Singleton not initializing correctly

I have the following code that I am calling using this statement: SQLiteDB *db = [[[SQLiteDB alloc] init] autorelease];

The problem is "sharedSQLiteDB" is not being called, but rather "allocWithZone" is, an开发者_Python百科d therefore "checkIfDatabaseExists" is not being called, which is where the database is created.

I don't understand why... (i.e. what am I doing wrong?)

#import "SQLiteDB.h"

static SQLiteDB *sharedSQLiteDB = nil;  //  makes this a singleton class

@implementation SQLiteDB

@synthesize searchPaths, documentPath, databasePath, cDatabasePath;

#pragma mark Singleton Methods

+ (SQLiteDB *) sharedSQLiteDB  {

    if(!sharedSQLiteDB)  {
        sharedSQLiteDB = [[SQLiteDB alloc] init];
        [sharedSQLiteDB checkIfDatabaseExists];  //  check to see if d/b exists
    }
    return sharedSQLiteDB;
}   

+(id)allocWithZone:(NSZone *)zone  {  //  makes sure another instance is not allocated
    if(!sharedSQLiteDB)  {
        sharedSQLiteDB = [super allocWithZone:zone];
        return  sharedSQLiteDB;
    }
    else {
        return nil;
    }
}   

-(id)copyWithZone:(NSZone *)zone  {
    return self;
}

-(void) release  {
    //  no-op
}

In the singleton pattern your use pattern should be:

SQLiteDB* db = [SQLiteDB sharedSQLiteDB];

They way you are calling it doesn't fit the singelton pattern. All access should be through your sharedSQLiteDB message.

In other words you shouldn't be initializing via typical Cocoa patterns (SQLiteDB *db = [[[SQLiteDB alloc] init] autorelease]; is incorrect and full of problems) outside the scope of the class.

In a singleton using the default initialization pattern for the language (alloc/init for ObjC or the default constructor for C++) should generate a compile time error message since the constructor/init method should be protected.

See the Wikipedia entry. consult the Design Pattern C++ bible. There is even a version for Cocoa

Good luck.

It isn't executing your + (SQLiteDB *) sharedSQLiteDB method because you're not actually calling that method anywhere.

As you've seen, when you call [[SQLiteDB alloc] init], the allocWithZone method is called.

Change your call to be SQLiteDB *db = [SQLiteDB sharedSQLiteDB], which will call your checkIfDatabaseExists method in this case. However, if [[SQLiteDB alloc] init] is called somewhere else, then the checkIfDatabaseExists method call will still be skipped.

Maybe consider moving the checkIfDatabaseExists method into an init method so that it will be called for both your singleton method and your allocWithZone.

Honestly I don't see any error... However I post the code I used to create a Singleton. It's from a source that now I don't remember the link... it's not my code.

static DataManager *_instance;

@implementation DataManager

+ (DataManager*)sharedInstance
{
    @synchronized(self) {

        if (_instance == nil) {

            _instance = [[super allocWithZone:NULL] init];

            // Allocate/initialize any member variables of the singleton class her
            // example
    //_instance.member = @"";
        }
    }
    return _instance;
}

#pragma mark Singleton Methods

+ (id)allocWithZone:(NSZone *)zone
{   
    return [[self sharedInstance]retain];   
}


- (id)copyWithZone:(NSZone *)zone
{
    return self;    
}

- (id)retain
{   
    return self;    
}

- (unsigned)retainCount
{
    return NSUIntegerMax;  //denotes an object that cannot be released
}

- (void)release
{
    //do nothing
}

- (id)autorelease
{
    return self;    
}

I hope it helps

I highly recommend using the SyntesizeSingleton header file first created by Matt Gallagher.

Find the latest version (that I know about) here:

https://github.com/cjhanson/Objective-C-Optimized-Singleton

It makes creating a singleton dead simple.

Here's an example header:

#import <Foundation/Foundation.h>
@interface Example : NSObject {}
+(Example*)sharedExample;
@end

And the corresponding .m:

#import "FMUser.h"
#import "SynthesizeSingleton.h"
@implementation Example
SYNTHESIZE_SINGLETON_FOR_CLASS(Example);
@end

[Example sharedExample] is created for you. It's pretty sweet.