undefined reference to typeinfo - C++ error message

There are some compiler and loader error messages that shout obviously as to their cause, but there are others that simply don't give the new user much of an indication as to what's really wrong. And most of those I get to know pretty quickly, so that I can whip around a room during a course, making suggestions to delegate to check for missing ; characters or double quotes, to check that they have used the right type of brackets for a list subscript and haven't unintentionally written a function call, etc.

Here's one of the more obscure messages from the Gnu C++ compiler - or rather from the loader:

g++ -o polygon shape.o circle.o square.o polygon.o

circle.o(.gnu.linkonce.r._ZTI6Circle+0x8): undefined reference to `typeinfo for Shape'

square.o(.gnu.linkonce.r._ZTI6Square+0x8): undefined reference to `typeinfo for Shape'

polygon.o(.gnu.linkonce.t._ZN5ShapeC2Ev+0x8): In function `Shape::Shape()':

: undefined reference to `vtable for Shape'

collect2: ld returned 1 exit status

And you can be scratching you head for hour over that one!

The error? shape.o contains a base class from which classes are derived in circle.o and square.o .. but virtual function(s) in shape's definition are missing null bodies.

The fix? You've got line(s) like

virtual float getarea() ;

that should read

virtual float getarea() {}

这个错误解决了