【C++修炼之路】9. string类的模拟实现

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发布于 2023-03-28 12:37:02

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发布于 2023-03-28 12:37:02

文章被收录于专栏：C++/Linux

string类的模拟实现

前言

本篇文章是衔接上一篇string，进行string的模拟实现，其中包含了众多重载函数，以及一些实现的细节，由于上篇已经知道具体函数的含义，这一篇就以纯代码的方式进行叙述。此外，这篇还对内置类型的知识进行了进一步的扩展。

代码：

1. string.h

#pragma once
#include<iostream>
#include<string.h>
#include<assert.h>

using namespace std;
namespace cfy
{
	class string
	{
	public:
		typedef char* iterator;

		iterator begin()
		{
			return _str;
		}

		iterator end()
		{
			return _str + _size;
		}
		/*string()
		{
			_str = new char[1];
			_str[0] = '\0';
			_capacity = _size = 0;
		}*/
		string(const char* str = "")
		{
			_size = strlen(str);
			_capacity = _size;
			_str = new char[_size + 1];

			strcpy(_str, str);
		}
		void swap(string& s)
		{
			std::swap(_str, s._str);
			std::swap(_size, s._size);
			std::swap(_capacity, s._capacity);
		}
		//s2(s1)
		//拷贝构造的现代写法
		string(const string& s)
			:_str(nullptr)//tmp指向随机值会有影响，未初始化会导致野指针的情况，因此在拷贝构造之前要给其初始化为nullptr
			,_size(0)
			,_capacity(0)
		{
			string tmp(s._str); // 是构造函数，因为s._str是一个值，不是一个对象
			//this->swap(tmp);
			swap(tmp);
		}//tmp作为临时变量除了作用域会自动调用析构，因此上面进行了解释。
		
		//s2(s1)
		//拷贝构造的传统写法
		/*string(const string& s)
		{
		
			_str = new char[s._capacity + 1];
			_capacity = s._capacity;
			_size = s._size;

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

				_size = s.size();
				_capacity = s._capacity;
			}

			return *this;
		}*/
		/*string& operator=(const string& s)
		{
			if (this != &s)
			{
				string tmp(s);
				swap(tmp);
			}
			return *this;
		}*/

		// s1 = s3
		string& operator=(string s)
		{
			swap(s);
			return *this;
		}

		~string()
		{
			delete[] _str;
			_str = nullptr;
			_size = _capacity = 0;
		}
		const char* c_str() const
		{
			return _str;
		}
		size_t size() const
		{
			return _size;
		}
		size_t capacity() const 
		{
			return _capacity;
		}
		//普通对象：可读可写
		char& operator[](size_t pos) const
		{
			assert(pos < _size);
			return _str[pos];
		}
		// const对象：只读
		char& operator[](int pos) 
		{
			assert(pos < _size);
			return _str[pos];
		}

		void reserve(size_t n)//扩展容量
		{
			if (n > _capacity)
			{
				char* tmp = new char[n + 1];
				strcpy(tmp, _str);
				delete[] _str;
				_str = tmp;
				_capacity = n + 1;
			}
		}

		void resize(size_t n, char ch = '\0')
		{
			if (n > _capacity)
			{
				reserve(n);
				for (size_t i = _size; i < n; i++)
				{
					_str[i] = ch;
				}

				_size = n;
				_str[_size] = '\0';
			}
			else
			{
				_str[n] = '\0';
				_size = n;
			}
		}
		
		void push_back(char ch)
		{
			if (_size == _capacity)
			{
				size_t newCapacity = _capacity == 0 ? 4 : _capacity * 2;
				reserve(newCapacity);
				_capacity = newCapacity;
			}
			_str[_size++] = ch;
			_str[_size] = '\0';//注意处理末尾
		}

		void append(const char* str)
		{
			size_t len = strlen(str);

			if (_size + len > _capacity)
			{
				reserve(_size + len);
			}

			strcpy(_str + _size, str);
			_size += len;
		}

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

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

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

			if (_size >= _capacity - 1)//改
			{
				size_t newCapacity = _capacity == 0 ? 4 : _capacity * 2;
				reserve(newCapacity);
			}

			size_t end = _size + 1;
			while (end > pos)
			{
				_str[end] = _str[end - 1];
				--end;
			}
			_str[pos] = ch;
			++_size;
			return *this;
		}
		string& insert(size_t pos, const char* str)
		{

			size_t len = strlen(str);

			if (_size + len > _capacity)
			{
				reserve(_size + len);
			}

			size_t end = _size + len;

			while (end > pos + len -1)
			{
				_str[end] = _str[end - len];
				--end;
			}
			strncpy(_str + pos, str, len);
			_size += len;
			return *this;
		}

		string& erase(size_t pos, size_t len = npos)
		{
			assert(pos < _size);

			if (len == npos || pos + len >= _size)
			{
				_str[pos] = '\0';
				_size = pos;
			}
			else
			{
				strcpy(_str + pos, _str + pos + len);
				_size -= len;
			}

			return *this;
		}
		size_t find(const char ch, size_t pos = 0) const
		{
			assert(pos < _size);
			while (pos < _size)
			{
				if (_str[pos] == ch)
				{
					return pos;
				}
				++pos;
			}
			return npos;
		}

		size_t find(const char* str, size_t pos = 0) const
		{
			assert(pos < _size);
			const char* ptr = strstr(_str + pos, str);
			if (ptr == nullptr)
			{
				return npos;
			}
			else
			{
				return ptr - _str;
			}

