C++模拟实现vector的示例代码

typedef T* iterator;
typedef const T* const_iterator;

iterator begin()
{
    return start;
}
iterator end()
{
    return finish;
}

const iterator cbegin()const
{
    return start;
}
const iterator cend()const
{
    return finish;
}

vector()
    :start(nullptr)
    , finish(nullptr)
    , endOfStorage(nullptr)
{}

vector(size_t n, const T& value = T())
    :start(new T[n])
    , finish(start)
    , endOfStorage(start + n)
{
    for (int i = 0; i < n; i++)
    {
        *finish++ = value;
    }
}

vector(int n, const T& value = T())
    :start(new T[n])
    , finish(start)
    , endOfStorage(start + n)
{
    for (int i = 0; i < n; i++)
    {
        *finish++ = value;
    }
}

//写成函数模板，可以接受任意类型的迭代器
template<typename Iterator>
vector(Iterator first, Iterator last)
{
    size_t n = ZH::distance(first, last);//获取长度
    start = new T[n];
    finish = start;
    endOfStorage = start + n;
    while (first != last){
        *finish = *first;
        first++;
        finish++;//完成赋值的同时也移动了finish的位置
    }
}

template<typename Iterator>
//此处的Iterator是模板参数，表示可以传任意类型的迭代器
size_t distance(Iterator first, Iterator last)
{
    //获取元素个数，暂时只考虑底层空间连续的情况
    int count = 0;
    while (first != last)
    {
        first++;
        count++;
    }
    return count;
}

//此处拷贝构造函数的形参是const类型
vector(const vector<T>& v)
    :start(nullptr)
    , finish(nullptr)
    , endOfStorage(nullptr)
{
    //▲用const类型的迭代器访问const变量
    vector<T> temp(v.cbegin(), v.cend());
    this->swap(temp);
}

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

~vector()
{
    delete[]start;
    start = nullptr;
    finish = nullptr;
    endOfStorage = nullptr;
}

size_t size()
{
    return finish - start;
}
size_t capacity()
{
    return endOfStorage - start;
}

size_t empty()
{
    return finish == start;
}

void resize(size_t newsize, const T& value = T())
{
    size_t oldsize = size();
    if (newsize > oldsize){
        if (newsize > capacity()){
            reserve(newsize);
        }
        for (size_t i = oldsize; i < newsize; i++)
        {
            start[i] = value;
        }
    }
    finish = start + newsize;//不用考虑增加或减小
}

void reserve(size_t newcapacity)
{
	size_t oldcapacity = capacity();
	if (newcapacity > oldcapacity)
	{
		size_t n = size();//保存size()的值
		T* temp = new T[newcapacity];
		//start不为空时才进行拷贝旧空间元素和释放的操作
		if (start)
		{
			//memcpy浅拷贝，当vector中存放的对象内部设计资源管理
			// 会有内存泄漏和野指针问题
			//memcpy(temp, start, sizeof(T) * n);

			for (size_t i = 0; i < n; i++)
			{
				temp[i] = start[i];//调用赋值运算符重载
			}
			delete[] start;
		}
		start = temp;
		//▲此处不能用satart+size(),因为size方法中有finish-start,而start值已经改变
		finish = start + n;
		endOfStorage = start + newcapacity;
	}
}

T& operator[](size_t index)
{
	assert(index < size());
	return start[index];
}

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

T& front()
{
    return start[0];
}
const T& front()const
{
    return start[0];
}

T& back()
{
    return *(finish - 1);
}
const T& back()const
{
    return *(finish - 1);
}

void push_back(const T& value)
{
    if (finish == endOfStorage)
    {
        //因为原来的capacity可能为0,所以要+3
        reserve(capacity() * 2 + 3);
    }
    *finish++ = value;
}

void pop_back()
{
    if (empty())
    {
        return;
    }
    finish--;
}

//第二个参数用const修饰，常量引用
//不用const修饰则为非常量引用
iterator insert(iterator pos, const T& value)
{
    int index = pos - start;
    assert(pos >= start && pos < finish);
    //判断空间是否足够
    if (finish == endOfStorage)
    {
        reserve(capacity() * 2);
    }
    pos = start + index;
    for (auto it = finish; it > pos; it--)
    {
        *it = *(it - 1);
    }
    *pos = value;
    finish++;
    return pos;
}

iterator erase(iterator pos)
{
	assert(pos >= start && pos < finish);

	auto it = pos;
	while (it < finish - 1)
	{
		*it = *(it + 1);
		it++;
	}
	finish--;
	return pos;
}

void clear()
{
    finish = start;
}

void swap(vector<T>& v)
{
    std::swap(start, v.start);
    std::swap(finish, v.finish);
    std::swap(endOfStorage, v.endOfStorage);
}

private:
    iterator start;
    iterator finish;
    iterator endOfStorage;