What should std::vector::data() return if the vector is empty?

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The convention for ranges is [inclusive, exclusive), that is if you iterate over a range [X,Y) you will conceptually do the following (pseudo-code):

for( iterator ii = X; ii != Y; ++ii) {
...
}

This permits to express an empty range as [X,X). Also this empty range is perfectly well defined for each address, no matter if it is valid or invalid.

That said the requirements for data() are (emphasis mine):

23.3.6.4 [vector.data]

T* data() noexcept;

const T* data() const noexcept;

Returns: A pointer such that [data(),data() + size()) is a valid range. For a
non-empty vector, data() == &front().

It seems to me that the only unconditional guarantee is that [data(),data() + size()) should be a valid range. For size() == 0 the member function data() may return any value and the range will be a valid empty range. Therefore I would say that an implementation is allowed to return a non-null pointer if size() is zero.

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