• Cecil Ward (6/6) Jun 24 2023 I would like to use scope guards but in the guard I need to get
• Paul Backus (15/21) Jun 26 2023 Scope guards are syntax sugar for try/catch/finally. For example,
• Cecil Ward (23/44) Jun 26 2023 Sorry for being stupid, but say you have something like this
• Steven Schveighoffer (5/29) Jun 26 2023 That will print the final value of `p`. `p` lives at the same address

Cecil Ward <cecil cecilward.com> writes:

I would like to use scope guards but in the guard I need to get 
access to some local variables at the end of the routine. This 
doesn’t really seem to make sense as to how it would work, 
because their values depend on the exact point where the scope 
guard is called at in the last, exiting line(s) of the routine. 
Am I misunderstanding?

Paul Backus <snarwin gmail.com> writes:

On Saturday, 24 June 2023 at 17:00:36 UTC, Cecil Ward wrote:
 I would like to use scope guards but in the guard I need to get 
 access to some local variables at the end of the routine. This 
 doesn’t really seem to make sense as to how it would work, 
 because their values depend on the exact point where the scope 
 guard is called at in the last, exiting line(s) of the routine. 
 Am I misunderstanding?

Scope guards are syntax sugar for try/catch/finally. For example, 
when you write

     auto f = open("foo");
     scope(exit) close(f);
     doSomethingWith(f);

...it gets transformed by the compiler into

     auto f = open("foo");
     try {
         doSomethingWith(f);
     } finally {
         close(f);
     }

I do not understand exactly what the problem is you are having, 
but hopefully this lets you figure out how to solve it.

Cecil Ward <cecil cecilward.com> writes:

On Monday, 26 June 2023 at 17:41:16 UTC, Paul Backus wrote:
 On Saturday, 24 June 2023 at 17:00:36 UTC, Cecil Ward wrote:
 I would like to use scope guards but in the guard I need to 
 get access to some local variables at the end of the routine. 
 This doesn’t really seem to make sense as to how it would 
 work, because their values depend on the exact point where the 
 scope guard is called at in the last, exiting line(s) of the 
 routine. Am I misunderstanding?

 Scope guards are syntax sugar for try/catch/finally. For 
 example, when you write

     auto f = open("foo");
     scope(exit) close(f);
     doSomethingWith(f);

 ...it gets transformed by the compiler into

     auto f = open("foo");
     try {
         doSomethingWith(f);
     } finally {
         close(f);
     }

 I do not understand exactly what the problem is you are having, 
 but hopefully this lets you figure out how to solve it.

Sorry for being stupid, but say you have something like this

==
{
size_t p = offset;
++p;
scope(exit) { writeOutput( 0, p );

++p
…
++p;

return;
}

==

The correctness of its behaviour depends on what the value of p 
is when it calls writeOutput(), and the value of p is being 
changed. To be correct, the final value of p needs to be passed 
to writeOutput( p ). That was what I was worrying about. I could 
have course introduce another variable to capture this final 
value and use that in the scope guard, but then I can’t make the 
scope guard general if I have more than one exit route. The 
compiler sounded as if it did not like the local variable p in 
the scope guard, but I need to try it again to get the error 
message.

Steven Schveighoffer <schveiguy gmail.com> writes:

On 6/26/23 2:42 PM, Cecil Ward wrote:

 ==
 {
 size_t p = offset;
 ++p;
 scope(exit) { writeOutput( 0, p );
 
 ++p
 …
 ++p;
 
 return;
 }
 
 ==
 
 The correctness of its behaviour depends on what the value of p is when 
 it calls writeOutput(), and the value of p is being changed. To be 
 correct, the final value of p needs to be passed to writeOutput( p ). 
 That was what I was worrying about. I could have course introduce 
 another variable to capture this final value and use that in the scope 
 guard, but then I can’t make the scope guard general if I have more than 
 one exit route. The compiler sounded as if it did not like the local 
 variable p in the scope guard, but I need to try it again to get the 
 error message.

That will print the final value of `p`. `p` lives at the same address 
for the entire function, and the scope exit prints the value from that 
address.

-Steve