Stack的模拟实现

栈的特性：后进先出
接口一般有：
push();   
pop();
size();
empty();
top();

#pragma once
#include <assert.h>
//const size_t N  = 10;
//#define NUM 20

template<class T>
class Stack
{
public:
    Stack()
        :_a(NULL)
        ,_size(0)
        ,_capacity(0)
    {}

    ~Stack()
    {
        delete[] _a;
        _a = NULL;
        _size = _capacity = 0;
    }

    Stack(const Stack<T>& s);
    Stack<T>& operator=(const Stack<T>& s);

public:
    void Push(const T& x)
    {
        _CheckCapacity();
        _a[_size++] = x;
    }

    void Pop()
    {
        assert(_size);

        --_size;
    }

    T& Top()
    {
        assert(_size > 0);
        return _a[_size-1];
    }

    const T& Top() const
    {
        assert(_size > 0);
        return _a[_size-1];
    }

    size_t Size()
    {
        return _size;
    }

    bool Empty()
    {
        return _size == 0;
    }

protected:
    void _CheckCapacity()
    {
        if (_size >= _capacity)
        {
            size_t newCapacity = _capacity*2 + 3;
            T* tmp = new T[newCapacity];

            // 类型萃取
            if (_a)
            {
                //memcpy(tmp, _a, _size*sizeof(T));
                for (size_t i = 0; i < _size; ++i)
                {
                    tmp[i] = _a[i];
                }
            }

            _capacity = newCapacity;

            delete[] _a;
            _a = tmp;
        }
    }

protected:
    T* _a;
    size_t _size;
    //size_t _top;
    size_t _capacity;
};