C++入门--vector
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目录
vector的介绍
vector是表示可变大小数组的序列容器。
vector的使用
对象的定义
void test_vector1()
{
vector<int> v1;
vector<int> v2(10, 8); //10个8
vector<int> v3(++v2.begin(), --v2.end());
vector<int> v4(v3);
string s("hello world");
vector<char> v5(s.begin(), s.end());
}遍历
有三种遍历方式分别是下标迭代器和范围for
void test_vector2()
{
vector<int> v;
v.push_back(1);
v.push_back(2);
v.push_back(3);
v.push_back(4);
//遍历
for (size_t i = 0; i < v.size(); ++i)
{
v[i] += 1;
cout << v[i] << " ";
}
cout << endl;
vector<int>::iterator it = v.begin();
while (it != v.end())
{
*it -= 1;
cout << *it << " ";
++it;
}
cout << endl;
for (auto e : v)
{
cout << e << " ";
}
cout << endl;
}reserve与resize（push_back)
[]和at对越界的处理方式不一样下标访问采用的是断言at使用的是抛异常。
void test_vector3()
{
vector<int> v;
v.push_back(1);
v.push_back(2);
v.push_back(3);
v.push_back(4);
cout << v.max_size() << endl;
v.reserve(100); //扩容
v.resize(100, 5);
v.resize(2);
/*v[3]; //断言
v.at(3);*/ //抛异常
}insert与erase（assign find 与clear)
插入和删除要注意迭代器失效。
void test_vector4()
{
vector<int> v;
v.push_back(1);
v.push_back(2);
v.push_back(3);
v.push_back(4);
//v.assign(10, 5); //改成10个5
vector<int>::iterator ret = find(v.begin(), v.end(), 3);
if (ret != v.end())
{
cout << "找到了" << endl;
v.insert(ret, 30);
//v.erase(ret); //不能这样删因为ret失效了。
}
v.insert(v.begin(), -1);
for (auto e : v)
{
cout << e << " ";
}
cout << endl;
vector<int>::iterator pos = find(v.begin(), v.end(),30);
if (pos != v.end())
{
v.erase(pos);
}
for (auto e : v)
{
cout << e << " ";
}
cout << endl;
v.clear();
for (auto e : v)
{
cout << e << " ";
}
cout << endl;
}迭代器失效
插入会导致vector扩容vector底层原理旧空间被释放掉 但是it还使用的是释放之间的旧空间在对it迭代器操作时实际操作的是一块已经被释放的空间而引起代码运行时崩溃。
解决方式在以上操作完成之后如果想要继续通过迭代器操作vector中的元素只需给it重新
赋值即可。
erase删除pos位置元素后pos位置之后的元素会往前搬移没有导致底层空间的改变理论上讲迭代 器不应该会失效但是如果pos刚好是最后一个元素删完之后pos刚好是end的位置而end位置是 没有元素的那么pos就失效了。因此删除vector中任意位置上元素时vs就认为该位置迭代器失效 了。
vector的模拟实现
#pragma once
namespace qwe
{
template<class T>
class vector
{
public:
typedef T* iterator;
typedef const T* const_iterator;
vector()
:_start(nullptr)
, _finish(nullptr)
, _endofstorage(nullptr)
{}
v2(v1)
传统写法
//vector(const vector<T>& v)
//{
// _start = new T[v.capacity()];
// _finish = _start + v.size();
// _endofstorage = _start + v.capacity();
// memcpy(_start, v._start, v.size()*sizeof(T));
//}
//一个类模板的成员函数又可以是一个函数模板
template <class InputIterator>
vector(InputIterator first, InputIterator last)
:_start(nullptr)
, _finish(nullptr)
, _endofstorage(nullptr)
{
while (first != last)
{
push_back(*first);
++first;
}
}
void swap(vector<T>& v)
{
std::swap(_start, v._start);
std::swap(_finish, v._finish);
std::swap(_endofstorage, v._endofstorage);
}
//v2(v1)
//现代写法
//vector(const vector& v)
vector(const vector<T>& v)
:_start(nullptr)
, _finish(nullptr)
, _endofstorage(nullptr)
{
vector<T> tmp(v.begin(), v._end());
swap(tmp);
