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

Question

According to the draft standard (23.3.6.4 vector data), data() points to the underlying array and [data(), data() + size()) must be a valid range:

T* data() noexcept;
const T* data() const noexcept;

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

But what if the vector is empty? When I construct a zero-size vector:

#include <vector>
#include <iostream>

int main() {
    const int NUM = 0*10;
    std::vector< double > v( NUM, 0.0 );
    std::cerr << "V : "<< v.data() << std::endl;
}

MSVC 2010 returns null, but on Linux (with GCC 4.2.1 and Intel 12.1) I get a non-null address.

Is vector::data() allowed to or should it return null? Could an implementation, for example, do a default-size initial allocation and return a (non-null) pointer to it?

Edit: Several answers focus on the validity of an empty-range. I fully agree there.

I would really like to see a good reference or explanation for: Is it allowed to, must it return null or may it also return non-null?

Answer 1

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.