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C++初阶——简单实现vector

article 2025/2/22 23:23:37

1、前言

2、Vector.h

3、Test.cpp

1、前言

简单实现std::vector类模板。

相较于前面的string，vector要注意：

深拷贝，因为vector的元素可能是类类型，类类型元素可以通过赋值重载，自己实现深拷贝。

迭代器失效，其实无非是两种(现在认为)：

1. 迭代器更新了，但是认为用的是之前的迭代器，可以先保存值。

2. 迭代器没有更新，但是认为是用更新后的迭代器，那就更新迭代器。

2、Vector.h

#pragma once

#include <iostream>
#include <vector>
#include <assert.h>

using namespace std;

namespace Lzc
{
	template <class T>
	class vector
	{
	public:
		typedef T* iterator;
		typedef const T* const_iterator;

		vector() // 有了拷贝构造，就不会生成默认构造，但是需要默认构造
		{}
		vector(size_t n, const T& val = T())// int有默认构造int(),为了兼容模板
		{
			reserve(n);
			for (size_t i = 0; i < n; i++)
			{
				push_back(val);
			}
		}
		template <class InputIterator> // 使用别的容器的迭代器初始化
		vector(InputIterator first, InputIterator last)
		{
			// reserve(last - first);
			// last - first 有迭代器不支持减
			while (first != last)
			{
				push_back(*first);
				++first;
			}
		}
		vector(const vector<T>& v)
		{
			reserve(v.size());
			for (int i = 0; i < v.size(); i++)
			{
				push_back(v[i]);
			}
		}
		void swap(vector<T>& tmp)
		{
			std::swap(_start, tmp._start);
			std::swap(_finish, tmp._finish);
			std::swap(_end_of_storage, tmp._end_of_storage);
		}
		vector<T>& operator=(const vector<T>& v)
		{
			vector tmp(v);
			swap(tmp);
			return *this;
		}
		~vector()
		{
			if (_start)
			{
				delete[] _start;
				_start = _finish = _end_of_storage = nullptr;
			}
		}

		iterator begin()
		{
			return _start;
		}
		iterator end()
		{
			return _finish;
		}
		const_iterator begin() const
		{
			return _start;
		}
		const_iterator end() const
		{
			return _finish;
		}

		size_t size() const
		{
			return _finish - _start;
		}
		size_t capacity() const
		{
			return _end_of_storage - _start;
		}
		void clear()
		{
			_finish = _start;
		}
		bool empty() const
		{
			return _finish == _start;
		}

		T& operator[](size_t pos)
		{
			assert(pos < size());
			return _start[pos];
		}
		const T& operator[](size_t pos) const
		{
			assert(pos < size());
			return _start[pos];
		}

		void resize(size_t n, const T& val = T()); // resize一般用于扩容+初始化

		void reserve(size_t n);

		void push_back(const T& val)
		{
			if (size() == capacity())
				reserve(capacity() == 0 ? 4 : 2 * capacity());
			*_finish = val;
			++_finish;
		}

		void pop_back()
		{
			assert(size() > 0);
			--_finish;
		}

		iterator insert(iterator pos, const T& val);

		iterator erase(iterator pos);

	private:
		iterator _start = nullptr;
		iterator _finish = nullptr;
		iterator _end_of_storage = nullptr;
	};

	template<class T>
	void vector<T>::resize(size_t n, const T& val) // resize一般用于扩容+初始化
	{
		if (n < size())
		{
			_finish = _start + n;
		}
		else
		{
			reserve(n);
			while (_finish < _start + n)
			{
				*_finish = val;
				++_finish;
			}
		}
	}

	template<class T>
	void vector<T>::reserve(size_t n)
	{
		if (n > capacity())
		{
			size_t old_size = size();
			T* tmp = new T[n];
			for (size_t i = 0; i < old_size; i++)
			{
				tmp[i] = _start[i]; // 若为类类型，会赋值重载，先释放，再深拷贝
			}
			delete[] _start;
			_start = tmp;
			_finish = tmp + old_size; // 不能调用size(),因为_start已更新，所以先保存size()
			_end_of_storage = tmp + n;
		}
	}

	// vector<T>::iterator编译器认为是类中的静态变量，typename告诉编译器是类型
	template<class T>
	typename vector<T>::iterator vector<T>::insert(iterator pos, const T& val)
	{
		assert(pos <= _finish);
		assert(pos >= _start);

		if (size() == capacity())
		{
			size_t old_pos = pos - _start; // 扩容后pos没有指向新空间
			reserve(capacity() == 0 ? 4 : 2 * capacity());
			pos = _start + old_pos;
		}

		iterator it = _finish;
		while (it != pos)
		{
			*it = *(it - 1); // 若为类类型，会赋值重载，先释放，再深拷贝
			--it;
		}

		*pos = val;

