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

Question

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.

Answer 1

You will need to provide a rebind member template and the other stuff that is listed in the allocator requirements in the C++ Standard. For example, you need a template copy constructor which accepts not only allocator<T> but also allocator<U>. For example, one code might do, which a std::list for example is likely to do

template<typename Allocator>
void alloc1chunk(Allocator const& alloc) {
    typename Allocator::template rebind<
        wrapper<typename Allocator::value_type>
      >::other ot(alloc);
    // ...
}

The code will fail if there either exist no correct rebind template, or there exist no corresponding copy constructor. You will get nowhere useful with guessing what the requirements are. Sooner or later you will have to do with code that relies on one part of those allocator requirements, and the code will fail because your allocator violates them. I recommend you take a look at them in some working draft your your copy of the Standard in 20.1.5.