Question

I have a class A, that is an abstract base class.(C++). Now, I have two classes B and C which inherit from A;

I have a virtual destructor in A;
The constructor in class A is protected.
Now, in the constructors of B and C, I have included a call to A’s constructor.

B::B():A()
{
//do something
}

similarly for C

C::C():A()
{
//do something
}

Now, while compiling I’m getting linking errors.

    B.obj : error LNK2019: unresolved external symbol "protected: __
thiscall A::A(void)" (??0A) referenced in function "protected: __thiscall B::B(void)" (??0B)

    C.obj : error LNK2001: unresolved external symbol "protected:
__thiscall A::A(void)" (??0A@XZ)
      Error.

Please suggest how to resolve this.

Thanks,
Karhtik.

Answer 1

Firstly, there’s really no need to “call” the base class constructor explicitly. The default constructor of the base class will be called for you automatically.

Secondly, as @DeadMG already noted, the error you are getting suggests that you explicitly declared the A::A() constructor, but forgot to define it.