C++ string类的模拟实现

这篇文章，主要就是展示模拟实现C++标准库中的string类的代码，下面就是具体的代码，同时也上传了具体的文件。

string.h(头文件)

// string.h
#pragma once
#define _CRT_SECURE_NO_WARNINGS 1

#include <iostream>
using namespace std;
#include <assert.h>

namespace Alen
{
	class string
	{
		// 类里面定义的成员函数默认是内敛函数，内敛不会放在符号表所以不会链接错误
	public:
		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;
		}
		string(const char* str = "")
		{
			_size = strlen(str);
			_capacity = _size;
			_str = new char[_capacity + 1];
			strcpy(_str, str);
		}
		 
		// 深拷贝问题
		// 写法一：老写法
		/*
		string(const string& s)
		{
			_str = new char[s._capacity + 1];
			strcpy(_str, s._str);
			_size = s._size;
			_capacity = s._capacity;
		}
		*/

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

		// 方法二：
		string(const string& s)
		{
			string tmp(s._str);
			swap(tmp);
		}

		//string& operator=(const string& s)
		//{
		//	if (this != &s)
		//	{
		//		/*delete[] _str;

		//		_str = new char[s._capacity + 1];
		//		strcpy(_str, s._str);
		//		_size = s._size;
		//		_capacity = s._capacity;*/

		//		//string tmp(s._str);
		//		string tmp(s);
		//		swap(tmp);
		//		
		//	}
		//	return *this;
		//}

		string& operator=(string tmp)
		{
			swap(tmp);
			return *this;
		}
		~string()	
		{
			if (_str)
			{
				delete[] _str;		// delete中对nullptr特别处理了，也可以不需要手动判断
				_str = nullptr;
				_size = _capacity = 0;
			}
		}

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

		void reserve(size_t newCapacity);
		void clear();

		void push_back(char c);
		string& operator+=(char c);
		void append(const char* s);
		string& operator+=(const char* s);

		void insert(size_t pos, char ch);
		void insert(size_t pos, const char* s);
		void erase(size_t pos, size_t len = npos);

		size_t find(char c, size_t pos = 0);
		size_t find(const char* s, size_t pos = 0);
		string substr(size_t pos = 0, size_t len = npos);
	private:
		char* _str = nullptr;
		size_t _capacity;
		size_t _size;
		static const size_t npos;
	};

	bool operator<(const string& s1, const string& s2);
	bool operator<=(const string& s1, const string& s2);
	bool operator>(const string& s1, const string& s2);
	bool operator>=(const string& s1, const string& s2);
	bool operator==(const string& s1, const string& s2);
	bool operator!=(const string& s1, const string& s2);

	// 加不加const取决于读还是写
	ostream& operator<<(ostream& out, const string& s);
	istream& operator>>(istream& in, string& s);

	istream& getline(istream& in, string& s, char delim = '\n');
}

string.cpp

// string.cpp
#include "string.h"
namespace Alen
{
	const size_t string::npos = -1;
	void string::reserve(size_t newCapacity)
	{
		if (newCapacity > _capacity)
		{
			char* tmp = new char[newCapacity + 1];	// 多一个空间存'\0'
			strcpy(tmp, _str);

			delete[] _str;
			_str = tmp;
			_capacity = newCapacity;
		}
	}

	void string::clear()
	{
		_str[0] = '\0';
		_size = 0;
	}

	void string::push_back(char c)
	{
		if (_size == _capacity)
			reserve(_capacity == 0 ? 4 : _capacity * 2);		// 进行2倍扩容

		_str[_size++] = c;
		_str[_size] = '\0';		// !!!
	}

	string& string::operator+=(char c)
	{
		push_back(c);
		return *this;
	}

	void string::append(const char* s)
	{
		size_t len = strlen(s);
		if (_size + len > _capacity)
		{
			reserve(_size + len > _capacity * 2 ? _size + len : _capacity * 2);
		}
		strcpy(_str + _size, s);
		_size += len;
	}

	string& string::operator+=(const char* s)
	{
		append(s);
		return *this;
	}


	void string::insert(size_t pos, char ch)
	{
		assert(pos <= _size);

		if (_size == _capacity)
			reserve(_capacity == 0 ? 4 : _capacity * 2);

		// 挪动数据
		// end 是int类型，pos是size_t类型，比较会将end隐式类型转换成size_t
		// 当end为-1的（即pos为0）时候，就成为了size_t的最大值
		//int end = _size;
		//while (end >= pos)		
		//{
		//	_str[end + 1] = _str[end];
		//	--end;
		//}
		//_str[pos] = ch;
		//++_size;

		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* s)
	{
		assert(pos <= _size);

		size_t len = strlen(s);
		if (_size + len > _capacity)
			reserve(_size + len > 2 * _capacity ? _size + len : 2 * _capacity);

