Re: Suggested method for returning a string from a C program?

On 22/03/2025 04:50, Keith Thompson wrote:

bart <bc@freeuk.com> writes:

This is a C program using one of the extensions from my old compiler:
>
   #include <stdio.h>
   #include <time.h>
>
   int main(void) {
       time_t t = clock();
       printf("%v\n", t);             # (used to be '?'; changed to 'v')
   }
>
The compiler replaces the 'v' format with a conventional format code
according to the type of the expression. For my 'time_t', it happens
to be 'lld'.

 That's nice.  Seriously, it's nice.  If it were added to a future
edition of the language, I'd likely use it (once I could count on it
being supported, which would take a while).
 

I am sure it could be done using variadic macros, _Generic, and newer C23 features like "typeof".  But it would be difficult to handle non-constant format strings, and may be difficult to do efficiently. (C++ now has something like that in the modern "format" library.  Of course, it has the added power and complexity of handling user-defined types too.)
It is certainly quite possible (in C11) to put together a macro so that you can do :
int x = ... ;
const char *s = ...
double d = ...;
print("The result of ", s, " is ", x, " and ", d, "\r\n);
I suspect the biggest challenge to getting that into the standard library would not be technical implementation in C11 - it would be getting C programmers to agree that it is a good idea!
It would certainly be nice to have a less cumbersome way to specify formats in C - especially for the size-specific integer types.  But I think it is likely that to do it /well/, you need a lot more apparatus than C has at the moment, or is likely to gain in the future.

It would be very nice if C had some kind of more generic I/O that
doesn't require remembering arbitrary format strings and qualifiers
for each type, and that doesn't provide format strings for a lot of
types in the standard library, and certainly not for types defined
in user code.  And I'd *love* it if a future C standard removed
the undefined behavior for numeric input using *scanf().  Other C
programmers will have different priorities than yours or mine.
If I want to print a time_t value in C++, I just write
`std::cout << t` and the compiler figures out which overloaded
function to call.

>
That's amazing.

 Not particularly.  C has programmer-defined operator (and function)

(You meant "C++" here?)

overloading as a language feature.  (There are IMHO some serious
flaws in C++'s use of overloaded "<<" for output, but I won't go
into that here.)
 [...]