Define global constant with username in Obj-C
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I want to define a constant file path that includes whoever's username (e.g. /Users/username/Desktop...).  Using the following I can get the user's username:

    char* userName;
    userName = getenv("USER");
    NSString *macUsername = [[NSString alloc] initWithCString:userName];

What's the best way of declaring this constant so my other classes can recognize it? I currently have a globals.h file to include for the classes that need to see the globals, but I'm not sure how to set the value.  `#define` obviously needs a hardcoded string literal, and I'm not sure if I can or how I would set the string using `extern const NSString*`.  I feel like this shouldn't be hard, but I'm at a loss.