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From: David Brown <david.brown@hesbynett.no>
Newsgroups: comp.lang.c
Subject: Re: C23 auto x C++ auto.
Date: Sun, 26 May 2024 18:10:32 +0200
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On 26/05/2024 16:22, Bonita Montero wrote:
> Am 26.05.2024 um 15:49 schrieb Thiago Adams:
>> I think most people is not aware of this:
>>
>>  From 3096 C23 draft
>>
>> "
>> 6.7.9 Type inference
>> ...
>>   2 For such a declaration that is the definition of an object the 
>> init-    declarator shall have the form
>>
>>     direct-declarator = assignment-expression
>> "
>>
>> Basically "direct-declarator" differs from "declarator" because it 
>> does not contains pointer.
>>
>> Then the type inference using auto and pointer is something undefined 
>> in C23.
>>
>> struct node{
>>      struct node * next;
>>
>> };
>> int main(){
>>     struct node node = {};
>>     auto * p = node.next;
>> }
>>
>> <source>:7:4: error: 'auto' requires a plain identifier, possibly with 
>> attributes, as declarator
>>      7 |    auto * p = node.next;
>>        |    ^~~~
>>
>> This differs from C++.
>>
>>
> 
> I don't know what type inference in C is good for since the type names
> in C are usually short. If I have short typenames in C++ I don't use
> type inference. Type-inference makes sense to make such things shorter
>      typename map<string, string>::const_iterator it = map.cbegin();
> This doesn't happen in C.
> 

"typeof" and "auto" have been available forever as gcc extensions (where 
"auto" was spelt "__auto_type", since of course "auto" had another 
meaning in C until C23).  One use-case for C is in macros that handle 
multiple types, but I expect people have done other things with them too.

It would have been nice to see statement expressions included in C23, as 
they have been in gcc for ages:

#define max(a,b) \
   ({ __auto_type _a = (a); \
       __auto_type _b = (b); \
     _a > _b ? _a : _b; })


In general, it's just another tool that could be useful in writing code 
that's a bit more flexible.