[cfe-dev] libc++: max_size() of a std::vector

[Mikael Persson]

Hi François,

I think you are absolutely correct. And this should definitely be
addressed. I think the best would be for you to file a bug report on both
libc++ and libstdc++.

Here are a few additional things I would point out:

Note, however, that this only affects vectors with elements of size 1 or
less (like vector of bool), because any other type, of size 2 or above, can
never have a pointer difference that would overflow ptrdiff_t. So, we are
really only talking about char, unsigned char, bool, and empty classes.

I see only one sensible option to address this, which is to restrict
max_size() to output the max positive value of ptrdiff_t. Something like
this:

    size_type max_size() const noexcept {
      return ( PTRDIFF_MAX < /* alloc max-size */ ? PTRDIFF_MAX : /* alloc
max-size */ );
    }

It could also be conditional upon sizeof(T) such that this extra step is
only done for elements of size 1.

The alternative of accounting for overflow on the calculation of size()
would not only have undue overhead on size(), but it would also imply the
same precaution be taken everywhere else (in iterators, in most mutating
member functions, etc..). And I'm also not sure that this would be standard
conforming since the difference_type would no longer be able to represent
all iterator differences.

Cheers,
Mikael.
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