Why doesn't this C++ STL allocator allocate?

I'm trying to write a custom STL allocator that is derived from std::allocator, but somehow all calls to allocate() go to the base class. I have narrowed it down to this code:

template <typename T> class a : public std::allocator<T> {
public:
    T* allocate(size_t n, const void* hint = 0) const {
        cout << "yo!";
        return 0;
    }
};

int main()
{
    vector<int, a<int>> v(1000, 42);
    return 0;
}

I expect "Yo!" to get printed, followed by some horrible error because I don't actually allocate anything. Instead, the program runs fine and prints nothing. What am I doing wrong?

I get the same results in gcc and VS2008.