C++易vector

很长一段时间没有动手编写C++计划。无非就是模仿后STL对，虽然达不到标准STL该程序。但简单的功能来实现。

STL事实上，深刻：泛型编程、容器、算法、适配器...有的是内容能够学。以下是依据STL源代码。写的一个非常easy的vector，实现了部分接口。事实上vector还是相对非常easy的容器了，元素按在内存中连续排列，仅仅须要三个指针就能实现非常多的接口。另一个就是内存的分配，这里採用了一个C++提供的allocator配置器。所以分配起来还是蛮简单的，SGI版本号的STL中的配置器为了达到效率的极致，使用了另外的分配器。相当复杂。这里就没有写了。

#ifndef __MYVECTOR_H__
#define __MYVECTOR_H__

template <class T>
class Vector {
public:
	typedef T value_type;
	typedef value_type* iterator;	// vector的迭代器就是原生指针
	typedef value_type* pointer;
	typedef value_type& reference;
	typedef size_t size_type;

public:
	Vector() : start(NULL), finish(NULL), end_of_storage(NULL)
	{
	}

	Vector(size_type n, const value_type &value)
	{
		start = alloc.allocate(n);
		end_of_storage = finish = start + n;
		uninitialized_fill_n(start, n, value);
	}

	Vector(size_type n)
	{
		start = alloc.allocate(n);
		end_of_storage = finish = start + n;
		uninitialized_fill_n(start, n, value_type());
	}

	~Vector()
	{
		// 顺序调用元素的析构函数
		for (iterator i = start; i != finish; ++i)
			alloc.destroy(i);
		
		// 销毁分配的空间
		if (start != NULL)
			alloc.deallocate(start, end_of_storage - start);
	}

	iterator begin() const
	{
		return start;
	}

	iterator end() const
	{
		return finish;
	}

	size_type size() const
	{
		return end() - begin();	// 使用接口函数，包裹性更好
	}

	size_type capacity() const
	{
		return end_of_storage - begin();	// 使用接口函数。包裹性更好
	}

	bool empty() const
	{
		return begin() == end();
	}

	// 返回的引用可被改动
	reference front()
	{
		return *(begin());
	}

	// 返回的引用可被改动
	reference back()
	{
		return *(end() - 1);
	}

	reference operator[] (const size_type n)
	{
		return *(begin() + n);
	}

	const reference operator[] (const size_type n) const
	{
		return *(begin() + n);
	}

	void push_back(const value_type &value)
	{
		if (finish == end_of_storage)
			reallocate();	// 存储空间已满。则又一次分配内存
		alloc.construct(finish, value);
		++finish;
	}

	void reallocate();

	void pop_back()
	{
		--finish;
		alloc.destroy(finish);	// 析构最后一个函数，但不释放空间
	}

	// 清除一个元素
	iterator erase(iterator position)
	{
		if (position + 1 != finish)
			copy(position + 1, finish, position);
		--finish;
		alloc.destroy(finish);
		return position;
	}

	// 清除一段元素
	iterator erase(iterator first, iterator last)
	{
		if (first < start || last > finish)
			throw exception("Invalid input.");

		copy(last, finish, first);
		int len = last - first;
		while (len--)
			alloc.destroy(--finish);
		return first;
	}

	void clear()
	{
		erase(begin(), end());
	}

private:
	iterator start;
	iterator finish;
	iterator end_of_storage;

private:
	static std::allocator<value_type> alloc;	// 空间配置器。採用静态属性节省空间
};

template <class Type>
std::allocator<Type> Vector<Type>::alloc;

template <class Type>
void Vector<Type>::reallocate()
{
	size_type oldsize = size();
	size_type newsize = 2 * (oldsize == 0 ? 1 : oldsize);
	
	// 分配新的内存空间
	iterator newstart = alloc.allocate(newsize);
	uninitialized_copy(start, finish, newstart);

	// 顺序调用每一个元素的析构函数
	for (iterator i = start; i != finish; ++i)
		alloc.destroy(i);

	// 销毁分配的空间，销毁之前主要检查是否为NULL
	if (start != NULL)
		alloc.deallocate(start, end_of_storage - start);

	// 更新下标
	start = newstart;
	finish = start + oldsize;
	end_of_storage = start + newsize;
}

#endif

insert操作应该算是最复杂的一个接口了，设计到元素的搬移、（可能）又一次分配内存等等，这里我仅仅实现了一个最简单的形式：

template <class Type>
void Vector<Type>::insert(iterator position, const value_type &value)
{
	size_type diff = position - start;

	if (finish == end_of_storage)
		reallocate();

	position = start + diff;

	// 注意，这里不能使用copy。由于目的地最后一个位置还没有被构造，
	// 赋值涉及析构操作，对未构造的对象进行析构，行为没有定义
	alloc.construct(finish, *(finish - 1));
	++finish;
	copy_backward(position, finish - 1, finish);

	// 不能使用uninitialized_copy。由于这个函数是从前向后构造。这会造成覆盖
	//uninitialized_copy(position, finish, position + 1);

	// 插入新对象，直接赋值就可以
	*position = value;
}

int main()
{
	Vector<int> v;

	v.push_back(1);
	cout << "size = " << v.size() << endl;
	cout << "capacity = " << v.capacity() << endl;
	
	v.push_back(2);
	cout << "size = " << v.size() << endl;
	cout << "capacity = " << v.capacity() << endl;

	v.push_back(3);
	cout << "size = " << v.size() << endl;
	cout << "capacity = " << v.capacity() << endl;

	v.push_back(4);
	cout << "size = " << v.size() << endl;
	cout << "capacity = " << v.capacity() << endl;

	v.push_back(5);
	cout << "size = " << v.size() << endl;
	cout << "capacity = " << v.capacity() << endl;

	Vector<int>::iterator iter1 = v.begin();
	Vector<int>::iterator iter2 = iter1 + 3;
	v.erase(iter1, iter2);

	cout << "size = " << v.size() << endl;
	cout << "capacity = " << v.capacity() << endl;

	v.clear();
	cout << "size = " << v.size() << endl;
	cout << "capacity = " << v.capacity() << endl;

	v.push_back(123);
	cout << "size = " << v.size() << endl;
	cout << "capacity = " << v.capacity() << endl;

	for (Vector<int>::iterator iter = v.begin(); iter != v.end(); ++iter)
		cout << *iter << endl;

	system("pause");
	return 0;
}

參考：

《STL源代码剖析》

《C++ primer》