Is it still safe to delete nullptr in c++0x?

c++ null language-lawyer delete-operator
Mankarse picture Mankarse · Jul 18, 2011 · Viewed 56.5k times · Source

In c++03 it is pretty clear that deleting a null pointer has no effect. Indeed, it is explicitly stated in §5.3.5/2 that:

In either alternative, if the value of the operand of delete is the null pointer the operation has no effect.

However, in the current draft for c++0x this sentence seems to be missing. In the rest of the draft I could only find sentences stating what happens if the operand of the delete-expression is not the null pointer constant. Is deleting the null pointer still defined in c++0x, and if so, where?

Notes:

There is significant circumstantial evidence to suggest that it is still well defined.

First, there are the two sentences in §5.3.5/2 stating that

In the first alternative (delete object), the value of the operand of delete may be a null pointer value, ...

and

In the second alternative (delete array), the value of the operand of delete may be a null pointer value or ...

These say that the operand is allowed to be null, but on their own do not actually define what happens if it is.

Second, changing the meaning of delete 0 is a major breaking change, and the standards committee would be very unlikely make this particular change. Furthermore there is no mention of this being a breaking change in the Compatibility Annex (Annex C) of the c++0x draft. Annex C is however an Informative section, so this has no bearing no the interpretation of the standard.

On the other hand, the fact that deleting the null pointer is required to have no effect implies an additional run-time check. In a lot of code the operand can never be null, so this runtime check is in conflict with the zero overhead principle. Maybe the committee just decided to change the behaviour to bring standard c++ more in line with the stated design goals of the language.

Answer

interjay picture interjay · Jul 18, 2011

5.3.5/7 says:

If the value of the operand of the delete-expression is not a null pointer value, the delete-expression will call a deallocation function (3.7.4.2). Otherwise, it is unspecified whether the deallocation function will be called.

And 3.7.4.2/3 says:

The value of the first argument supplied to a deallocation function may be a null pointer value; if so, and if the deallocation function is one supplied in the standard library, the call has no effect.

So the behavior is well defined, as long as the standard deallocation function is used, or a user-provided deallocation function handles null pointers correctly.

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