Question

Suppose I’m getting a C string from some function:

char * mystring = SomeCFunction(...);

And I own this string (I’m responsible for freeing it when I’m done).

If, in Objective-C, I create an NSString * using:

NSString * mynsstring = [NSString stringWithCString:mystring 
                                           encoding:NSUTF8StringEncoding];

Am I still responsible for freeing the original C string?

I’m assuming the answer is yes, but I can’t find a definitive answer in the documentation – and the functionality of the opposite method (cStringUsingEncoding), while sensical, does give me pause, because it handles the cString freeing for you.

If the answer is yes, then am I also responsible for making sure I don’t free the c string before I’m done using the NSString *, or does the function copy the string for me? I ask this because the documentation for stringWithCString says it:

Returns a string containing the bytes in a given C array, interpreted
according to a given encoding.

Which still leaves me wondering if it actually copied the bytes, or is just pointing at them internally (and I’m basically just doing a cast).

Answer 1

It does copy the bytes.

If you don’t want the bytes to be copied, you may use - (id)initWithBytesNoCopy:(void *)bytes length:(NSUInteger)length encoding:(NSStringEncoding)encoding freeWhenDone:(BOOL)flag instead.

If you use - (id)initWithBytes:(const void *)bytes length:(NSUInteger)length encoding:(NSStringEncoding)encoding, or any commodity method like stringWithCString:encoding: that itself calls this low-level method, the bytes are copied.