但是如何传递匿名变量指针？为什么会编译？

你可能使用的编译器不符合C ++标准。

可以不使用r值（临时）对象的地址。这不应该编译。

但是， operator& 重载，以便它可以在临时对象上调用，例如：

  struct A 
 {
 A * operator&（）{return this; } 
}; 
 $ p> 



在C ++ 11中，临时对象可以是绑定到r值引用。之后，r值引用的行为类似于l值，因此临时对象的地址可以取：
  struct A {}; 

 void foo（A *）; 

 void foo（A&& a）{foo（& a）; } 

 int main（）{
 foo（A {}）; 
} 
 
 
We know that we can pass temporary objects to functions by const reference, like this:
class A
{
public:
    A(int _b = 0) 
    {
        b = _b;
    }

    int b;
};

void foo(A& a) {printf("%d", a.b);}
void cfoo(const A& a) {printf("%d", a.b);}

int main(void)
{
    //foo(A(4)); doesn't compile
    cfoo(A(5));
}
but what about passing by pointer?
why does this compile?
void pfoo(A* pa) {pa->b = 19;}

int main(void)
{
    pfoo(&A(5));
}


  
but what about passing anonymous variables pointer? why does this compile?
You are probably using a compiler that does not honour C++ standard.

No address of an r-value (temporary) object can be taken. That should not compile.

However, operator& can be overloaded, so that it can be invoked on a temporary object, e.g.:
struct A
{
    A* operator&() { return this; }
};


In C++11 a temporary object can be bound to an r-value reference. After that r-value reference behaves like an l-value and hence the address of a temporary object can be taken:
struct A {};

void foo(A*);

void foo(A&& a) { foo(&a); }

int main() {
    foo(A{});
}