重点：

1.长时间不使用static会忘记，记录一下，static变量的设置使用。

class Test
{
public:
	Test() = default;
	~Test() = default;

public:
	int GetNum() { return m_num; };
	void SetNum(int num) { m_num = num; };
private:
	static int m_num;
};

int Test::m_num = 10;

int main()
{
	Test t;
	std::cout << t.GetNum() << std::endl;
	t.SetNum(20);

	Test m;
	std::cout << m.GetNum() << std::endl;
	return 0;
}