		++_finish;

		return pos;
	}

	template<class T>
	typename vector<T>::iterator vector<T>::erase(iterator pos)
	{
		assert(pos >= _start);
		assert(pos < _finish);

		iterator it = pos + 1;
		while (it != end())
		{
			*(it - 1) = *it; // 若为类类型，会赋值重载，先释放，再深拷贝
			++it;
		}
		--_finish;

		return pos;
	}
	
	template<class Container> // 什么容器都能打印
	void print_Container(const Container& v)
	{
		for (const auto& e : v)
		{
			cout << e << " ";
		}
		cout << endl;
	}
}

3、Test.cpp

#include "Vector.h"
#include <list>
#include <string>

namespace Lzc
{
	void test_vector1()
	{
		vector<int> v;
		v.push_back(1);
		v.push_back(2);
		v.push_back(3);
		v.push_back(4);
		v.push_back(5);

		for (size_t i = 0; i < v.size(); i++)
		{
			cout << v[i] << " ";
		}
		cout << endl;
		
		//vector<int>::iterator it = v.begin();
		auto it = v.begin();
		while (it != v.end())
		{
			cout << *it << " ";
			++it;
		}
		cout << endl;

		for (auto e : v)
		{
			cout << e << " ";
		}
		cout << endl;

		vector<double> vd;
		vd.push_back(1.1);
		vd.push_back(1.2);
		vd.push_back(1.3);
		vd.push_back(1.4);
		vd.push_back(1.5);

		print_Container(vd);
	}

	void test_vector2()
	{
		vector<int> v;
		v.push_back(1);
		v.push_back(2);
		v.push_back(3);
		v.push_back(4);
		v.push_back(5);

		print_Container(v);

		// 若iterator insert(iterator& pos, const T& val)
		// 下面的临时变量(具有常性)，不能传引用
		/*v.insert(v.begin() + 2, 30);
		print_vector(v);*/

		int x;
		cin >> x;
		auto p = find(v.begin(), v.end(), x);
		if (p != v.end())
		{
			// insert以后p就是失效，不要直接访问，要访问就要更新这个失效的迭代器的值
			/*v.insert(p, 20);
			(*p) *= 10;*/

			p = v.insert(p, 40);
			(*(p + 1)) *= 10;
		}
		print_Container(v);
	}

	void test_vector3()
	{
		vector<int> v;
		v.push_back(1);
		v.push_back(2);
		v.push_back(3);
		v.push_back(4);
		//v.push_back(5);

		print_Container(v);

		// 删除所有的偶数
		auto it = v.begin();
		while (it != v.end())
		{
			if (*it % 2 == 0)
			{
				it = v.erase(it); // 删除了，相当于++了
			}
			else
			{
				++it;
			}
		}

		print_Container(v);
	}

	void test_vector4()
	{
		int i = int(); // int有默认构造int(),为了兼容模板
		int j = int(1);
		int k(2);

		vector<int> v;
		v.resize(10, 1);
		v.reserve(20);

		print_Container(v);
		cout << v.size() << endl;
		cout << v.capacity() << endl;

		v.resize(15, 2);
		print_Container(v);

		v.resize(25, 3);
		print_Container(v);

		v.resize(5);
		print_Container(v);
	}

	void test_vector5()
	{
		vector<int> v1;
		v1.push_back(1);
		v1.push_back(2);
		v1.push_back(3);
		v1.push_back(4);
		print_Container(v1);

		vector<int> v2 = v1;
		print_Container(v2);

		vector<int> v3;
		v3.push_back(10);
		v3.push_back(20);
		v3.push_back(30);

		v1 = v3;
		print_Container(v1);
		print_Container(v3);
	}

	void test_vector6()
	{
		vector<int> v1;
		v1.push_back(1);
		v1.push_back(2);
		v1.push_back(3);
		v1.push_back(4);
		v1.push_back(4);
		v1.push_back(4);

		vector<int> v2(v1.begin(), v1.begin() + 3);
		print_Container(v1);
		print_Container(v2); // 1 2 3

		list<int> lt;
		lt.push_back(10);
		lt.push_back(10);
		lt.push_back(10);
		lt.push_back(10);
		vector<int> v3(lt.begin(), lt.end()); 
		print_Container(lt);
		print_Container(v3);// 10 10 10 10

		vector<string> v4(10, "1111111");
		print_Container(v4);

		vector<int> v5(10);
		print_Container(v5);

		vector<int> v6(10u, 1); // 指定构造函数，不然编译器会使用模板函数。
		print_Container(v6);    // vector(size_t n, const T& val = T())

		//vector<int> v7(10, 1); // 编译器会使用模板函数，报错
		//print_Container(v7); // 可以再重载一个vector(int n, const T& val = T())
	}

	void test_vector7() // 扩容后需深拷贝
	{
		vector<string> v;
		v.push_back("11111111111111111111");
		v.push_back("11111111111111111111");
		v.push_back("11111111111111111111");
		v.push_back("11111111111111111111");
		print_Container(v);

		v.push_back("11111111111111111111");
		print_Container(v);
	}
}

int main()
{
	Lzc::test_vector7();
	return 0;
}

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