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STD-DISCUSSION |
Subject: Accessing an object with a char pointer.
From: yo mizu (mizu2594_at_[hidden])
Date: 2020-05-13 09:03:01
http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2019/p1839r2.pdf
According to P1839R2, the following code has undefined behavior as per
[expr.pre] p4.
int a = 420;
char b = *reinterpret_cast<char*>(&a);
P1839R2 states, "When the lvalue-to-rvalue conversion is applied to
the initializer expression of b, the behavior is undefined as per
[expr.pre] p4 because the result of such a conversion would be the
value of the int object (420), which is not a value representable by
char."
So is the behavior of the other example below defined?
int c = 4;
char d = *reinterpret_cast<char*>(&c);
The result of the lvalue-to-rvalue conversion would be the value of
the int object (4).
It is not an object of char, but the value of 4 can be represented by char.
If this behavior is defined, and the value of "d" is 4, I think it is
at odds with the behavior of many existing compilers for big endian
architectures.
And how about the following example.
struct E {} e {};
char f = *reinterpret_cast<char*>(&e);
Is the behavior of this example defined?
If this is undefined behavior, what description in the standard is it based on?
-- Thank you and best regards, Yo Mizu
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