Delete objects of incomplete type
-
29-09-2019 - |
문제
This one made me think:
class X;
void foo(X* p)
{
delete p;
}
How can we possibly delete p
if we do not even know whether X
has visible destructor? g++ 4.5.1 gives three warnings:
warning: possible problem detected in invocation of delete operator: warning: 'p' has incomplete type warning: forward declaration of 'struct X'
And then it says:
note: neither the destructor nor the class-specific operator delete will be called, even if they are declared when the class is defined.
Wow... are compilers required to diagnose this situation like g++ does? Or is it undefined behavior?
해결책
From the standard [expr.delete]:
If the object being deleted has incomplete class type at the point of deletion and the complete class has a non-trivial destructor or a deallocation function, the behavior is undefined.
So, it's UB if there's nontrivial stuff to do, and it's ok if there isn't. Warnings aren't neccessary for UB.
다른 팁
It is undefined behavior.
However, you can make the compiler check for incomplete types, like boost:
// verify that types are complete for increased safety
template<class T> inline void checked_delete(T * x)
{
// intentionally complex - simplification causes regressions
typedef char type_must_be_complete[ sizeof(T)? 1: -1 ];
(void) sizeof(type_must_be_complete);
delete x;
}
Applying sizeof
to an incomplete type should trigger an error, and I suppose if that passes with some compiler, then an array of negative size would trigger an error.
It is undefined behaviour, and a common gotcha when implementing the pImpl pattern. To the best of my knowledge, there is simply no such thing as a warning that the compiler is required to emit. Warnings are elective; they're there because the compiler writer thought they would be useful.