Pointers in c++ after delete

Question

After reading many posts about this, I want to clarify the next point:

A* a = new A();
A* b = a;

delete a;

A* c = a; //illegal - I know it (in c++ 11)
A* d = b; //I suppose it's legal, is it true?

So the question is about using the value of copy of deleted pointer.

I've read, that in c++ 11 reading the value of a leads to undefined behaviour - but what about reading the value of b?

Trying to read the value of the pointer (note: this is different to dereferencing it) causes implementation-defined behaviour since C++14, which may include generating a runtime fault. (In C++11 it was undefined behaviour) What happens to the pointer itself after delete?

Answer 1

Both:

A* c = a;
A* d = b;

are undefined in C++11 and implementation defined in C++14. This is because a and b are both "invalid pointer values" (as they point to deallocated storage space), and "using an invalid pointer value" is either undefined or implementation defined, depending on the C++ version. ("Using" includes "copying the value of").

The relevant section ([basic.stc.dynamic.deallocation]/4) in C++11 reads (emphasis added):

If the argument given to a deallocation function in the standard library is a pointer that is not the null pointer value (4.10), the deallocation function shall deallocate the storage referenced by the pointer, rendering invalid all pointers referring to any part of the deallocated storage. The effect of using an invalid pointer value (including passing it to a deallocation function) is undefined.

with a non-normative note stating:

On some implementations, it causes a system-generated runtime

In C++14 the same section reads:

If the argument given to a deallocation function in the standard library is a pointer that is not the null pointer value (4.10), the deallocation function shall deallocate the storage referenced by the pointer, rendering invalid all pointers referring to any part of the deallocated storage. Indirection through an invalid pointer value and passing an invalid pointer value to a deallocation function have undefined behavior. Any other use of an invalid pointer value has implementation-defined behavior.

with a non-normative note stating:

Some implementations might define that copying an invalid pointer value causes a system-generated runtime fault