C++ – vector类模拟实现
0. 成员变量
template<class T> typedef T* iterator; typedef const T* const_iterator; iterator _start; iterator _finish; iterator _end_of_storage;
- 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 <class InputIterator> 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; } }
- 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; } }
操作：
是否扩容
拷贝数据
变化成员变量
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; }
- 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
- 对没有改动pos进行测试：
void test1() { int arr[] = { 1,2,3,4}; vector<int> 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<T>& 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(); }
- 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 <assert.h> namespace myvector { template<class T> class 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 <class InputIterator> vector(InputIterator first, InputIterator last) :_start(nullptr) , _finish(nullptr) , _end_of_storage(nullptr) { while (first != last) { push_back(*first); ++first; } } /*vector(const vector<T>& 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<T>& 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<T>& 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<T>& v) { for (size_t i = 0; i < v.size(); ++i) { cout << v[i] << " "; } cout << endl; vector<T>::const_iterator it = v.begin(); while (it != v.end()) { cout << *it << " "; ++it; } cout << endl; } private: iterator _start; iterator _finish; iterator _end_of_storage; }; }
