Re: C23 auto x C++ auto.

Em 5/26/2024 1:10 PM, David Brown escreveu:

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.
 

I am trying to remember the situation where typeof cannot be used, justifying the existence of auto other than "easy to read".
  #define max(a,b) \
     ({ typeof(a) _a = (a); \
         typeof(a) _b = (b); \
       _a > _b ? _a : _b; })
I think for function calls typeof can be a little confusing because the arguments.
typeof(f(arg1, arg2)) r = f(arg1, arg2);
auto r = f(arg1, arg2);