以下定义错误的是: struct A{A _a}; struct A{A* _a;}; struct A{A& _a;}; struct B;struct A{B& _b;};struct B{A& _a;};
struct A{A _a};
struct A{A* _a;};
struct A{A& _a;};
struct B;struct A{B& _b;};struct B{A& _a;};