foo.cc: In function 'bool comp(const vector3&, const vector3&)':
foo.cc:34:21: error: passing 'const vector3' as 'this' argument discards qualifiers [-fpermissive]
  return a.getlength()>b.getlength();
                     ^
foo.cc:22:6: note:   in call to 'int vector3::getlength()'
  int getlength()
      ^~~~~~~~~
foo.cc:34:35: error: passing 'const vector3' as 'this' argument discards qualifiers [-fpermissive]
  return a.getlength()>b.getlength();
                                   ^
foo.cc:22:6: note:   in call to 'int vector3::getlength()'
  int getlength()
      ^~~~~~~~~

自豪的采用jd5进行评测(github.com/masnn/jd5)