eric <cneric12lin0@xxxxxxxxx> writes: > actually that book even did not specially define Superhero::walk(), > that is I add by myself to escape my compile's error > (is that right? or book's is right?) I see you did not define Superhero::eat(), however. You must define all the virtual functions of class Superhero, even if the program never calls them. http://gcc.gnu.org/faq.html#vtables http://www.codesourcery.com/public/cxx-abi/abi.html#vague-vtable and http://gcc.gnu.org/onlinedocs/gcc-4.5.2/gcc/Vague-Linkage.html explain that GCC emits the vtable in the same translation unit as the first non-inline, non-pure virtual function of the class. In class Superhero, that function is Superhero::eat(). Because you did not define Superhero::eat(), GCC did not emit the vtable, and the linker then failed because the vtable was missing. If you add a definition for Superhero::eat(), you get different errors from the linker: /tmp/ccuVi68j.o: In function `main': Example8-11.eat.cpp:(.text+0x58): undefined reference to `Superhero::~Superhero()' /tmp/ccuVi68j.o:(.rodata._ZTV9Superhero[vtable for Superhero]+0x18): undefined reference to `Superhero::sleep()' /tmp/ccuVi68j.o:(.rodata._ZTV9Superhero[vtable for Superhero]+0x28): undefined reference to `Superhero::jump()' /tmp/ccuVi68j.o:(.rodata._ZTV9Superhero[vtable for Superhero]+0x38): undefined reference to `Superhero::up()' /tmp/ccuVi68j.o:(.rodata._ZTV9Superhero[vtable for Superhero]+0x40): undefined reference to `Superhero::down()' /tmp/ccuVi68j.o:(.rodata._ZTV9Superhero[vtable for Superhero]+0x48): undefined reference to `Superhero::~Superhero()' /tmp/ccuVi68j.o:(.rodata._ZTV9Superhero[vtable for Superhero]+0x50): undefined reference to `Superhero::~Superhero()' /tmp/ccuVi68j.o:(.rodata._ZTV9Superhero[vtable for Superhero]+0x70): undefined reference to `non-virtual thunk to Superhero::up()' /tmp/ccuVi68j.o:(.rodata._ZTV9Superhero[vtable for Superhero]+0x78): undefined reference to `non-virtual thunk to Superhero::down()' collect2: ld returned 1 exit status Now it tells you which functions are missing. So, when you get a linker error about an undefined reference to a vtable, do check that you have defined all the declared functions in that class.
