capacity){reserve(_size + n);}for (int i = 0; i < n; i++){_str[_size + i] = c;}_size += n;_str[_size] = '\0';}string& operator+=(const string& str){this->append(str);return *this;}string& operator+=(const char* s){this->append(s);return *this;}string& operator+=(char c){this->push_back(c);return *this;}

2、insert、erase

insert重载也比较多，比较冗余；这里也只实现其中的一部分；

erase这里只实现一个

//insert 、erase
void insert(size_t pos, const string& str)
{assert(pos >= 0 && pos < _size);size_t len = str._size;size_t n = _size + len;if (n > _capacity){reserve(n);}//挪动数据for (size_t i = n; i >= pos + len; i--){_str[i] = _str[i - len];}memcpy(_str + pos, str._str, str._size);_size += str._size;_str[_size] = '\0';
}
void insert(size_t pos, const char* s)
{assert(pos >= 0 && pos < _size);size_t len = strlen(s);size_t n = _size + len;if (n > _capacity){reserve(n);}//挪动数据for (size_t i = n; i >= pos + len; i--){_str[i] = _str[i - len];}memcpy(_str + pos, s, len);_size += len;_str[_size] = '\0';
}
void insert(size_t pos, size_t n, char c)
{assert(pos >= 0 && pos < _size);if (_size + n > _capacity){reserve(_size + n);}//挪动数据for (size_t i = _size+n; i >= pos + n; i--){_str[i] = _str[i - n];}for (size_t i = 0; i < n; i++){_str[pos + i] = c;}_size += n;_str[_size] = '\0';
}
void erase(size_t pos, size_t len = npos)
{assert(pos >= 0 && pos < _size);if (len == npos){_str[0] = '\0';_size = 0;return;}for (size_t i = pos; (len + i) < _size; i++){_str[i] = _str[i + len];}_size -= len;_str[_size] = '\0';
}

3、find

		//findsize_t find(const string& str, size_t pos = 0){assert(pos >= 0 && pos < _size);char* tmp = strstr(_str + pos, str._str);if (tmp == nullptr){return -1;}return tmp - _str;}size_t find(const char* s, size_t pos = 0){assert(pos >= 0 && pos < _size);char* tmp = strstr(_str + pos, s);if (tmp == nullptr){return -1;}return tmp - _str;}size_t find(char c, size_t pos = 0){assert(pos >= 0 && pos < _size);for (size_t i = pos; i < _size; i++){if (_str[i] == c){return i;}}return -1;}

4、swap

swap作为string的成员函数，交换两个string类类型的对象。

		//swapvoid swap(string& str){Swap(_str, str._str);Swap(_size, str._size);Swap(_capacity, str._capacity);}

这里swap函数内部也可以调用库里面的swap模版（这里我自己写了一个模版Swap）

有了swap函数，上面赋值运算符重载中就可以这样写了：

		string& operator= (const string& str){string tmp(str);swap(tmp);return *this;}

四、字符串操作函数

1、c_str

c_str()函数返回string类对象中的字符串;

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

2、substr

获得，sting对象中字符串的子串。

//substr
string substr(size_t pos = 0, size_t len = npos) const
{assert(pos >= 0 && pos < _size);size_t n = 0;if (len == npos || pos + len > _size){n = _size - pos;}else{n = len;}string ret;for (size_t i = 0; i < n; i++){ret += _str[pos + i];}return ret;
}

五、其他成员函数

这里max_size是返回容器可以容纳的最大元素的数量，这里就不进行实现了。

reserve在增加元素前已经实现了。（扩容）

//其他成员函数
size_t size() const
{return _size;
}
size_t length()const
{return _size;
}
size_t capacity()const
{return _capacity;
}
void clear()
{_str[0] = '\0';_size = 0;
}
bool empty()const
{return _size == 0;
}
void resize(size_t n)
{if (n > _capacity){reserve(n);}_size = n;_str[_size] = '\0';
}
void resize(size_t n, char c)
{if (n > _capacity){reserve(n);}for (size_t i = _size; i < n; i++){_str[i] = c;}_size = n;_str[_size] = '\0';
}

六、流插入、流提取

因为成员函数有一个隐藏的this指针，会和istream 和istream 类对象抢占第一个参数的位置，所以我们不能将流插入和流提取写成string类的成员函数。

string中实现了访问元素方成员函数，就可以不将流插入、流提取写成string类的友元函数。

1、流插入运算符重载

	std::ostream& operator<<(std::ostream& out, const HL::string& str){//for (int i = 0; i < str.size(); i++)//{//	out << str[i];//}//return out;for (auto ch : str){out << ch;}return out;}

