使用 nullptr 作为 std::unordered_map 的键有什么后果吗？

Question

Are there any side effects / problems that could arise in doing someMap.insert(make_pair(nullptr, nullptr)) ? Not including the dangers of dereferencing the null pointer.

Answer 1

There is no violations in someMap.insert(make_pair(nullptr, nullptr)) . But you need to pay attention when use this element to not dereference the nullptr or your program will have undefined behaviour (and may trigger a Segmentation Fault).

Note: Obviously the map need to have a pointer in the key and a pointer in the value, or the expression can cause a compile error or in the worst case an Undefined Behaviour.

Answer 2

This is fine.

A null pointer is a valid value for a pointer to hold. You can do anything with it that you can do with any other valid pointer, (except dereference it).

Do make sure that you pointers are valid. If they are not, merely evaluating them has undefined behaviour ( implementation-defined behaviour since C++17 ). But evaluating a null pointer is fine because it's not "invalid", just undereferencable.

Answer 3

The validity of this is entirely dependent on what exactly someMap is.

Let's assume someMap has a key-type which is an actual pointer (that is, not a pointer-like type, but a T* of some kind).

Then the question becomes what the validity is of comparing two pointers if one of them is nullptr . Which means asking... how does the map compare its key type?

std::map 's key comparison is configurable. By default, it uses std::less<Key> .

This is important because normal ordering comparison of pointers using < or whatever only imposes a partial ordering between pointer values . Pointers within the same array can be compared, as well as pointers to subobjects within the same object (ordered by declaration, for standard-layout types). Therefore, order-comparisons between a null pointer and any pointer value other than another null pointer is not permitted.

Fortunately for you, std::less has a specialization for pointers which imposes a total order to all pointer values . by contrast defines a total order for all pointers.

However, if you have provided a different comparison type, one which is based on invoking < rather than std::less , then you cannot insert nullptr into such a map .

If the key-type is some kind of pointer-like type, one which does comparisons based on the value of the pointer, then again, it had better be using std::less instead of < , or you're in trouble.