One possible reason is because you are declaring a virtual function without defining it.
When you declare it without defining it in the same compilation unit, you’re indicating that it’s defined somewhere else – this means the linker phase will try to find it in one of the other compilation units (or libraries).
An example of defining the virtual function is:
virtual void fn() { /* insert code here */ }
In this case, you are attaching a definition to the declaration, which means the linker doesn’t need to resolve it later.
The line
virtual void fn();
declares fn() without defining it and will cause the error message you asked about.
It’s very similar to the code:
extern int i;
int *pi = &i;
which states that the integer i is declared in another compilation unit which must be resolved at link time (otherwise pi can’t be set to it’s address).