文章目录

• C++ -- vector类模拟实现
• • 0. 成员变量
  • 1. 构造函数
  • 2. 析构函数
  • 3. reserve方法
  • 5. resize方法
  • 6. insert方法
  • 7. erase方法
  • 8. push_back方法
  • 9. pop_back方法
  • 10. []运算符重载
  • 11. 拷贝构造
  • 12. 迭代器
  • 13. 完整代码

C++ – vector类模拟实现

0. 成员变量

template
typedef T* iterator;
typedef const T* const_iterator;
iterator _start;
iterator _finish;
iterator _end_of_storage;

1. 为什么这里需要模板呢？

因为我们使用数组可能是int、char等等类型，这里就依靠是顺序存储的特性，就直接用指针来充当成员变量。

1. 构造函数

vector():_start(nullptr), _finish(nullptr), _end_of_storage(nullptr)
{}vector(size_t n, const T& val = T()) //T()->T类型的构造 && const T&引用修饰->起别名延长了生命周期:_start(nullptr), _finish(nullptr), _end_of_storage(nullptr)
{reserve(n);for (size_t i = 0; i < n; ++i){push_back(val);}
}template 
vector(InputIterator first, InputIterator last) //迭代器区间:_start(nullptr), _finish(nullptr), _end_of_storage(nullptr)
{while (first != last){push_back(*first);++first;}
}

1. 为什么这里的const T& val = T()？

这里的写法是为了给一个默认值，T()是针对自定义类型:string、vector等等。内置类型int()、char()等都是由编译器来初始化

2. 析构函数

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

3. reserve方法

void reserve(size_t capa)
{if (capa > capacity()){size_t len = size(); //OK:记录当前长度T* tmp = new T[capa];if (_start != nullptr){for (size_t i = 0; i < len; ++i){tmp[i] = _start[i];}delete[] _start;}_start = tmp;_finish = _start + len;_end_of_storage = _start + capa;}
}

1. 操作：

   • 是否扩容
   • 拷贝数据
   • 变化成员变量

2. 为什么_finish = _start + len，而不是 _finish = _start + size()？

因为size() = _finish - _start , _finish = _start + size() --> _finish = _start + _finiosh - _start

5. resize方法

void resize(size_t capa, T val = T())
{if (capa < size()){_finish = _start + capa;}else{if (capa > capacity()){reserve(capa);}while (_finish != _start + capa){*_finish = val;++_finish;}}
}

6. insert方法

iterator insert(iterator pos, const T& val)
{assert(pos >= _start && pos <= _finish);if (_finish == _end_of_storage){size_t len = pos - _start; //记录当前pos距离_start的距离reserve(capacity() == 0 ? 4 : capacity() * 2);pos = _start + len; //扩容:因为new的新空间会导致pos不在空间中导致后面while循环出错(pos失效解决方案)}iterator end = _finish - 1;while (end >= pos){*(end + 1) = *end;--end;}*pos = val;++_finish;return pos;
}

1. 为什么需要接受pos呢？

扩容问题会导致new出新的空间给_start，连续插入会导致pos迭代器失效

2. 对没有改动pos进行测试：

void test1()
{int arr[] = { 1,2,3,4};vector v(arr, arr + sizeof(arr) / sizeof(int));v.insert(v.end(), 10);for (auto i : v){cout << i << " ";}cout << endl;
}

reserve后pos不在[_start, _finish)范围内，解决方案就是改动pos

7. erase方法

iterator erase(iterator pos)
{assert(pos >= _start && pos < _finish);iterator start = pos + 1;while (start != _finish){*(start - 1) = *start;++start;}--_finish;return pos; //没有返回pos就导致迭代器失效
}

visual studio2019中的erase机制是一定是会pos迭代器失效。

8. push_back方法

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

9. pop_back方法

void pop_back()
{assert(!empty());--_finish;
}

10. []运算符重载

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

11. 拷贝构造

vector(const vector& v):_start(nullptr), _finish(nullptr), _end_of_storage(nullptr)
{reserve(v.capacity());_start = new T[v.capacity()];for (size_t i = 0; i < v.size(); ++i){_start[i] = v._start[i];}_finish = _start + v.size();_end_of_storage = _start + v.capacity();
}

1. 为什么用for循环依次拷贝数据，而不是memcpy()呢？

假如是string类型，拷贝的时候就需要深拷贝，但是memcpy是浅拷贝

12. 迭代器

typedef T* iterator;
typedef const T* const_iterator;
iterator begin()
{return _start; //传值返回:临时对象具有常性
}
iterator end()
{return _finish;
}
const_iterator begin() const
{return _start;
}
const_iterator end() const
{return _finish;
}

