I have this code:
- (BOOL)isConnected {
return !!_sessionKey;
}
where _sessionKey is defined earlier as:
NSString* _sessionKey;
the code comes from the facebook-connect for iphone.
Since I am learning Objective-C by looking at code written by other people. The !! used in the isConnection function seems useless to me, or am I missing something? What does it do?
The !! converts the result to either YES or NO.
Using !!x is an idiom from C. The result of this expression is:
!!x == 0 when x == 0 // x is zero!!x == 1 when x != 0 // x is non-zeroAt least in C, you can use any non-zero expression as a value which satisfies the condition of an if () or other conditional control flow. However, sometimes it is nice to know that the "true value" is represented by 1 rather than merely "non-zero".
In Objective-C, YES is defined as 1 rather than as "non-zero". Thus, in Objective-C, this C idiom becomes more useful.
Another way of putting it:
!!x == NO when x == NO!!x == YES when x != NO