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From: David Brown <david.brown@hesbynett.no>
Newsgroups: comp.lang.c
Subject: Re: constexpr keyword is unnecessary
Date: Tue, 29 Oct 2024 15:13:16 +0100
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On 29/10/2024 13:21, Thiago Adams wrote:
> On 29/10/2024 09:16, Thiago Adams wrote:
> 
> Edit to fix the sample
> 
> // Here we acknowledge the truncation
> void f(int i) {
>      ...
>      const unsigned char c = i; [[!truncation]];
> }
> 
> 
> // Warning: there is no truncation to acknowledge.
> void f(unsigned char i) {
>      ...
>      const unsigned char c = i; [[!truncation]];
> }
> 

This should give you what you are asking for, if I have understood you 
correctly:

#define truncate(type, value) \
     _Generic((value), \
         type : (void) 0, \
         default : (type) value \
     )



// OK
void f1(int i) {
	const auto c = truncate(unsigned char, i);
}

// Error
void f1(unsigned char i) {
	const auto c = truncate(unsigned char, i);
}


I am not at all convinced this is a good idea.  I am /certainly/ not 
convinced "truncate" is a good name - the general term, AFAIK, for a 
conversion that might try to squeeze a large value into a smaller type 
is "narrowing" rather than "truncating".

You could expand the idea further and have a "truncate" macro that 
checks for bounds at run time or compile time (I'd make use of the gcc 
extension __builtin_constant_p here, but there may be a fully standard 
way to do this).