		}
		void clear()
		{
			_size = 0;
			_str[0] = '\0';
		}
	private:
		char* _str;
		size_t _size;
		size_t _capacity;

		const static size_t npos = -1;//只有这个是特例
	};

	ostream& operator<<(ostream& out, const string& s) 
	{
		for (size_t i = 0; i < s.size(); i++)
		{
			out << s[i];
		}
		return out;
	}
	istream& operator>>(istream& in, string& s)
	{
		s.clear();
		/*char ch = in.get();
		while (ch != ' ' && ch != '\n')
		{
			s += ch;
			ch = in.get();
		}
		return in;*/

		char buff[128] = { '\0' };//防止扩容代价大，通过buff暂时储存，一段一段进
		size_t i = 0;
		char ch = in.get();
		while (ch != ' ' && ch != '\n')
		{
			if (i < 127)
			{
				buff[i++] = ch;
			}
			else
			{
				s += buff;
				i = 0;
				buff[i++] = ch;
			}
			ch = in.get();
		}
		if (i > 0)
		{
			buff[i] = '\0';
			s += buff;
		}
	}
	void test_string1()
	{
		string s1("hello world");
		cout << s1.c_str() << endl;

		for (size_t i = 0; i < s1.size(); i++)
		{
			s1[i]++;
		}
		cout << s1.c_str() << endl;


		string::iterator it1 = s1.begin();
		while (it1 != s1.end())
		{
			(*it1)--;
			it1++;
		}
		cout << s1.c_str() << endl;

		for (auto ch : s1)//底层是调用迭代器iterator，官方库就是这样
		{
			cout << ch << " ";
		}
		cout << endl;

		s1.push_back('!');
		s1.push_back('!');
		s1.push_back('!');
		cout << s1.c_str() << endl;
	}

	void test_string2()
	{
		string s1("hello");
		s1 += ' ';
		s1 += '!';
		s1 += '!';
		s1 += "happy";
		cout << s1.c_str() << endl;

		string s2;

		s2 += 'x';
	
		cout << s2.c_str() << endl;
	}

	void test_string3()
	{
		string s1("hello world");

		s1.insert(6, " cfy ");
		cout << s1.c_str() << endl;
		s1.insert(0, "");
		cout << s1.c_str() << endl;
	}

	void test_string4()
	{
		string s1("hello hello world");
		s1.erase(0, 6);
		cout << s1.c_str() << endl;
	}

	void test_string5()
	{
		string s1("hello hello world");
		s1.resize(25, 'x');
		cout << s1.c_str() << endl;
	}

	void test_string6()
	{
		string s1("hello world");
		
		cout << s1 << endl;
		cout << s1.c_str() << endl;

		s1.insert(5, '\0');
		cout << s1.size() << endl;
		cout << s1.capacity() << endl;
		cout << s1 << endl;
		cout << s1.c_str() << endl;

		//string s2;
		cout << s1 << endl;
		cin >> s1;
		cout << s1 << endl;

	}
	void test_string7()
	{
		string s1("hello world");
		string s2(s1); // 默认的拷贝构造是浅拷贝，指针指向同一个位置，对开辟的空间的析构会析构多次，导致错误。
		cout << s2 << endl;
	}
	void test_string8()
	{
		string s1("hello");
		cout << s1 << endl;

		string s2("world");
		cout << s2 << endl;

		s1.swap(s2);
		cout << "s1:" << s1 << endl;
		cout << "s2:" << s2 << endl;
	}
}

2. test.cpp

#define _CRT_SECURE_NO_WARNINGS 1
#include"string.h"


int main()
{
	cfy::test_string8();

	/*int i(10);
	cout << i << endl;
	int j = int();
	cout << j;*/
	return 0;
}

扩展：内置类型的拷贝构造

对于C++来说，我们知道其具有默认的拷贝构造函数，这是对自定义的类实现的，但由于C++含有泛型模板template<T>，我们发现其也可以作为类，因此也具有构造和拷贝构造、析构等默认成员函数，因此这也让内置类型支持了拷贝构造，因为我们可以将T替换成相应的内置类型时间比如我们耳熟能详的int、char、double，那我们就来看一下具体做法：

#inclue<iostream>
using namespace std;
int main()
{
	int i(10);
	cout << i << endl;
	int j = int();
	cout << j;
	return 0;
}

#inclue<iostream>
using namespace std;
int main()
{
	double d(10.0);
	double j = double();
	cout << d << " " << j << endl;
	char a('A');
	cout << a << endl;
	return 0;
}