2、流提取运算符重载

1、 s.clear()清理缓冲区（上次cin流提取的剩余）；

2、创建一个数组，防止多次去开空间（输入到128或者输入结束（‘ ’或者‘\n’）才添加到str中）。

3、下面的代码处理缓冲区的空格。

        char ch;
       ch = in.get();
       while (ch == ' ' || ch == '\n')
       {
           ch = in.get();
       }

4、最后循环里if是遇到空格或者换行结束，将s中输入添加到s中，末尾添加'\0'。

std::istream& operator>>(std::istream& in, HL::string& str)
{char s[128] = { 0 };char ch;ch = in.get();while (ch == ' ' || ch == '\n'){ch = in.get();}str.clear();int i = 0;while (ch != '\n'){s[i] = ch;i++;if (i == 127){s[i] = '\0';str += s;i = 0;}ch = in.get();}if (i){str += s;}return in;
}

HL :: string 源码

#pragma once
#include<iostream>
#include<cassert>template <typename T>
void Swap(T& x, T& y)
{T tmp = x;x = y;y = tmp;
}
namespace HL
{class string{friend std::ostream& operator<<(std::ostream& out, const HL::string& str);friend std::istream& operator>>(std::istream& in, HL::string& str);public://构造函数/*string(){_str = new char[1];_str[0] = '\0';_size = 0;_capacity = 0;}string(char ch){_str = new char[1];_str[0] = ch;_size = 0;_capacity = 0;}*/string(char ch = '\0'){_str = new char[2];_str[0] = ch;_str[1] = '\0';_size = 1;_capacity = 1;}string(const char* s){size_t len = strlen(s);_str = new char[len + 1];memcpy(_str, s, len + 1);_size = len;_capacity = len;}string(const char* s, size_t n){size_t len = strlen(s);if (n > len){n = len;}_str = new char[n + 1];memcpy(_str, s, n);_str[n] = '\0';_size = n;_capacity = n;}string(const string& str){_str = new char[str._capacity + 1];memcpy(_str, str._str, str._size + 1);_size = str._size;_capacity = str._capacity;}string(size_t n, char c){_str = new char[n + 1];for (size_t i = 0; i < n; i++){_str[i] = c;}_str[n] = '\0';_size = n;_capacity = n;}~string(){delete[] _str;_str = nullptr;_size = _capacity = 0;}/*string& operator=(const string& str){delete[] _str;_str = new char[str._capacity];memcpy(_str, str._str, str._size + 1);_size = str._size;_capacity = str._capacity;return *this;}*//*string& operator= (const string& str){string tmp(str);Swap(_str, tmp._str);Swap(_size, tmp._size);Swap(_capacity, tmp._capacity);return *this;}*/string& operator= (const string& str){string tmp(str);swap(tmp);return *this;}/*string& operator= (const char* s){size_t len = strlen(s);delete[] _str;_str = new char[len + 1];memcpy(_str, s, len + 1);_size = _capacity = len;return *this;}*/string& operator= (const char* s){string tmp(s);Swap(_str, tmp._str);Swap(_size, tmp._size);Swap(_capacity, tmp._capacity);return *this;}string& operator= (char c){delete[] _str;_str = new char[2];_str[0] = c;_str[1] = '\0';_size = _capacity = 1;return *this;}//string& operator= (char c)//{//	string tmp(c);//	Swap(_str, tmp._str);//	Swap(_size, tmp._size);//	Swap(_capacity, tmp._capacity);//	return *this;//}//迭代器typedef char* iterator;typedef const char* const_iterator;iterator begin(){return _str;}iterator end(){return (_str + _size);}const_iterator begin() const{return _str;}const_iterator end() const{return (_str + _size);}//下标访问 [ ]char& operator[] (size_t pos){assert(pos >= 0 && pos < _size);return *(_str + pos);}const char& operator[] (size_t pos) const{assert(pos >= 0 && pos < _size);return *(_str + _size);}//扩容void reserve(size_t n){if (n > _capacity){char* s = new char[n + 1];memcpy(s, _str, _size);delete[] _str;_str = s;_capacity = n;}}//增void push_back(char c){if (_size >= _capacity){reserve((_capacity == 0) ? 