		// 挪动数据
		size_t end = _size + len;
		while (end > pos + len - 1)
		{
			_str[end] = _str[end - len];
		}

		for (size_t i = 0; i < len; i++)
		{
			_str[pos + i] = s[i];
		}
	}
	void string::erase(size_t pos, size_t len)
	{
		assert(pos < _size);
		if (len >= _size - pos)
		{
			_str[pos] = '\0';
			_size = pos;
		}
		else
		{
			for (size_t i = pos + len; i <= _size; i++)
			{
				_str[i - len] = _str[i];
			}
			_size -= len;
		}
	}

	size_t string::find(char c, size_t pos)
	{
		for (size_t i = pos; i < _size; i++)
		{
			if (_str[i] == c)
			{
				return i;
			}
		}

		return npos;
	}

	// 可以使用KMP、BM算法
	size_t string::find(const char* s, size_t pos)
	{
		assert(pos < _size);

		const char* ptr = strstr(_str + pos, s);
		if (ptr == nullptr)
		{
			return npos;
		}
		else
		{
			return ptr - _str;
		}
	}

	string string::substr(size_t pos, size_t len)
	{
		assert(pos < _size);

		if (len > _size - pos)
		{
			len = _size - pos;
		}

		string sub;
		sub.reserve(len);
		for (size_t i = 0; i < len; i++)
		{
			sub += _str[pos + i];
		}
		return sub;
	}

	bool operator<(const string& s1, const string& s2)
	{
		return strcmp(s1.c_str(), s2.c_str()) < 0;
	}
	bool operator<=(const string& s1, const string& s2)
	{
		return s1 < s2 || s1 == s2;
	}
	bool operator>(const string& s1, const string& s2)
	{
		return !(s1 <= s2);
	}
	bool operator>=(const string& s1, const string& s2)
	{
		return !(s1 < s2);
	}
	bool operator==(const string& s1, const string& s2)
	{
		return strcmp(s1.c_str(), s2.c_str()) == 0;
	}
	bool operator!=(const string& s1, const string& s2)
	{
		return !(s1 == s2);
	}

	ostream& operator<<(ostream& out, const string& s)
	{
		for (auto ch : s)
		{
			out << ch;
		}
		return out;
	}

	void IN(istream& in ,string& s, char option = '\n')
	{
		char ch;
		const int N = 256;
		char buff[N];	// 利用缓存区的思路，来减少string的扩容操作，增加效率
		int i = 0;

		ch = in.get();	// = C语言的getc一个字符一个字符读
		while (option == ' ' ? ch != ' ' && ch != '\n' : ch != option)
		{
			buff[i++] = ch;
			if (i == N - 1)
			{
				buff[i] = '\0';
				s += buff;
				i = 0;
			}
			//s += ch;
			ch = in.get();
		}
		if (i > 0)
		{
			buff[i] = '\0';
			s += buff;
		}
	}
	istream& operator>>(istream& in, string& s)
	{
		s.clear();
		/*
		char ch;
		//in >> ch;		// 将 ' ' 与 '\n'作为分隔符，直接忽视不会读取 
		//while (ch != ' ' && ch != '\n')
		//{
		//	s += ch;
		//	in >> ch;
		//}
		const int N = 256;
		char buff[N];	// 利用缓存区的思路，来减少string的扩容操作，增加效率
		int i = 0;

		ch = in.get();	// = C语言的getc一个字符一个字符读
		while (ch != ' ' && ch != '\n')
		{
			buff[i++] = ch;
			if (i == N - 1)
			{
				buff[i] = '\0';
				s += buff;
				i = 0;
			}
			//s += ch;
			ch = in.get();
		}
		if (i > 0)
		{
			buff[i] = '\0';
			s += buff;
		}
		*/
		IN(in, s, ' ');
		return in;
	}

	istream& getline(istream& in, string& s, char delim)
	{
		s.clear();
		IN(in, s, delim);
		return in;
	}

}

Test.cpp(测试文件 / 主函数)

// Test.cpp
#include "string.h"

namespace Alen
{
	void teststring1()
	{
		string s("hello world");
		s += 's';
		s += "abc";
		for (auto& ch : s)
		{
			ch += 1;
		}
		cout << s.c_str() << endl;
	}

	void teststring2()
	{
		string s("hello world");
		s.erase(6, 3);
		cout << s.c_str() << endl;
	}

	void teststring3()
	{
		string s("test.cpp.zip");
		size_t pos1 = s.find('.');
		size_t pos2 = s.find("cpp");
		string suffix1 = s.substr(pos1);
		string suffix2 = s.substr(pos2);

		cout << suffix1.c_str() << endl;
		cout << suffix2.c_str() << endl;
	}

	void teststring4()
	{
		string s1("hello world");
		string s2("hello world");

		cout << (s1 < s2) << endl;
		cout << (s1 == s2) << endl;
		cout << ("hello world" < s2) << endl;
		cout << (s1 == "hello world") << endl;	// 将 "hello world"隐式类型转换成string临时对象
		cout << ("hello world" == "hello world") << endl;	// 两个指针的比较（运算符重载必须有一个对应的对象识别）
	}

	void teststring5()
	{
		string s("hello world");
		string s1 = s;
		getline(cin, s);
		getline(cin, s, '*');
		cout << s << endl;
	}

}

int main()
{
	//Alen::teststring5();
	return 0;
}