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POObjC: Alloc instance of Class and performing selectors on it leads to __CFRequireConcreteImplementation
 

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  1. POObjC: Alloc instance of Class and performing selectors on it leads to __CFRequireConcreteImplementation
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    <p>I'm new to Objective-C and I'd like to abstract my database access using a model class like this:</p> <pre><code> @interface LectureModel : NSMutableDictionary { } -(NSString*)title; -(NSDate*)begin; ... @end </code></pre> <p>I use the dictionary methods <code>setValue:forKey:</code> to store attributes and return these in the getters. Now I want to read these models from a sqlite database by using the Class dynamically.</p> <pre><code> + (NSArray*)findBySQL:(NSString*)sql intoModelClass:(Class)modelClass { NSMutableArray* models = [[[NSMutableArray alloc] init] autorelease]; sqlite3* db = sqlite3_open(...); sqlite3_stmt* result = NULL; sqlite3_prepare_v2(db, [sql UTF8String], -1, &result, NULL); while(sqlite3_step(result) == SQLITE_ROW) { id modelInstance = [[modelClass alloc] init]; for (int i = 0; i &lt; sqlite3_column_count(result); ++i) { NSString* key = [NSString stringWithUTF8String:sqlite3_column_name(result, i)]; NSString* value = [NSString stringWithUTF8String:(const char*)sqlite3_column_text(result, i)]; if([modelInstance respondsToSelector:@selector(setValue:forKey:)]) [modelInstance setValue:value forKey:key]; } [models addObject:modelInstance]; } sqlite3_finalize(result); sqlite3_close(db); return models; } </code></pre> <p>Funny thing is, the <code>respondsToSelector:</code> works, but if I try (in the debugger) to step over <code>[modelInstance setValue:value forKey:key]</code>, it will throw an exception, and the stacktrace looks like:</p> <pre> #0 0x302ac924 in ___TERMINATING_DUE_TO_UNCAUGHT_EXCEPTION___ #1 0x991d9509 in objc_exception_throw #2 0x302d6e4d in __CFRequireConcreteImplementation #3 0x00024d92 in +[DBManager findBySQL:intoModelClass:] at DBManager.m:114 #4 0x0001ea86 in -[FavoritesViewController initializeTableData:] at FavoritesViewController.m:423 #5 0x0001ee41 in -[FavoritesViewController initializeTableData] at FavoritesViewController.m:540 #6 0x305359da in __NSFireDelayedPerform #7 0x302454a0 in CFRunLoopRunSpecific #8 0x30244628 in CFRunLoopRunInMode #9 0x32044c31 in GSEventRunModal #10 0x32044cf6 in GSEventRun #11 0x309021ee in UIApplicationMain #12 0x00002988 in main at main.m:14 </pre> <p>So, what's wrong with this? Presumably I'm doing something really stupid and just don't see it...</p> <p>Many thanks in advance for your answers,<br> Arakyd :..</p>
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    1. COWhere do you implement setValue:forKey: or call NSMutableDictionary's init ?
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      1. POObjC: Alloc instance of Class and performing selectors on it leads to __CFRequireConcreteImplementation
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    2. COHm, good point Mark. Originally, I'm a C++ programmer and I thought that not implementing `-init` would lead to a compiler generated call to `[super init]`. Is that wrong? Also, the line `id modelInstance = [[modelClass alloc] init]` should initialize the `NSMutableDictionary`...
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