【c++】vector的增删查改

1.先定义一个类对象vector

为了防止和库里面发生冲突，定义一个命名空间，将类对象放在命名空间 里面

#include<iostream>
using namespace std;
namespace zjw {class vector {public:private:};
}

2.定义变量，需要一个迭代器，为了便于修改，变成任意类型的迭代器，我们使用函数模版，三个迭代器变量 _start用于指向顺序表的头，_finish指向顺序表的结尾的下一位，_end_of_storage保存总容量，在初始化列表中先设置为空

#include<iostream>
using namespace std;
namespace zjw {template<class T>class vector {public:typedef T * iterator;//迭代器类型vector():_start(nullptr), _finish(nullptr), _end_of_storage(nullptr){}private:iterator _start;iterator _finish;iterator _end_of_storage};
}

3.定义函数

1.返回指向顺序表开始的迭代器函数
2.返回指向顺序表结尾的迭代器函数
3.返回容量大小的函数
4.返回顺序表元素多少
5.判断顺序表为空地的函数
5.一个运算符重载的函数（返回给定下标下的值）

#include<iostream>
using namespace std;
namespace zjw {template<class T>class vector {public:typedef T * iterator;//迭代器类型vector():_start(nullptr), _finish(nullptr), _end_of_storage(nullptr){}iterator begin()//1{return _start;}iterator end()//2{return  _finish;}size_t capacity()//3{return _end_of_storage - _start;}size_t size()//4{return _finish - _start;}bool empty()//5{return _finish == _start;}T& operator[](size_t pos)//6{assert(pos < size());return _start[pos];}private:iterator _start;iterator _finish;iterator _end_of_storage;};
}

4.reserve函数开空间

  void reserve(size_t n){if (n > capacity){T* tmp = new T[n];if (_start){memcpy(tmp, _start, sizeof(T) * size());delete[] _start;}_start = tmp;_finish = _start + size();_end_of_storage = _start + n;}}

5.push_back函数尾插，以及尾删函数

 void push_back(const T& x){if (_finish == _end_of_storage){reserve(capacity() == 0 ? 4 : capacity() * 2);}*_finish = x;_finish++;}void pop_back(){assert(!empty());--_finish;}

6.测试尾插，尾删，下标遍历，迭代器遍历，范围for遍历，以及运算符重载返回对应下标的元素

 void test1(){vector<int>v1;v1.push_back(1);v1.push_back(2);v1.push_back(3);v1.push_back(4);v1.push_back(5);for (int i = 0; i < v1.size(); i++){cout << v1[i] << " ";}cout << endl;v1.pop_back();v1.pop_back();vector<int>::iterator it = v1.begin();while (it != v1.end()){cout << *it << " ";++it;}cout << endl;v1.pop_back();for (auto e : v1){cout << e << " ";}cout << endl;}

这里开空间的函数会发生问题

7.修改扩容函数

  void reserve(size_t n){if (n > capacity){   int sz=size();T* tmp = new T[n];if (_start){memcpy(tmp, _start, sizeof(T) * size());delete[] _start;}_start = tmp;_finish = _start + sz;_end_of_storage = _start + n;}}

8.resize函数

参数小于容量大小，相当于缩容，参数大于容量大小，相当于扩容，超过_finish的用值初始化
在讲resize之前看看模版会不会给匿名变量初始化，内置类型是否能初始化.

 template <class T>void f(){T x = T();cout << x << endl;}void test3(){f<int>();f<int*>();f<double>();}

发现可以，所以我们在resize里面当n>capacity(),我们可以把顺序表外的用来初始化，自定义类型相当于调用自己的构造函数，内置函数我们在这假装理解为调用自己的构造函数.

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

9.测试resize

 void test2(){vector<int>v1;v1.push_back(1);v1.push_back(2);v1.push_back(3);v1.push_back(4);v1.push_back(5);cout << v1.size() << endl;cout << v1.capacity() << endl;v1.resize(10);cout << v1.size() << endl;cout << v1.capacity() << endl;for (auto e : v1){cout << e << " ";}cout << endl;v1.resize(3);for (auto e : v1){cout << e << " ";}}}

10.封装一个打印函数用于任何vector对象

 void func(const vector<int>& v){for (int i = 0; i < v.size(); i++)//下标{cout << v[i] << " ";}cout << endl;vector<int>::iterator it = v.begin();//迭代器while (it != v.end()){cout << *it << " ";++it;}cout << endl;for (auto e : v)//范围for{cout << e << " ";}cout << endl;}

