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So, I've been messing around with the objc-runtime again (surprise surprise), and I found an interesting block of code here:
const char *sel_getName(SEL sel) {
#if SUPPORT_IGNORED_SELECTOR_CONSTANT
if ((uintptr_t)sel == kIgnore) return "<ignored selector>";
#endif
return sel ? (const char *)sel : "<null selector>";
}
So, what this tells me is that a SEL is equivalent to a C-string, in every mannerism. Doing a hex dump of the first 16 bytes of SEL that contains @selector(addObject:) gives the following:
61 64 64 4F 62 6A 65 63 74 3A 00 00 00 00 00 00
Which is equal to the C-string addObject:.
With that said, why does this code crash when I use the C-string as the selector?
SEL normalSEL = @selector(addObject:);
SEL cStringSEL = (SEL) "addObject:";
NSMutableArray *arr = [NSMutableArray arrayWithObjects:@"1", @"2", nil];
[arr performSelector:normalSEL withObject:@"3"];
[arr performSelector:cStringSEL withObject:@"4"];
NSLog(@"%@", arr);
As far as I can tell, the contents of the selectors are the same, so why the crash on the second one with the following error message?
***Terminating app due to uncaught exception 'NSInvalidArgumentException', reason: '-[__NSArrayM addObject:]: unrecognized selector sent to instance 0x101918720'***
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C-strings are simply implemented as a char array which is terminated by a null character (aka 0 ). This last part of the definition is important: all C-strings are char arrays, but not all char arrays are c-strings. C-strings of this form are called “string literals“: const char * str = "This is a string literal.
In C++, the old C-style strings are often called C-Strings. Most C++ gurus would advise you to avoid C-Strings and just use std::string . And it is true that std::string is safer and easier to use than C-Strings.
In C programming, the collection of characters is stored in the form of arrays. This is also supported in C++ programming. Hence it's called C-strings. C-strings are arrays of type char terminated with null character, that is, \0 (ASCII value of null character is 0).
C-string (clothing), a specific type of thong, or a brand of women shorts.
Selectors are interned C strings and are compared by their address, not their contents. The string contents is only used for converting to/from an external string representation. Interning is done to improve performance--when the runtime is looking up the method implementation that matches a selector it can compare the selector pointers directly instead of dereferencing each pointer and comparing the characters.
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