delete expression

Question

Reference here

That destructor will also implicitly call the destructor of the auto_ptr object. And that will delete the pointer it holds, that points to the C object - without knowing the definition of C! That appeared in the .cpp file where struct A's constructor is defined.

This was curious and then

5.3.5/5 states - "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."

My question is that why isn't such a program which attempts to delete a pointer to an incomplete type treated as ill-formed? Why is it pushed into the realm of conditional (and the complete class has a non-trivial destructor..) 'undefined behavior'?

What does the 'and' imply?

EDIT 2:

Is the code below well-formed? VS and Gcc/CLang compile, but Comeau gives a warning. I guess all this is part of the undefined behavior mentioned in the Standard. My question is 'why is this not ill-formed but is undefined'?

#include <iostream>
#include <memory>
using namespace std;

struct C;
                        // Is this the POI for auto_ptr<C>? $14.6.4.1/3
struct A{
    A();
    auto_ptr<C> mc;
    ~A(){}             // how does it link to C::~C at this point?
};

struct C{};

A::A():mc(new C){}

int main(){
    A a;
}

Answer 1

As I'm writing this your text says "Reference [here][1]" with no reference.

But essentially, the standard allows you to delete a pointer to incomplete type so that you can leverage knowledge that the compiler doesn't have, namely that the type's destructor does nothing.

std::auto_ptr is an example where this is a problem, especially for the PIMPL idiom (an infamous example of getting it wrong was Herb Sutter's GOTW on PIMPL, where he incorrectly used std::auto_ptr). boost::shared_ptr is an example where it isn't a problem (in general). That's because the constructor of boost::shared_ptr stores a deleter function, and the complete type of the pointee must necessarily be known at the point of construction.

Cheers & hth.,