关于自定义类型转换的问题

[ndjk454164628]

class A
{
public:
	A(){};
	class B
	{
	public:
		B();
		operator int(){ return int(0);}
	};
	B f(){ return B();}
};

int main(void)
{
    A a;
    int i = a.f();
    
    return 0;
}

error LNK2001: unresolved external symbol "public: __thiscall A::B::B(void)" (??0B@A@@QAE@XZ)

不晓得怎么回事？