In C++, local instances (these not allocated by hand) have their destructors run as soon as they go out of scope. This behavior is explored in a few different schemes, besides its obvious intent of destructing the instance. The most known one is the auto_ptr class, part of the STL. It is merely a memory [...]

delete p is a two-step process: it calls the destructor, then releases the memory. The code generated for delete p is functionally similar to this (assuming p is of type Fred*):
// Original code: delete p;
if (p != NULL) {
p->~Fred();
operator delete(p);
}
The statement p->~Fred() calls the destructor for the Fred object pointed to by p.
The statement operator delete(p) calls the memory deallocation primitive, void operator [...]

It is perfectly legal to use

malloc() and delete in the same program, or to use new and free() in the same program. But it is illegal, immoral, and despicable to call free() with a pointer allocated via new, or to call delete on a pointer allocated via malloc().
Constructors/destructors: are aware of constructors and destructors, [...]