4 : 2 * _capacity);}_str[_size] = c;_size++;_str[_size] = '\0';}void append(const string& str){size_t n = _size + str._size;if (n > _capacity){reserve(n);}for (int i = 0; i < str._size; i++){_str[_size + i] = str._str[i];}_size += str._size;_str[_size] = '\0';}void append(const char* s){size_t len = strlen(s);size_t n = _size + len;if (n > _capacity){reserve(n);}for (int i = 0; i < len; i++){_str[_size + i] = s[i];}_size = n;_str[_size] = '\0';}void append(size_t n, char c){if (_size + n > _capacity){reserve(_size + n);}for (int i = 0; i < n; i++){_str[_size + i] = c;}_size += n;_str[_size] = '\0';}string& operator+=(const string& str){this->append(str);return *this;}string& operator+=(const char* s){this->append(s);return *this;}string& operator+=(char c){this->push_back(c);return *this;}//insert 、erasevoid insert(size_t pos, const string& str){assert(pos >= 0 && pos < _size);size_t len = str._size;size_t n = _size + len;if (n > _capacity){reserve(n);}//挪动数据for (size_t i = n; i >= pos + len; i--){_str[i] = _str[i - len];}memcpy(_str + pos, str._str, str._size);_size += str._size;_str[_size] = '\0';}void insert(size_t pos, const char* s){assert(pos >= 0 && pos < _size);size_t len = strlen(s);size_t n = _size + len;if (n > _capacity){reserve(n);}//挪动数据for (size_t i = n; i >= pos + len; i--){_str[i] = _str[i - len];}memcpy(_str + pos, s, len);_size += len;_str[_size] = '\0';}void insert(size_t pos, size_t n, char c){assert(pos >= 0 && pos < _size);if (_size + n > _capacity){reserve(_size + n);}//挪动数据for (size_t i = _size+n; i >= pos + n; i--){_str[i] = _str[i - n];}for (size_t i = 0; i < n; i++){_str[pos + i] = c;}_size += n;_str[_size] = '\0';}void erase(size_t pos, size_t len = npos){assert(pos >= 0 && pos < _size);if (len == npos){_str[0] = '\0';_size = 0;return;}for (size_t i = pos; (len + i) < _size; i++){_str[i] = _str[i + len];}_size -= len;_str[_size] = '\0';}//findsize_t find(const string& str, size_t pos = 0){assert(pos >= 0 && pos < _size);char* tmp = strstr(_str + pos, str._str);if (tmp == nullptr){return -1;}return tmp - _str;}size_t find(const char* s, size_t pos = 0){assert(pos >= 0 && pos < _size);char* tmp = strstr(_str + pos, s);if (tmp == nullptr){return -1;}return tmp - _str;}size_t find(char c, size_t pos = 0){assert(pos >= 0 && pos < _size);for (size_t i = pos; i < _size; i++){if (_str[i] == c){return i;}}return -1;}//swapvoid swap(string& str){Swap(_str, str._str);Swap(_size, str._size);Swap(_capacity, str._capacity);}//c_strconst char* c_str() const{return _str;}	char* c_str(){return _str;}//substrstring substr(size_t pos = 0, size_t len = npos) const{assert(pos >= 0 && pos < _size);size_t n = 0;if (len == npos || pos + len > _size){n = _size - pos;}else{n = len;}string ret;for (size_t i = 0; i < n; i++){ret += _str[pos + i];}return ret;}//其他成员函数size_t size() const{return _size;}size_t length()const{return _size;}size_t capacity()const{return _capacity;}void clear(){_str[0] = '\0';_size = 0;}bool empty()const{return _size == 0;}void resize(size_t n){if (n > _capacity){reserve(n);}_size = n;_str[_size] = '\0';}void resize(size_t n, char c){if (n > _capacity){reserve(n);}for (size_t i = _size; i < n; i++){_str[i] = c;}_size = n;_str[_size] = '\0';}private:char* _str;size_t _size;size_t _capacity;const static size_t npos;};const size_t HL::string::npos = -1;std::ostream& operator<<(std::ostream& out, const HL::string& str){//for (int i = 0; i < str.size(); i++)//{//	out << str[i];//}//return out;for (auto ch : str){out << ch;}return out;}std::istream& operator>>(std::istream& in, HL::string& str){char s[128] = { 0 };char ch;ch = in.get();while (ch == ' ' || ch == '\n'){ch = in.get();}str.clear();int i = 0;while (ch != '\n'){s[i] = ch;i++;if (i == 127){s[i] = '\0';str += s;i = 0;}ch = in.get();}if (i){str += s;}return in;}
};