因为打印不同对象的元素，为了区分是有一个this指针的，但是参数是const 无法产生this指针

解决方法，在这些函数后面加 const ,如果是const对象调用的话，就是权限平移，如果是非const的话，是权限缩小，都可以调用了，如果要限制别人去修改这个*it,我们可以定义一个const的迭代器

void func(const vector<int>& v){for (int i = 0; i < v.size(); i++)//下标{cout << v[i] << " ";}cout << endl;vector<int>::iterator it = v.begin();//迭代器while (it != v.end()){cout << *it << " ";++it;}cout << endl;for (auto e : v)//范围for{cout << e << " ";}cout << endl;}

 void test4(){vector<int>v1;v1.push_back(1);v1.push_back(2);v1.push_back(3);v1.push_back(4);v1.push_back(5);func(v1);}

11.insert函数实现

   void insert(iterator pos, const T& val){assert(pos >= _start);assert(pos <= _finish);if (_finish == _end_of_storage){reserve(capacity() == 0 ? 4 : capacity() * 2);}iterator end = _finish - 1;while (end >= pos){*(end + 1) = *end;--end;}*pos = val;++_finish;}

测试insert

 void test5(){vector<int>v1;v1.push_back(1);v1.push_back(2);v1.push_back(3);v1.push_back(4);v1.push_back(5);for (auto e : v1)//范围for{cout << e << " ";}cout << endl;auto pos = find(v1.begin(), v1.end(), 3);//找到3的位置if (pos != v1.end()){v1.insert(pos, 30);//3前插入30}for (auto e : v1)//范围for{cout << e << " ";}cout << endl;(*pos)++;//让3+1for (auto e : v1)//范围for{cout << e << " ";}cout << endl;}}

pos保存的之前是3的地址，如果在3前插入一个30，pos所指向的位置就变成了30，*pos++,就变成了31；

如果我们不使用push_back（5）的话

 void test5(){vector<int>v1;v1.push_back(1);v1.push_back(2);v1.push_back(3);v1.push_back(4);//v1.push_back(5);for (auto e : v1)//范围for{cout << e << " ";}cout << endl;auto pos = find(v1.begin(), v1.end(), 3);//找到3的位置if (pos != v1.end()){v1.insert(pos, 30);//3前插入30}for (auto e : v1)//范围for{cout << e << " ";}cout << endl;(*pos)++;//让3+1for (auto e : v1)//范围for{cout << e << " ";}cout << endl;}}

会出现报错，到底是怎么一回事？？


如何修改？？
我们需要在未扩容前记录好pos距离_start的位置，拷到新空间，他与新_start也保持这个距离，更新pos

   void insert(iterator pos, const T& val){assert(pos >= _start);assert(pos <= _finish);if (_finish == _end_of_storage){int len=pos - _start;reserve(capacity() == 0 ? 4 : capacity() * 2);pos = _start + len;}iterator end = _finish - 1;while (end >= pos){*(end + 1) = *end;--end;}*pos = val;++_finish;}

这里的*pos里面的数据被清理，如果不清理应该是3++，应该是4，我们可以注释掉reserve中的delete看看

这里的问题被叫做迭代器失效

12.earse函数

 void eraase(iterator pos){assert(pos >= _start);assert(pos < _finish);iterator start = pos + 1;while (start != _finish){*(start - 1) = *start;++start;}--_finish;}

earse函数测试

void test6(){vector<int>v1;v1.push_back(1);v1.push_back(2);v1.push_back(3);v1.push_back(4);for (auto e : v1)//范围for{cout << e << " ";}cout << endl;auto pos = find(v1.begin(), v1.end(), 2);if (pos != v1.end()){v1.erase(pos);//删除2}(*pos)++;2位置上的元素++for (auto e : v1)//范围for{cout << e << " ";}}

这里2删除后2的地址上存的就是3，然后给3++；在std中这个也算迭代器失效，由于这里是自己写的，所以没报错.

 void test6(){std::vector<int>v1;v1.push_back(1);v1.push_back(2);v1.push_back(3);v1.push_back(4);for (auto e : v1)//范围for{cout << e << " ";}cout << endl;auto pos = find(v1.begin(), v1.end(), 2);if (pos != v1.end()){v1.erase(pos);}(*pos)++;for (auto e : v1)//范围for{cout << e << " ";}}}

上面会调用vector里面的函数

在不同编译器下有不同的处理情况。在vs中std我们认为迭代器失效，如果删除的是4的话，以我们实现的earse后，–_finish ;4那里的值没有被清理，所以后面的（*pos）++也不会出错，但在vs中我们使用vector里面的实现认为迭代器失效.
我们可以将代码拷过去在linux下试一下g++是怎么回事？？

在g++中使用库中的vector函数删除数据4没有报错，对于迭代器失效，不同编译器也有不同处理.

