Re: how cast works?

Thiago Adams <thiago.adams@gmail.com> writes:

Em 8/10/2024 9:10 PM, Keith Thompson escreveu:

Thiago Adams <thiago.adams@gmail.com> writes:

Em 8/10/2024 1:14 PM, Bart escreveu:

>
Bart, Does your compiler support the `bool` type, where the value
is always either 1 or 0?

There is a bool type, but it is treated like unsigned char, so is
non-conforming.

>
I do the same in my compiler , when I transpile from C99 to C89.
I was thinking how to make it conforming.
For instance on each write.
>
bool b = 123; -> unsigned char b = !!(123);
>
The problem this does not fix unions, writing on int and reading from char.

I don't think you need to fix that.

>
[....]
>

Summary:
Conversion from any scalar type to _Bool is well defined, and must
yield
0 or 1.

>
>
I will fix in terns of expressions types.
>
 - In this case cast to bool
 - Assignment to bool

You need to cover all cases where a scalar value is converted to _Bool.
That includes (explicit) casts, assignment, initialization, argument
passing, and returning from a _Bool function.

Ideally you'd have one place in your code that handles conversions, but
you'll want to test all those cases and more.

It's possible to force a representation other than 0 or 1 into a _Bool
object, bypassing any value conversion.
Conversion from _Bool to any scalar type is well defined if the
operand is a _Bool object holding a representation of 0 or 1.
Conversion from _Bool to any scalar type for an object holding some
representation other than 0 or 1 either yields 0 or 1 (depending
on the low-order bit) or has undefined behavior.

>
I did a sample now..
>
#include <stdio.h>
>
int main() {
    union {
        int i;
        _Bool b;
    } data;
    data.i = 123;
    printf("%d", data.b);
}
>
it printed 123 not 1.

I get 123 if I compile with gcc, 1 if I compile with clang.
Both results are valid.

So I think the assignment and cast covers all/most cases.
(From some previous tests I thought this was printing 1)
>
The motivation for C89 in cake was not to support old compilers, but
generate code that is compatible with C++98. In this aspect bool was
already there in C++98.(This just gave me idea to add target c++98)

C++ may have different rules, which you can discuss in comp.lang.c++.

>
>
>
>
>

A lot of your articles have multiple blank lines at the end.  Can you
try to avoid that?

--
Keith Thompson (The_Other_Keith) Keith.S.Thompson+u@gmail.com
void Void(void) { Void(); } /* The recursive call of the void */