coredump when calling virtual method of a dynamic allocated object

Question

The snippet below will coredumped in fun() method. Why calling c->b.f() works in main() but failed in the function call?

class A {
 public:
  inline virtual void f() const {printf("A\n");}
};

class B : public A {
 public:
  inline void f() const override {printf("B\n");}
};

class C {
 public:
  B b;
};

void fun(const B &b) {
  b.f();                         // coredump
}

int main() {
  C *c = (C*)malloc(sizeof(C));
  c->b.f();                      // print "B"
  fun(c->b);
  return 0;
}

I guess it's because the virtual table is not initialized properly.

Answer 1

You are allocating memory for class C using malloc, but this doesn't create any object of class C. your program don't have any valid object and have undefined behavior.

As it is undefined behavior, your program may fail or may not be at c->b.f();

If you really want to use malloc, you should use placement new in your program to create object of your class.

C *c = (C*)malloc(sizeof(C));
new (c) C();

And to destroy your object and free the allocated memory, you should do

c->~C();
free(c);