Re: question on vector<char>::difference_type

James Kanze <>
Sat, 1 Aug 2009 01:30:24 -0700 (PDT)
On Jul 31, 2:31 pm, "Bo Persson" <> wrote:

James Kanze wrote:

On Jul 31, 2:26 am, Victor Bazarov <> wrote:, India wrote:

I wrote the following program for learning purpose only.
Why is the difference_type not large enough to hold the
distance between any two iterators ?

That's a good question you should most likely be asking the
implementors of the standard library you're using. The
quantities and the types are implementation-defined. Some
implementors *could* decide to use a larger signed type for
'difference_type' just to satisfy that requirement.
Apparently in the particular implementation you use, they
didn't. Why?... How should we know?

Does the implementation actually have a choice? I can't
find any concrete requirement in the C standard saying that
ptrdiff_t must have the same size as size_t, but this would
seem to be the intent. Particularly as the standard says
that subtraction of two pointers is undefined behavior if
the result is not representable in a ptrdiff_t---the
standard explicitly caters to the case in question.

Of course, the code in question dealt with std::vector and a
difference between two iterators. In this case, the C++
standard is somewhat less explicit, but it does say that a
precondition to the expression is "there exists a value n of
Distance such that a + b == b[...]"; in the case in
question, the pre-condition is not met, so the behavior is

Logically, of course, the error is using two types, size_t
and ptrdiff_t, rather than just ptrdiff_t. Still, an
implementation could detect the error (although I suspect
that most consider it too rare to be worth the bother), or
simply refuse to allocate a container so large that the
error could occur, making max_size() equal to:
    min( numeric_limits< ptrdiff_t >::max(),
         numeric_limits< size_t >::max() / sizeof( T ) )

In addition to this, i belive the OP's result is Linux
specific, in that it allows him to actually pretend to
allocate a vector that large.

Not necessarily Linux specific, but it obviously depends on the
OS and the hardware. And I'm not sure what you mean by
"pretend"; if Linux is configured correctly (it usually isn't),
or if there's enough memory and other processes aren't using it
all (usually the case on the machines I use), there's no
problem; the vector is real, and fully usable.

On Windows I would expect either a length_error exception
telling us that we are out of user address space, or (if
allocating a few bytes less) a bad_alloc saying that it isn't
available anyway.

Yes. Depending on the implementation of the library, I'd expect
either length_error or bad_alloc under Windows, because Windows
limits 32 bit programs to half of the address space. But that's
just a particularity of Windows---Linux limits them to 3/4 (I
think), and Solaris allows 100% of the address space, at least
on a Sparc.

That's just another instance of the undefined behavior, of

If you attempt to extend the vector beyond max_size(), the
behavior isn't undefined. Nor if the vector can't allocate the
necessary memory.

James Kanze (GABI Software)
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