c++ - Delete objects of incomplete type -

this 1 made me think:

class x;  void foo(x* p) {     delete p; } 

how can possibly delete p if not know whether x has visible destructor? g++ 4.5.1 gives 3 warnings:

warning: possible problem detected in invocation of delete operator: warning: 'p' has incomplete type warning: forward declaration of 'struct x' 

and says:

note: neither destructor nor class-specific operator delete called, if declared when class defined.

wow... compilers required diagnose situation g++ does? or undefined behavior?

from standard [expr.delete]:

if object being deleted has incomplete class type @ point of deletion , complete class has non-trivial destructor or deallocation function, behavior undefined.

so, it's ub if there's nontrivial stuff do, , it's ok if there isn't. warnings aren't neccessary ub.