Question

I have the following code to read a credit card number from a text field and store it in a datamodel. The card number is correct when it is stored but later when I try to read from it it has become gibberish. Now I am guessing this is because the memory is reclaimed and it no longer makes sense.

Code to set the card number from the label:

        [cardInfo setCardNumber: [textField.text cStringUsingEncoding:[NSString defaultCStringEncoding]]];

Code in the data model:

    - (void) setCardNumber:(char *)number{
            cardNumber = number;
            //Value of cardNumber here is correct
    }

Everything looks correct until I use the property to retrieve the value at a later time.

Answer 1

From the docs for -[NSString cStringUsingEncoding:]:

The returned C string is guaranteed to be valid only until either the
receiver is freed, or until the current autorelease pool is emptied,
whichever occurs first. You should copy the C string or use
getCString:maxLength:encoding: if it needs to store the C string
beyond this time.

Also, you should never use +[NSString defaultCStringEncoding]. It’s completely unreliable and can’t generally represent arbitrary strings. From the docs:

in general this encoding should be used rarely, if at all