Setting pointer to NULL before delete

Question

So I stumbled upon this bit of code and while I've been coding in C/C++ for about 5 years now, I cannot fathom why anybody would want to do this. I understand why you'd want to set the pointer to NULL after deallocating memory, but I certainly don't understand why someone would want to do the opposite (looks like a memory leak to me).

Second, I'm pretty sure it's not necessary to check if the pointer is NULL before setting it to NULL and deleting it, as discussed here.

if( m_pio )
{
    m_pio = NULL;
    delete m_pio;
}

Answer 1

Double deleting an object can result in the destructor being called twice, so some people favor setting it to NULL after deletion. This prevents the destructor from being called on memory which could have been reallocated from the pointer's previous memory address.

As shown here though, setting to NULL before deleting is a memory leak. Fixed code without memory leak:

if( m_pio ) {
    delete m_pio;
    m_pio = NULL;
}

Note that calling delete on NULL (or nullptr in C++11), is legal so you code could just be written as this instead:

delete m_pio;
m_pio = NULL