Is there still a reason to use `int` in C++ code? [duplicate]

There was a discussion on the C++ Core Guidelines what to use:

https://github.com/isocpp/CppCoreGuidelines/pull/1115

Herb Sutter wrote that gsl::index will be added (in the future maybe std::index), which will be defined as ptrdiff_t.

hsutter commented on 26 Dec 2017 •

(Thanks to many WG21 experts for their comments and feedback into this
note.)

Add the following typedef to GSL

namespace gsl { using index = ptrdiff_t; }

and recommend gsl::index for all container indexes/subscripts/sizes.

Rationale

The Guidelines recommend using a signed type for subscripts/indices.
See ES.100 through ES.107. C++ already uses signed integers for array
subscripts.

We want to be able to teach people to write “new clean modern code”
that is simple, natural, warning-free at high warning levels, and
doesn’t make us write a “pitfall” footnote about simple code.

If we don’t have a short adoptable word like index that is competitive
with int and auto, people will still use int and auto and get their
bugs. For example, they will write for(int i=0; i<v.size(); ++i) or
for(auto i=0; i<v.size(); ++i) which have 32-bit size bugs on widely
used platforms, and for(auto i=v.size()-1; i>=0; ++i) which just
doesn’t work. I don’t think we can teach for(ptrdiff_t i = ... with a
straight face, or that people would accept it.

If we had a saturating arithmetic type, we might use that. Otherwise,
the best option is ptrdiff_t which has nearly all the advantages of a
saturating arithmetic unsigned type, except only that ptrdiff_t still
makes the pervasive loop style for(ptrdiff_t i=0; i<v.size(); ++i)
emit signed/unsigned mismatches on i<v.size() (and similarly for
i!=v.size()) for today’s STL containers. (If a future STL changes its
size_type to be signed, even this last drawback goes away.)

However, it would be hopeless (and embarrassing) to try to teach
people to routinely write for (ptrdiff_t i = ... ; ... ; ...). (Even
the Guidelines currently use it in only one place, and that’s a “bad”
example that is unrelated to indexing`.)

Therefore we should provide gsl::index (which can later be proposed
for consideration as std::index) as a typedef for ptrdiff_t, so we can
hopefully (and not embarrassingly) teach people to routinely write for
(index i = ... ; ... ; ...).

Why not just tell people to write ptrdiff_t? Because we believe it
would be embarrassing to tell people that’s what you have to do in
C++, and even if we did people won’t do it. Writing ptrdiff_t is too
ugly and unadoptable compared to auto and int. The point of adding the
name index is to make it as easy and attractive as possible to use a
correctly sized signed type.

Edit: More rationale from Herb Sutter

Is ptrdiff_t big enough? Yes. Standard containers are already required
to have no more elements than can be represented by ptrdiff_t, because
subtracting two iterators must fit in a difference_type.

But is ptrdiff_t really big enough, if I have a built-in array of char
or byte that is bigger than half the size of the memory address space
and so has more elements than can be represented in a ptrdiff_t? Yes.
C++ already uses signed integers for array subscripts. So use index as
the default option for the vast majority of uses including all
built-in arrays. (If you do encounter the extremely rare case of an
array, or array-like type, that is bigger than half the address space
and whose elements are sizeof(1), and you’re careful about avoiding
truncation issues, go ahead and use a size_t for indexes into that
very special container only. Such beasts are very rare in practice,
and when they do arise often won’t be indexed directly by user code.
For example, they typically arise in a memory manager that takes over
system allocation and parcels out individual smaller allocations that
its users use, or in an MPEG or similar which provides its own
interface; in both cases the size_t should only be needed internally
within the memory manager or the MPEG class implementation.)