comma before ?:

[Fraser Ross]

int
a= 4,
c= 0,
x= 100,
y= 200;
c ? ++a, x : y ; // comma before ?:

Should a be 5 at the end? If this is not an example of comma precedence
over ?: can someone give one?

Fraser.

 

[Ron Natalie]

int
a= 4,
c= 0,
x= 100,
y= 200;
c ? ++a, x : y ; // comma before ?:

Should a be 5 at the end? If this is not an example of comma precedence
over ?: can someone give one?

First off, anybody who uses ?: and discards the value should
have their fingers broken. That's what if/else is for.
C++ parsing is not strictly determined by operator precedence,
the ? operator is a pretty good examle of that.

The question isn't one of precedence anyhow. Precedence doesn't
change the order and when things are executed, just how they are
parsed. If c is false, the stuff before the colon is NOT evaluated
and no side-effects occur.

 

[Victor Bazarov]

int
a= 4,
c= 0,
x= 100,
y= 200;
c ? ++a, x : y ; // comma before ?:

Should a be 5 at the end?
No.

If this is not an example of comma
precedence over ?: can someone give one?

It *is* an example of comma precedence over ?:. The last line
is interpreted as if it were written

c ? (++a, x) : y ;

HOWEVER, since 'c' is 0, the parenthesised (here) expression is
not evaluated. 'a' will be 4. Change 'c' to be initialised with
non-zero value, and 'a' will get incremented.

V

 

[ralpe]

int
a= 4,
c= 0,
x= 100,
y= 200;
c ? ++a, x : y ; // comma before ?:

++a, x means "increase a by one and return x".

Should a be 5 at the end?

c is zero, so ++a, x is not executed.

Ralpe