实现一个可变长数组

#include<iostream>
#include<vector>
#include<map>
#include<string>
#include<cstring>
#include<cstdio>
#include<algorithm>
#include<set>
#include<queue>
#include<unordered_map>
#include<cmath>
using namespace std;
class CArray
{
	int st = 32;
	int size, temp;
	int* ptr;
public:
	CArray(int s = 0);
	CArray(CArray& a);
	~CArray();
	void push_back(int v);
	void resize(int v);
	void Resize(int v);
	CArray& operator=(const CArray& a);
	int length() { return max(size,temp); };
	int& operator [](int i)
	{
		return ptr[i];
	}
};
CArray::CArray(int s):size(s)
{
	if (s == 0)
		ptr = NULL;
	else
		ptr = new int[s];
}
CArray::CArray(CArray& a)
{
	if (!a.ptr)
	{
		ptr = NULL;
		size = 0;
		return;
	}
	ptr = new int[a.size];
	memcpy(ptr, a.ptr, sizeof(int) * a.size);
	size = a.size;
}
CArray::~CArray()
{
	if (ptr) delete[]ptr;
}
CArray& CArray::operator=(const CArray& a)
{
	if (ptr == a.ptr)
		return *this;
	if (a.ptr == NULL)
	{
		if (ptr) delete[]ptr;
		ptr = NULL;
		size = 0;
		return *this;
	}
	if (size < a.size)
	{
		if (ptr) delete[]ptr;
		ptr = new int[a.size];
	}
	memcpy(ptr, a.ptr, sizeof(int) * a.size);
	size = a.size;
	return *this;
}
void CArray::resize(int v)
{
	int* temptr = new int[v];
	memcpy(temptr, ptr, sizeof(int) * size);
	delete[]ptr;
	ptr = temptr;
	if (v > size)
	{
		temp = size;
		while (temp < v)
		{
			ptr[temp++] = 0;
		}
	}
}
void CArray::Resize(int v)
{
	int* temptr = new int[v];
	memcpy(temptr, ptr, sizeof(int) * size);
	delete[]ptr;
	ptr = temptr;
}
void CArray::push_back(int v)
{
	if (ptr)
	{
		if(size<st)
			ptr[size++] = v;
		else
		{
			st = st * 2;
			Resize(st);
			ptr[size++] = v;
		}
	}
	else
	{
		ptr = new int[st];
		ptr[size++] = v;
	}

}
int main()
{
	CArray a, a2, a3, a5;
	for (int i = 0; i < 123; i++)
	{
		a.push_back(i);
	}
	a2 = a;
	for (int i = 0; i < a2.length(); i++)
	{
		cout << a2[i] << " ";
	}
	cout << endl;
	a2 = a3;
	for (int i = 0; i < a2.length(); i++)
	{
		cout << a2[i] << " ";
	}
	cout << endl;
	a[3] = 100;
	CArray a4(a);
	for (int i = 0; i < a4.length(); i++)
	{
		cout << a4[i] << " ";
	}
	cout << endl;
	a5.resize(7);
	a5[5] = 1;
	for (int i = 0; i < a5.length(); i++)
	{
		cout << a5[i] << " ";
	}
	return 0;
}`