On 16 August 2011 00:30, Jeffrey Walton wrote:
> Hi All,
>
> I'm testing a custom allocator. The allocator works well with vector,
> but fails to link with basic_string (see below).
>
> I looked through Stroustrup and could not find info on _Alloc_hider.
It starts with an underscore followed by an upper case letter, so it's
in the implementation namespace i.e. it's an implementation detail, so
you're not going to find it in a book.
> Searching online doesn't reveal much either (perhaps I have not waded
> through enough irrelevant fodder). I did find [1], buts its just
> DoOxygen with no real content (its written in STL terseness with no
> comments).
>
> I believe I need another constructor, but I'm not sure what it takes
> or how to write it:
>
> inline explicit zallocator() { }
> inline ~zallocator() { }
> inline explicit zallocator(zallocator const&) { }
>
> template<typename U>
> inline explicit zallocator(zallocator<U> const&) { }
>
> How does one one provided an allocator for use by a basic_string?
By writing a type that meets the C++03 Allocator requirements, which
yours doesn't.
If you look at the source you'll see that _Alloc_hider is a type
derived from the string's allocator_type, which is done to benefit
from the Empty Base Optimization. Since it is derived from the
string's allocator type, it is convertible to the allocator type, and
this constructor should work:
template<typename U>
inline explicit zallocator(zallocator<U> const&) { }
The problem is that you've made it explicit, take the explicit keyword
out and it should work.
