What is the different between NSCFString and NSConstantString?

Question

I declare the object variable as a NSString But when I use the XCode to look into my object, I saw there are two type of String, it seems that the system automatically transfer to another:

What are the different between them? Are they interchangeable to one and others. Also, what is the condition two change to another?

Thanks.

Answer 1

They're both concrete subclasses of NSString. __NSCFString is one created during runtime via Foundation or Core Foundation, while __NSCFConstantString is either a CFSTR("...") constant or an @"..." constant, created at compile-time.

Their interfaces are private. Treat them both as NSString and you should have no trouble.