Question

I’m learning objective-C and Cocoa and have come across this statement:

The Cocoa frameworks expect that global string constants rather than string literals are used for dictionary keys, notification and exception names, and some method parameters that take strings.

I’ve only worked in higher level languages so have never had to consider the details of strings that much. What’s the difference between a string constant and string literal?

Answer 1

In Objective-C, the syntax @'foo' is an immutable, literal instance of NSString. It does not make a constant string from a string literal as Mike assume.

Objective-C compilers typically do intern literal strings within compilation units — that is, they coalesce multiple uses of the same literal string — and it’s possible for the linker to do additional interning across the compilation units that are directly linked into a single binary. (Since Cocoa distinguishes between mutable and immutable strings, and literal strings are always also immutable, this can be straightforward and safe.)

Constant strings on the other hand are typically declared and defined using syntax like this:

// MyExample.h - declaration, other code references this extern NSString * const MyExampleNotification;  // MyExample.m - definition, compiled for other code to reference NSString * const MyExampleNotification = @'MyExampleNotification'; 

The point of the syntactic exercise here is that you can make uses of the string efficient by ensuring that there’s only one instance of that string in use even across multiple frameworks (shared libraries) in the same address space. (The placement of the const keyword matters; it guarantees that the pointer itself is guaranteed to be constant.)

While burning memory isn’t as big a deal as it may have been in the days of 25MHz 68030 workstations with 8MB of RAM, comparing strings for equality can take time. Ensuring that most of the time strings that are equal will also be pointer-equal helps.

Say, for example, you want to subscribe to notifications from an object by name. If you use non-constant strings for the names, the NSNotificationCenter posting the notification could wind up doing a lot of byte-by-byte string comparisons when determining who is interested in it. If most of these comparisons are short-circuited because the strings being compared have the same pointer, that can be a big win.