Re: [Boost-users] [MPL][MP11] Return value of at_c for out of range index

AMDG On 3/4/19 2:41 PM, Mateusz Loskot via Boost-users wrote:

Hi,

While porting some code from MPL to MP11, the latter helped to find some bugs in the ported code [1]

I looked closer at those and found 'behavioural' differences between MPL and MP11 when accessing sequences with index value that is out of range.

Could anyone help me to understand the following?

1. Why this compiles?

using R = at_c, 100>::type;

Tested in C++11 using GCC, clang, MSVC from VS2017/2019

It compiles because there's no range check. It isn't something that you're supposed to do, however.

2. Why this compiles in GCC and clang, but fails with MSVC?

using V = at_c, 100>::type::type;

It's essentially equivalent to: auto V = std::vector<int>{1, 2, 3}[100]; which has undefined behavior. It would be better if it consistently produced an error, of course, but the short answer is don't do that.

3. For GCC and clang, why this compiles

static_assert(std::is_same>::value, "");

but this fails?

static_assert(std::is_same>::value, "");

Don't vector_c and range_c yield equivalent sequences?

They're only equivalent if you say within bounds.

By the way, MP11's mp_at_c yield void, as expected, there also no ::type or ::value member, so diagnostics is clearer.

Yep. In Christ, Steven Watanabe