13.整体代码

#include<iostream>
#include<assert.h>
#include<vector>
using namespace std;
namespace zjw {template<class T>class vector {public:typedef T * iterator;//迭代器类型typedef T* const_iterator;//迭代器类型vector():_start(nullptr), _finish(nullptr), _end_of_storage(nullptr){}iterator begin()const {return _start;}iterator end()  const{return  _finish;}size_t capacity(){return _end_of_storage - _start;}size_t size()  const{return _finish - _start;}bool empty(){return _finish == _start;}T& operator[](size_t pos)  const{assert(pos < size());return _start[pos];}void reserve(size_t n){int sz = size();if (n > capacity()){T* tmp = new T[n];if (_start){memcpy(tmp, _start, sizeof(T) * size());delete[] _start;}_start = tmp;_finish = _start + sz;_end_of_storage = _start + n;}}void push_back(const T& x){if (_finish == _end_of_storage){reserve(capacity() == 0 ? 4 : capacity() * 2);}*_finish = x;_finish++;}void pop_back(){assert(!empty());--_finish;}void resize(size_t n, T val = T()){if (n < size()){_finish = _start + n;}else{if (n > capacity())reserve(n);while (_finish != _start + n){*_finish = val;++_finish;}}}void insert(iterator pos, const T& val){assert(pos >= _start);assert(pos <= _finish);if (_finish == _end_of_storage){int len=pos - _start;reserve(capacity() == 0 ? 4 : capacity() * 2);pos = _start + len;}iterator end = _finish - 1;while (end >= pos){*(end + 1) = *end;--end;}*pos = val;++_finish;}void erase(iterator pos){assert(pos >= _start);assert(pos < _finish);iterator start = pos + 1;while (start != _finish){*(start - 1) = *start;++start;}--_finish;}private:iterator _start;iterator _finish;iterator _end_of_storage;};void test1(){vector<int>v1;v1.push_back(1);v1.push_back(2);v1.push_back(3);v1.push_back(4);v1.push_back(5);for (int i = 0; i < v1.size(); i++){cout << v1[i] << " ";}cout << endl;v1.pop_back();v1.pop_back();vector<int>::const_iterator it = v1.begin();while (it != v1.end()){cout << *it << " ";++it;}cout << endl;v1.pop_back();for (auto e : v1){cout << e << " ";}cout << endl;}void test2(){vector<int>v1;v1.push_back(1);v1.push_back(2);v1.push_back(3);v1.push_back(4);v1.push_back(5);cout << v1.size() << endl;cout << v1.capacity() << endl;v1.resize(10);cout << v1.size() << endl;cout << v1.capacity() << endl;for (auto e : v1){cout << e << " ";}cout << endl;v1.resize(3);for (auto e : v1){cout << e << " ";}}void func(const vector<int>& v){for (int i = 0; i < v.size(); i++)//下标{cout << v[i] << " ";}cout << endl;vector<int>::iterator it = v.begin();//迭代器while (it != v.end()){cout << *it << " ";++it;}cout << endl;for (auto e : v)//范围for{cout << e << " ";}cout << endl;}void test4(){vector<int>v1;v1.push_back(1);v1.push_back(2);v1.push_back(3);v1.push_back(4);v1.push_back(5);func(v1);}void test5(){vector<int>v1;v1.push_back(1);v1.push_back(2);v1.push_back(3);v1.push_back(4);// v1.push_back(5);for (auto e : v1)//范围for{cout << e << " ";}cout << endl;auto pos = find(v1.begin(), v1.end(), 3);//找到3的位置if (pos != v1.end()){v1.insert(pos, 30);//3前插入30}for (auto e : v1)//范围for{cout << e << " ";}cout << endl;(*pos)++;//让3+1cout << *pos << endl;for (auto e : v1)//范围for{cout << e << " ";}cout << endl;}void test6(){std:: vector<int>v1;v1.push_back(1);v1.push_back(2);v1.push_back(3);v1.push_back(4);for (auto e : v1)//范围for{cout << e << " ";}cout << endl;auto pos = find(v1.begin(), v1.end(), 4);if (pos != v1.end()){v1.erase(pos);}(*pos)++;for (auto e : v1)//范围for{cout << e << " ";}}}int main()
{// zjw::test1();zjw::test6();// zjw::test3();
}