How to make a simple error reporter that is thread safe

main-snipit.m

/**
 * What I'm attempting to do is to init an object. I want that object to
 * default to a value if there was an error. In other words because the object
 * is a model it should not attempt to send a message to the user as this is
 * the responsability of the view (or controller to decide what to do). So it
 * needs a mechinism to commiunicate an error to the caller without ever
 * throwing an exception. I'm using the errorString and weather it is nil or
 * not. However it is possible that more than one thread accesses errorString
 * and there for causing false positives.
 *
 * After carefull self review it dawned on me that the init method was thread
 * safe. Because errorString is encapsulated as an instance variable and it is
 * readonly there is no way for another thread to mutate this. If it were not
 * readonly or if there was a mechinism to reset it then were would be worried
 * about threads.
 *
 * Case in point the saveToFile: selector does just that. It resets the
 * errorString. Now any thread could attempt to save and mutate errorString.
 */

#import <Foundation/Foundation.h>
#import "SomeObject.h"


int main(int argc, char *argv[]) {

    NSAutoreleasePool * pool = [[NSAutoreleasePool alloc] init];


    SomeObject *myNewObject = [[SomeObject alloc] initWithFile:@"test.plist"];

    if (myNewObject.errorString != nil)
    {
        // Warn the user
    }

    // Continue on with default values.

    // Not thread safe
    if ( [myNewObject saveToFile:@"foo.plist"] )
    {
        // We're good to go
    }
    else
    {
        // There was an error
        NSLog(@"Error Message: %@", myNewObject.errorString);
    }


    // Thread safe
    NSString *errMsg = nil;
    [myNewObject saveToFile:@"bar.plist" withErrorString:&errMsg];

    if ( errMsg != nil )
    {
        // We're good to go
    }
    else
    {
        // There was an error
        NSLog(@"Error Message: %@", errMsg);
    }


    [myNewObject release];
    [pool release];

    return 0;
}

SomeObject.h

#import <Foundation/Foundation.h>


@interface SomeObject : NSObject {
    NSString *errorString;
    NSArray *testArray;
}

@property (nonatomic, readonly) NSString *errorString;
@property (nonatomic, readonly) NSArray *testArray;

- (id)initWithFile:(NSString *)filename;
- (BOOL)saveToFile:(NSString *)filename;
- (void)saveTofile:(NSString *)filename withErrorString:(NSString **)errMsg;

@end

SomeObject.m

#import "SomeObject.h"


@implementation SomeObject

@synthesize errorString;
@synthesize testArray;


- (id)initWithFile:(NSString *)filename {
    
    if (self = [super init])
    {
        // Do some file IO thing.

        if ( YES ) // There was an error
        {
            errorString = @"There was an error.";

            // Set testArray to an empty array. (Fresh start)
            testArray = [[[NSArray alloc] init] retain];
        }
        else // All is good
        {
            // Set testArray to something usefull based on file IO.
        }
    }

    return self;
}

- (BOOL)saveToFile:(NSString *)filename {
    
    // Clear errorString
    [errorString release];
    errorString = nil;

    // Attempt to save

    if (YES) // Some thing went wrong
    {
        errorString = @"There was an error.";

        return NO;
    }

    // everything worked
    return YES;
}

- (void)saveTofile:(NSString *)filename withErrorString:(NSString **)errMsg {
    
    // Attempt to save file

    if (YES) // Something went wrong
    {
        *errMsg = @"There was an error";
    }
    else
    {
        *errMsg = nil;
    }
}

- (void)dealloc {
    
    [errorString release];

    [super dealloc];
}


@end