Compiler error: "initializer element is not a compile-time constant"

Question

When compiling this code, I get the error "initializer element is not a compile-time constant". Can anyone explain why?

#import "PreferencesController.h"  @implementation PreferencesController  - (id)init {     self = [super init];     if (self) {         // Initialization code here.     }      return self; }   NSImage* imageSegment = [[NSImage alloc] initWithContentsOfFile:@"/User/asd.jpg"];//error here 

Answer 1

When you define a variable outside the scope of a function, that variable's value is actually written into your executable file. This means you can only use a constant value. Since you don't know everything about the runtime environment at compile time (which classes are available, what is their structure, etc.), you cannot create objective c objects until runtime, with the exception of constant strings, which are given a specific structure and guaranteed to stay that way. What you should do is initialize the variable to nil and use +initialize to create your image. initialize is a class method which will be called before any other method is called on your class.

Example:

NSImage *imageSegment = nil; + (void)initialize {     if(!imageSegment)         imageSegment = [[NSImage alloc] initWithContentsOfFile:@"/User/asd.jpg"]; } - (id)init {     self = [super init];     if (self) {         // Initialization code here.     }      return self; }