Why does Objective C provide both class NSString and subclass NSMutableString rather than just provide NSMutableString? Isn't a NSString equivalent to "const NSMutableString"?
In C++, you have only one string class, std::string, and if you want a constant you declare a const std:string.
I'm interested in knowing why I shouldn't just use NSMutableString everywhere and never bother with NSString? There must be a reason, or the language designers wouldn't provide both. maybe it takes up less storage or something?
The reason for both classes is the same reason that you sometimes use a std::string and sometimes use a const std::string. However, unlike C++, Objective-C doesn't have const methods, so they instead separate const- from non-const- methods into two different classes. This is also seen in many of the core classes, such as NSArray (NSMutableArray), NSDictionary (NSMutableDictionary), etc.
It is very possible, and even likely, that there are optimizations in place that are only allowed when strings are immutable.
In fact running
NSString *A = @"Bob";
NSString *B = @"Bob";
in the debugger immediately shows that they are both pointers to the same string. In fact
NSString *C = [NSString stringWithString:@"Bob"];
NSString *D = [A copy];
both point to the same memory address as well. Meanwhile
NSString *E = [NSMutableString stringWithString:@"Bob"];
points to a different string.
So yes, using NSStrings are more efficient in some cases. And in general cocoa lends itself to returning a new copy of a string rather than an edited one. However, I can't really argue that you shouldn't use a mutable string everywhere, but it does seem to go against the general guidelines for the framework.
In my own work I tend to only use mutable variants where I need to edit things directly. It's just a little backwards from the C/C++ style of everything mutable unless you need a const, everything is const unless you need mutability.
I would say the general rule is "don't use a class whose purpose is to provide functionality you don't need". If you need to change the contents of a string directly, use NSMutableString. If not, use NSString. In terms of size and how much heap space they take up, the classes themselves should be pretty similar.
I think the usage of an immutable string is a hint for the compiler which can perform optimizations by knowning it won't change.
来源：https://stackoverflow.com/questions/1749571/what-is-the-purpose-of-having-both-nsmutablestring-and-nsstring