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[wg14/wg21 liaison] C trap representations and unspecified values versus C++ indeterminate values

From: Hubert Tong <hubert.reinterpretcast_at_[hidden]>
Date: Fri, 9 Oct 2020 18:04:09 -0400
Older thread appears to end here:
http://open-std.org/JTC1/SC22/WG14/17199, "(SC22WG14.17199) terminology:
indeterminate value"

UB from uninitialized values emanates from indeterminate values in C++ and
trap representations in C.
C further has unspecified values that become unspecified at a specific
point in time but can be copied without mutation of the value. C++ received
a National Body comment for C++14 where such cases were removed:

It is further an issue that the definition of trap representation does not
admit trap representations for the exact-width integer types (int16_t,
etc.) because the lack of padding bits, combined with two's complement
representation, means that every possible object representation represents
a value of the type.

I think it would really help if the C committee could record a decision to
move towards harmonizing with C++ on this.


Hubert Tong

Received on 2020-10-09 17:04:28