CODEWITHSUNDEEP
c++language-lawyer
Baum mit Augen
Baum mit Augen

Reputation: 50061

Meaning of valid pointer

In the comments to this answer I stumbled about a discussion about the meaning of "valid pointer". Since I think that that is interesting in general:

What is a "valid pointer" in C++?

In particular:

Is reinterpret_cast<const void*>(0x1) a valid pointer?

Upvotes: 7

Views: 853

Answers (1)

Ben Voigt
Ben Voigt

Reputation: 283733

The Standard places implementations into two general categories:

  • Those with strict pointer safety
  • Those with relaxed pointer safety

Your expression definitely is not a safely-derived pointer, so it's invalid in the first.

Quote from 3.7.4.3:

An implementation may have relaxed pointer safety, in which case the validity of a pointer value does not depend on whether it is a safely-derived pointer value. Alternatively, an implementation may have strict pointer safety, in which case a pointer value referring to an object with dynamic storage duration that is not a safely-derived pointer value is an invalid pointer value unless the referenced complete object has previously been declared reachable (20.7.4). [ Note: the effect of using an invalid pointer value (including passing it to a deallocation function) is undefined, see 3.7.4.2. This is true even if the unsafely-derived pointer value might compare equal to some safely-derived pointer value. — end note ] It is implementation defined whether an implementation has relaxed or strict pointer safety.

For implementations with relaxed safety, it doesn't matter how the pointer value is gotten, just that (3.9.2):

A valid value of an object pointer type represents either the address of a byte in memory (1.7) or a null pointer.

Is 0x1 a valid memory address on your system? Well, for some embedded systems it is. For most OSes using virtual memory, the page beginning at zero is reserved as invalid.

Upvotes: 10

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