学步园

the   following   may   help   determine   wheter   an   object   is   a   lvalue   or   rvalue:   if   the   object   obtained   from   an   expression   has   a   name,   then   it   must   be   lvalue;   otherwise   it   is   rvalue.   this   means   all
  temporary   objects   must   be   rvalues.   example: 

++a;       //   the   result   is   'a ',   which   has   a   name   --   'a ',   so   it   is   lvalue. 
a++;       //   the   result   is   a   temporary   object   --   rvalue. 
a+5;       //   again,   a   temporary   object   --   rvalue. 
a=3;       //   the   result   is   'a ',   which   has   a   name,   lvalue.

An   lvalue   is   an   expression   denoting   a   nonconstant   object.   
                                                                                                                      -TCPL