学步园

class A
{
public:
A(int m,int n)// 没有默认构造函数
{
m_m = m;
m_n = n;
}

private:
int m_m;
int m_n;
};

class B
{
public:
B():a(1,2)
{

}

private:
A a;// 只能在初始化列表里初始化，调用方法为a(1,2)
};