类组合涉及初始化问题

下面代码中，类B里组合了类A，运行时会报错，

class A
{
public:
    A(int _a)
    {
        a = _a;
        cout << "构造函数" << "a" << a << endl;
    }
        ~A()
    {
        cout << "析构函数" << "a" << a << endl;
    }
protected:
private:
    int a;
};
class B
{
public:
protected:
private:
    int b1;
    int b2;
    A a2;
    A a1;
}; 
void obj10play()
{
    A a1(10);
    B ojbB;
    return ;
}
void main()
{
    obj10play();
    system("pause");
} 

出错根源在于给类B分配内存时，因其含有类A作为成员变量一部分，