Re: int a = a (Was: Bart's Language)

On 20/03/2025 12:59, bart wrote:

On 20/03/2025 08:39, David Brown wrote:

On 19/03/2025 22:29, Chris M. Thomasson wrote:

>
The scares me a bit:
_____________________
#include <stdio.h>
>
int main() {
>
     int a = 666;
>
     {
         // Humm... Odd to me...
         int a = a;
>
         printf("%d", a);
     }
>
     return 0;
}

>
Yes, that is an unfortunate consequence of the scoping in C - on the line "int a = a;", the new "a" is initialised with its own unspecified value, not with the value of the outer "a".

 So the inner 'a' wasn't supposed to mysteriously inherit the outer a's value? (Perhaps due to sharing the same location.) Since the compilers I tried just showed zero not 666.

No, the inner "a" is completely new and is initialised with itself.  As Keith has pointed out, this is UB - but of course compilers can choose how to implement the code in question.  gcc seems to pick "initialise to 0" as a default when you write "int a;" and then read "a", and does the same for "int a = a;".  Other compilers could choose to do something different, such as pick a register for "a" and leave the value in the register unchanged - in which case it might, by luck, pick up the value of the outer "a".

 

If the scope of the new variable did not start until after the initialisation, then it would have allowed a number of possibilities that would be a lot more useful than the "disable warnings about uninitialised variables" effect or initialisers which refer to their own address.  For example :
>
     int a = 123;
     {
         int a = a;
         // local copy that does not affect the outer one
     ...
>
>
     int a = 123;
     {
         long long int a = a;
         // Local copy that with expanded size
     ...
>
     for (int a = 0; a < 100; a++) {
         const int a = a;
         // "Lock" the loop index to catch errors
>
As it is, you need to do something like :
>
     for (int a = 0; a < 100; a++) {
         const int _a = a;
         const int a = _a;
         // "Lock" the loop index to catch errors

 You mean locking the loop index so it can't be modified?

Yes.