C++___vector的模拟实现

1.vector深度剖析及模拟实现

1.1 std::vector的核心框架接口的模拟实现myVector::vector

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

namespace myVector
{
	template<class T>
	class vector
	{
	public:
		// Vector的迭代器是一个原生指针
		typedef T* iterator;
		typedef const T* const_iterator;

		iterator begin()
		{
			return _start;
		}

		iterator end()
		{
			return _finish;
		}

		const_iterator begin() const
		{
			return _start;
		}

		const_iterator end() const
		{
			return _finish;
		}

		vector()
			:_start(nullptr)
			, _finish(nullptr)
			, _endOfStorage(nullptr)
		{}

		template<class InputIterator>
		vector(InputIterator first, InputIterator last)
			:_start(nullptr)
			,_finish(nullptr)
			,_endOfStorage(nullptr)
		{
			reverse(last - first);
			while (first != last)
			{
				push_back(*first);
				++first;
			}
		}

		void swap(const vector<T>& tmp)
		{
			std::swap(_start, tmp._start);
			std::swap(_finish, tmp._finish);
			std::swap(_endOfStorage, tmp._endOfStorage);   
		}

		//v1(v2)
		vector(const vector<T>& v)
			:_start(nullptr)
			,_finish(nullptr)
			,_endOfStorage(nullptr)
		{
			vector<T> tmp(v.begin(), v._end());
			swap(tmp);//this->swap(tmp);
		}

		//v1 = v2
		vector<T>& operator=(const vector<T> v)
		{
			this->swap(v);
			return *this;
		}

		~vector()
		{
			delete[] _start;
			_start = _finish = _endOfStorage;
		}

		size_t capacity()
		{
			return _endOfStorage - _start;
		}

		size_t size()
		{
			return _finish - _start;
		}

		T& operator[](size_t pos)
		{
			assert(pos < size());

			return _start[pos];
		}

		//int i = int();
		void resize(size_t n, const T& val = T())
		{
			if (n > capacity())
			{
				reverse(n);
			}

			if (n < size())
			{
				_finish = _start + n;
			}
			else
			{
				while (_finish != _start + n)
				{
					*_finish = val;
					_finish++;
				}
			}
		}

		void reverse(size_t n)
		{
			if (n > capacity())
			{
				size_t sz = size();
				T* tmp = new T[n];
				
				//1.memcpy(tmp, _start, sizeof(T) * size());//内置类型 int char 浅拷贝
				
				//2.自定义类型 for + opereator= 深拷贝
				for (size_t i = 0; i < size(); ++i)
				{
					tmp[i] = _start[i];
				}

				delete[] _start;

				_start = tmp;
				_finish = _start + sz;
				_endOfStorage = _start + n;
			}
		}

		void check_capacity()
		{
			if (_finish == _endOfStorage)
			{
				size_t newcapacity = capacity() == 0 ? 4 : capacity() * 2;
				reverse(newcapacity);
			}
		}

		void push_back(const T& x)
		{
			check_capacity();

			*_finish = x;
			++_finish;
		}

		void pop_back()
		{
			assert(_finish > _start);
			_finish--;
		}

		//返回删除的位置
		iterator insert(iterator pos, const T& val)
		{
			assert(pos >= _start && pos <= _finish);

			size_t posi = pos - _start;//提前保存pos位置，防止增容pos失效
			
			check_capacity();

			pos = _start + posi;

			iterator end = _finish - 1;
			while (end >= pos)
			{
				*(end + 1) = *end;
				--end;
			}
			
			*pos = val;
			++_finish;

			return pos;
		}

		iterator erase(iterator pos)
		{
			assert(pos >= _start && pos < _finish);

			iterator it = pos + 1;
			while (it != _finish)
			{
				*(it - 1) = *it;
				++it;
			}
			--_finish;

			return pos;
		}

	private:
		iterator _start;// 指向数据块的开始
		iterator _finish;// 指向有效数据的尾
		iterator _endOfStorage;// 指向存储容量的尾
	};

}

1.2 使用memcpy拷贝问题

假设模拟实现的vector中的reserve接口中，使用memcpy进行的拷贝，以下代码会发生什么问题?

int main()
{
	myVector::vector<string> v;
	v.push_back("1111");
	v.push_back("2222");
	v.push_back("3333");
	v.push_back("4444");
	v.push_back("5555");
	return 0;
}

问题分析:

1. memcpy是内存的二进制格式拷贝，将一段内存空间中内容原封不动的拷贝到另外一段内存空间中
2. 如果拷贝的是内置类型的元素，memcpy即高效又不会出错，但如果拷贝的是自定义类型元素，并且自定义类型元素中涉及到资源管理时，就会出错，因为memcpy的拷贝实际是浅拷贝。（比如string）