Objective-C convention to prevent "local declaration hides instance variable" warning

Question

I am using the following code ...

-(id) initWithVariableName:(NSString*)variableName withComparisonValue:(NSString*)comparisonValue {

    // super init
    self = [super init];
    if (!self) return nil;

    // set instance variables
    self.mustExist = NO;
    self.reverseCondition = NO;
    self.regularExpression = NO;
    self.variableName = variableName; // generates warning
    self.comparisonValue = comparisonValue; // generates warning

    return self;
}

which generated the following two warnings ...

• Local declaration of 'variableName' hides instance variable
• Local declaration of 'comparisonValue' hides instance variable

Is there a common or accepted convention for dealing with these warnings?

I understand that it is simply to inform the user that they should specify an instance when referring to the class member, but its annoying.

Answer 1

Unfortunately, no there's no "good" way to prevent this error. The common pattern is to use a slightly stupid parameter name like

-(id) initWithVariableName:(NSString*)theVariableName 
       withComparisonValue:(NSString*)theComparisonValue {
    self.variableName = theVariableName;
    self.comparisonValue = theComparisonValue;

    return self;
}

Answer 2

You should generally prefix instance variables with something like an underscore (e.g. _variableName) to avoid compiler warnings like this.

Otherwise just slightly change the names in your method signature, there is no hard defined naming convention.

Answer 3

Either give the local a more descriptive name (e.g. initialVariableName) or give instance variables a different notation (e.g. myClass_variableName). I prefer the latter in most cases because it calls attention to when I'm using class internals rather than the proper interface.

Answer 4

If your method truly is an initialiser, don't forget to do your self = [super init];.

- (id) initWith...
{
    self = [super init];
    if (!self) return nil;

    // do stuff

    return self;
}

I have never personally encountered a situation where self has changed to nil or another value, but it's the Objective-C Initialiser Idiom™.