Re: "A diagram of C23 basic types"

On 03/04/2025 10:45, Muttley@DastardlyHQ.org wrote:

On Wed, 02 Apr 2025 16:16:27 GMT
scott@slp53.sl.home (Scott Lurndal) wibbled:

Muttley@DastardlyHQ.org writes:

On Wed, 2 Apr 2025 16:59:45 +0200
David Brown <david.brown@hesbynett.no> wibbled:

On 02/04/2025 16:05, Muttley@DastardlyHQ.org wrote:

ist first.

>

18. "unreachable()" is now standard.

>
Googled it - don't see the point.

>
That's a defect in your understanding, not a defect in the standard.
>
I've found  the gcc equivelent useful often in standalone
applications (OS, Hypervisor, standalone utilities, etc).

 Enlighten me then.
 

I can't tell you what Scott uses it for, but I have used gcc's __builtin_unreachable() a fair number of times in my coding.  I use it to inform both the compiler and human readers that a path is unreachable:
switch (x) {
case 1 : ...
case 2 : ...
case 3 : ...
default : __builtin_unreachable();
}
I can also use it to inform the compiler about data :
if ((x < 0) || (x > 10)) __builtin_unreachable();
// x must be 1 .. 10
Mostly I have it wrapped in macros that let me conveniently have run-time checking during testing or debugging, and extra efficiency in the code when I am confident it is bug-free.
Good use of __builtin_unreachable() can result in smaller and faster code, and possibly improved static error checking.  It is related to the C++23 "assume" attribute (which is also available as a gcc extension in any C and C++ version).