未定义的vtable C ++

Question

#include <stdio.h>

class A {
public:
  virtual void print();
  A();
};

class B :public A {
public:
  void print();
  B();
};

class C :public B {
public:
  void print();
  C();
};

A::A(){
}

B::B(){
}

C::C(){
}

void B::print() {
  printf("From B\n");
}

void C::print() {
  printf("From C\n");
}

int main() {
  B* object = new C;
  object->print();

  return 0;
}

When I try to compile this C++ file, I get the following error. Can't figure out the reason. I tried reading through similar undefined vtable questions on SO.

/tmp/ccpOkVJb.o: In function `A::A()':
test1.cpp:(.text+0xf): undefined reference to `vtable for A'
/tmp/ccpOkVJb.o:(.rodata._ZTI1B[_ZTI1B]+0x10): undefined reference to `typeinfo for A'
collect2: error: ld returned 1 exit status

Answer 1

If A::print() isn't meant to be implemented, declare it as pure:

class A {
public:
  virtual void print() = 0;
  A();
};

Otherwise, implement it.

Answer 2

You're declaring that there is supposed to be an A::print method implemented (you probably meant to make it pure virtual using = 0 ), but you're not implementing it.

Since the first implemented virtual method makes the compiler instantiate the vtable and you've not implemented any in A, A's vtable is missing, leading to undefined reference to 'vtable for A' .

Answer 3

You need to declare A::print() as pure virtual, or provide an implementation:

class A {
public:
  virtual void print()=0;
  A();
};

or

class A {
public:
  virtual void print() {}
  A();
};