Undefined Reference to ‘vtable for XXX' / ‘typeinfo for XXX'

This page answers questions like these:

• Why am I getting “undefined reference to ‘vtable for XXX'” error?
  
• Why am I getting “undefined reference to ‘typeinfo for XXX'” error?
  
• Why doesn’t my code compile?
  
• Why doesn’t my code link?
  

undefined reference to ‘typeinfo for XXX' / ‘vtable for XXX'

• A virtual function must either (1) be defined (even if empty), or (2) be declared a pure virtual function.
• virtual void func() {}
• virtual void func() = 0;