iOS 6 Programming Cookbook

1.14. Defining Two or More Methods with the Same Name

Problem

You would like to implement two or more methods with the same name in one object. In object-oriented programming, this is called method overloading. However, in Objective-C, method overloading does not exist in the same way as it does in other programming languages such as C++.

Solution

Use the same name for your method, but keep the number and/or the names of your parameters different in every method:

- (void) drawRectangle{

  [self drawRectangleInRect:CGRectMake(0.0f, 0.0f, 4.0f, 4.0f)];

}

- (void) drawRectangleInRect:(CGRect)paramInRect{

  [self drawRectangleInRect:paramInRect
                  withColor:[UIColor blueColor]];

}

- (void) drawRectangleInRect:(CGRect)paramInRect
             withColor:(UIColor*)paramColor{

  [self drawRectangleInRect:paramInRect
                  withColor:paramColor
                  andFilled:YES];

}

- (void) drawRectangleInRect:(CGRect)paramInRect
                   withColor:(UIColor*)paramColor
                   andFilled:(BOOL)paramFilled{

  /* Draw the rectangle here */

}

This example shows a typical pattern in overloading. Each rectangle can be drawn either filled (solid color) or empty (showing just its boundaries). The first procedure is a “convenience procedure” that allows the caller to avoid specifying how to fill the rectangle. In our implementation of the first procedure, we merely call the second procedure, making the choice for the caller (andFilled:YES) The second procedure gives the caller control over filling.

Discussion

You can define two methods with the same name so long as they differ in the parameters they accept. One reasons for doing this is one function offers more customization (through parameterization) than the other function.

Method overloading is a programming language feature supported by Objective-C, C++, Java, and a few other languages. Using this feature, programmers can create different methods with the same name, in the same object. However, method overloading in Objective-C differs from that which can be used in C++. For instance, in C++, to overload a method, the programmer needs to assign a different number of parameters to the same method and/or change a parameter’s data type.

In Objective-C, however, you simply change the name of at least one parameter. Changing the type of parameters will not work:

- (void) method1:(NSInteger)param1{

  /* We have one parameter only */

}

- (void) method1:(NSString *)param1{

  /* This will not compile as we already have a
   method called [method1] with one parameter */

}

Changing the return value of these methods will not work either:

- (int) method1:(NSInteger)param1{

  /* We have one parameter only */
  return param1;

}

- (NSString *) method1:(NSString *)param1{

  /* This will not compile as we already have a
   method called [method1] with one parameter */
  return param1;

}

As a result, you need to change the number of parameters or the name of (at least) one parameter that each method accepts. Here is an example where we have changed the number of parameters:

- (NSInteger) method1:(NSInteger)param1{

  return param1;

}

- (NSString*) method1:(NSString *)param1
            andParam2:(NSString *)param2{

  NSString *result = param1;

  if ([param1 length] > 0 &&
      [param2 length] > 0){
    result = [result stringByAppendingString:param2];
  }

  return result;

}

Here is an example of changing the name of a parameter:

- (void) drawCircleWithCenter:(CGPoint)paramCenter
                       radius:(CGFloat)paramRadius{

  /* Draw the circle here */

}

- (void) drawCircleWithCenter:(CGPoint)paramCenter
                       Radius:(CGFloat)paramRadius{

  /* Draw the circle here */

}

Can you spot the difference between the declarations of these two methods? The first method’s second parameter is called radius (with a lowercase r) whereas the second method’s second parameter is called Radius (with an uppercase R). This will set these two methods apart and allows your program to get compiled. However, Apple has guidelines for choosing method names as well as what to do and what not to do when constructing methods. For more information, please refer to the “Coding Guidelines for Cocoa” Apple documentation available here.

Here is another example of two methods that draw a circle but have different names for their second parameter:

- (void) drawCircleWithCenter:(CGPoint)paramCenterPoint
               radiusInPoints:(CGFloat)paramRadiusInPoints{
  /* Draw the circle here */
}

- (void) drawCircleWithCenter:(CGPoint)paramCenterPoint
          radiusInMillimeters:(CGFloat)paramRadiusInMillimeters{
  /* Draw the circle here */
}

Here is a concise extract of the things to look out for when constructing and working with methods:

• Have your method names describe what the method does clearly, without using too much jargon and abbreviations. A list of acceptable abbreviations is in the Coding Guidelines.

• Have each parameter name describe the parameter and its purpose. On a method with exactly three parameters, you can use the word and to start the name of the last parameter if the method is programmed to perform two separate actions. In any other case, refrain from using and to start a parameter name. An example of the name of a method that performs two actions and uses the word and in its name is prefixFirstName:withInitials:andMakeInitialisUppercase:, where the method can prefix a first name (of type NSString) with the initials (of type NSString again) of that individual. In addition, the method accepts a boolean parameter named andMakeInitialsUppercase which, if set to YES, will prefix the first name with an uppercase equivalent of the initials passed to the method. If this parameter is set to NO, the method will use the initials it is given, without changing their case, to prefix the first name parameter.

• Start method names with a lowercase letter.

• For delegate methods, start the method name with the name of the class that invokes that delegate method.

See Also

Recipe 1.13