13. 完整代码

#pragma once
#include namespace myvector
{templateclass vector{public:typedef T* iterator;typedef const T* const_iterator;vector():_start(nullptr), _finish(nullptr), _end_of_storage(nullptr){}vector(size_t n, const T& val = T()) //T()->T类型的构造 && const T&引用修饰->起别名延长了生命周期:_start(nullptr),_finish(nullptr),_end_of_storage(nullptr){reserve(n);for (size_t i = 0; i < n; ++i){push_back(val);}}vector(int n, const T& val = T()) //T()->T类型的构造 && const T&引用修饰->起别名延长了生命周期:_start(nullptr), _finish(nullptr), _end_of_storage(nullptr){reserve(n);for (int i = 0; i < n; ++i){push_back(val);}}template vector(InputIterator first, InputIterator last):_start(nullptr), _finish(nullptr), _end_of_storage(nullptr){while (first != last){push_back(*first);++first;}}/*vector(const vector& v):_start(nullptr), _finish(nullptr), _end_of_storage(nullptr){reserve(v.capacity());for (auto i : v){push_back(i);}}*///vector<自定义类型>-->两次析构会程序崩溃//vector(const vector& v)//	:_start(nullptr)//	, _finish(nullptr)//	, _end_of_storage(nullptr)//{//	reserve(v.capacity());//	_start = new T[v.capacity()];//	memcpy(_start, v._start, sizeof(T) * v.size()); //memcpy也是浅拷贝//	_finish = _start + v.size();//	_end_of_storage = _start + v.capacity();//}vector(const vector& v):_start(nullptr), _finish(nullptr), _end_of_storage(nullptr){reserve(v.capacity());_start = new T[v.capacity()];for (size_t i = 0; i < v.size(); ++i){_start[i] = v._start[i];}_finish = _start + v.size();_end_of_storage = _start + v.capacity();}iterator begin(){return _start; //传值返回:临时对象具有常性}iterator end(){return _finish;}const_iterator begin() const{return _start;}const_iterator end() const{return _finish;}T& operator[](const size_t& pos){assert(pos < size());return _start[pos];}const T& operator[](const size_t& pos) const{assert(pos < size());return _start[pos];}size_t capacity() const{return _end_of_storage - _start;}size_t size() const{return _finish - _start;}void resize(size_t capa, T val = T()){if (capa < size()){_finish = _start + capa;}else{if (capa > capacity()){reserve(capa);}while (_finish != _start + capa){*_finish = val;++_finish;}}}//void reserve(size_t capa)//{//	if (capa > capacity())//	{//		size_t len = size(); //OK:记录当前长度//		T* tmp = new T[capa];//		if (_start != nullptr)//		{//			memcpy(tmp, _start, sizeof(T) * size()); //memcpy是浅拷贝//			delete[] _start;//		}//		_start = tmp;//		_finish = _start + len;//		//_finish = _start + size(); //err:size() = _finish - _start --> _finish = _finish//		_end_of_storage = _start + capa;//	}//}void reserve(size_t capa){if (capa > capacity()){size_t len = size(); //OK:记录当前长度T* tmp = new T[capa];if (_start != nullptr){for (size_t i = 0; i < len; ++i){tmp[i] = _start[i];}delete[] _start;}_start = tmp;_finish = _start + len;//_finish = _start + size(); //err:size() = _finish - _start --> _finish = _finish_end_of_storage = _start + capa;}}void push_back(const T& val){if (_finish == _end_of_storage){reserve(capacity() == 0 ? 4 : capacity() * 2);}*(_finish) = val;++_finish;}bool empty(){return _start == _finish;}void pop_back(){assert(!empty());--_finish;}iterator insert(iterator pos, const T& val){assert(pos >= _start && pos <= _finish);if (_finish == _end_of_storage){size_t len = pos - _start; //记录当前pos距离_start的距离reserve(capacity() == 0 ? 4 : capacity() * 2);pos = _start + len; //扩容:因为new的新空间会导致pos不在空间中导致后面while循环出错(pos失效解决方案)}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 start = pos + 1;while (start != _finish){*(start - 1) = *start;++start;}--_finish;return pos; //没有返回pos就导致迭代器失效}~vector(){delete[] _start;_start = _finish = _end_of_storage = nullptr;}void out(const vector& v){for (size_t i = 0; i < v.size(); ++i){cout << v[i] << " ";}cout << endl;vector::const_iterator it = v.begin();while (it != v.end()){cout << *it << " ";++it;}cout << endl;}private:iterator _start;iterator _finish;iterator _end_of_storage;};
}