}
//v3=v1
//vector& operator=(vector v)
vector<T>& operator=(vector<T> v)
{
swap(v);
return *this;
}
~vector()
{
if (_start)
{
delete[] _start;
_start = _finish = _endofstorage = nullptr;
}
}
const_iterator begin()const
{
return _start;
}
const_iterator end()const
{
return _finish;
}
iterator begin()
{
return _start;
}
iterator end()
{
return _finish;
}
const T& operator[](size_t i)const
{
assert(i < size());
return _start[i];
}
T& operator[](size_t i)
{
assert(i < size());
return _start[i];
}
size_t size()const
{
return _finish - _start;
}
size_t capacity()const
{
return _endofstorage - _start;
}
void reserve(size_t n)
{
if (n > capacity())
{
size_t sz = size();
T* tmp = new T[n];
if (_start)
{
//memcpy(tmp, _start, sizeof(T)*size());
for (size_t i = 0; i < sz; ++i)
{
//T是int一个一个拷贝没问题
//T是自定义类型深拷贝赋值
tmp[i] = _start[i];
}
delete[] _start;
}
_start = tmp;
_finish = _start + sz;
_endofstorage = _start + n;
}
}
void resize(size_t n, const T& val = T())
{
if (n < size())
{
_finish = _start + n;
}
else
{
if (n>capacity())
{
reserve(n);
}
while (_finish != _start + n)
{
*_finish = val;
++_finish;
}
}
}
iterator insert(iterator pos, const T& x)
{
assert(pos >= _start);
assert(pos <= _finish);
//满了扩容
if (_finish == _endofstorage)
{
//扩容会导致pos失效
size_t len = pos - _start;
reserve(capacity() == 0 ? 4 : capacity() * 2);
pos = _start + len;
}
iterator end = _finish - 1;
while (end >= pos)
{
*(end + 1) = *end;
--end;
}
*pos = x;
++_finish;
return pos;
}
iterator erase(iterator pos)
{
assert(pos >= _start);
assert(pos <= _finish);
iterator begin = pos + 1;
while (begin < _finish)
{
*(begin - 1) = *begin;
++begin;
}
--_finish;
return pos;
}
void push_back(const T& x)
{
if (_finish == _endofstorage)
{
reserve(capacity() == 0 ? 4 : capacity() * 2);
}
*_finish = x;
++_finish;
}
void pop_back()
{
assert(_finish > _start);
--_finish;
}
private:
iterator _start;
iterator _finish;
iterator _endofstorage;
};
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);
v.push_back(6);
for (size_t i = 0; i < v.size(); ++i)
{
cout << v[i] << " ";
}
cout << endl;
vector<int>::iterator it = v.begin();
while (it != v.end())
{
cout << *it << " ";
++it;
}
cout << endl;
for (auto e : v)
{
cout << e << " ";
}
cout << endl;
}
void test_vector2()
{
vector<int> v;
v.push_back(1);
v.push_back(2);
v.push_back(3);
v.push_back(4);
vector<int>::iterator it = find(v.begin(), v.end(), 2);
if (it != v.end())
{
v.insert(it,20); //it会失效
}
for (auto e : v)
{
cout << e << " ";
}
cout << endl;
}
void test_vector3()
{
//1 2 3 4 5
//1 2 3 4
//1 2 3 4 5
vector<int> v1;
v1.push_back(1);
v1.push_back(2);
v1.push_back(3);
v1.push_back(4);
v1.push_back(5);
//删掉使用的偶数
vector<int>::iterator it = v1.begin();
while (it != v1.end())
{
if (*it % 2 == 0)
{
it = v1.erase(it);
}
else
{
++it;
}
}
for (auto e : v1)
{
cout << e << " ";
}
cout << endl;
}
}