What Happens to Pointers After Deleting the Memory They Point To in C ?

Pointers Behavior in C after Delete

In C , deleting a pointer deallocates the memory it points to. However, the behavior of pointers pointing to that deleted memory becomes undefined or implementation-defined, depending on the C version.

Consider the following code:

<code class="cpp">A* a = new A();
A* b = a;

delete a;

A* c = a; // Assuming undefined or implementation-defined in C++11
A* d = b; // Supposed to be legal</code>

Reading Value of Copy of Deleted Pointer (b)

In C 11, reading the value of b after a has been deleted is undefined behavior. However, in C 14, it is implementation-defined. This is because the pointer b itself becomes an "invalid pointer value" after a is deleted.

Implementation-Defined Behavior

In C 14, using an invalid pointer value, including copying it, has implementation-defined behavior. This means the compiler is allowed to decide what to do in such cases. In some implementations, it may generate a runtime fault, while in others, it may allow the operation but result in undefined behavior when the pointer is subsequently used.

Legal or Undefined?

Therefore, both A* c = a; and A* d = b; are undefined in C 11 and implementation-defined in C 14. The copied pointer value (b) is considered an invalid pointer value, and its use is subject to the implementation's definition.

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