CMPT 212 Object-Oriented Applications Design in C++ Spring 1998

Preventing Multiple Header File Inclusion (continued)

• Example: The first time the preprocessor goes through a header file, a value will be defined. On subsequent passes through the file, the contents of the file will be ignored.

• I expect you to use this technique on all your header files.

Function Overloading

• Function overloading is when the same name is used for more than one function. Function overloading is also called function polymorphism. (Do not confuse function polymorphism with object polymorphism, which we will see later.)

• Functions of the same name are distinguished by the number of parameters and types of parameters. Note that parameter names are irrelevant.

• Examples:

     // file display.h
     void display(char c);          // first display function.
     void display(int i);           // second display function.
     bool display(char c, int i);   // third display function.
  
     // file one.cpp
     ...
     char letter;
     int  width;
     bool result;
     ...
     display(letter);                  // calls first display function.
     result = display(letter, width);  // calls third display function.
     display(width);                   // calls second display function.
  	 

• You cannot have two functions with the same name and same parameters but which have different return types.
  • Example:

       void display(char c, int i);
       bool display(char c, int i);    // causes a compiling error.
    	       

• When you call an overloaded function with actual parameters that do not exactly match any of the formal parameters, the compiler will try to convert the actual parameters to another type in order to match some function definition.
  • If it cannot get a match or if there is more than one possible match, then an error results.

  • Example: unique match.

       void display(float f);
       void display(bool b);
       ...
       int i;
       ...
       display(i);          // calls display(float).
    	       

  • Example: no match.

       void display(float f);
       void display(bool b);
       ...
       char c; 
       ...
       display(c);          // causes a compiling error.
    	       

  • Example: ambiguous match.

       void display(long x);
       void display(int i);
       ...
       unsigned int i;
       ...
       display(i);          // causes a compiling error.
    	       

Function Default Parameters

• Default parameters are parameters which have default values that are used if actual parameters are not given when the function is called.

• Only the latter part of the parameter list can be defaulted.

• It is an error to omit parameters when calling a function if those parameters have not been defaulted.

• Example:

     void display(char c, int size = 12, int x = 0, int y = 0);
     ...
     display('W');              // same as display('W', 12, 0, 0).
     display('W', 5);
     display('W', 5, 16, 10);
     display();                 // causes a compiling error.
  	 

• Using default parameters in your program can easily lead to confusion. You should use them carefully and only when they will make your program easier to understand.

• Make sure you understand the difference between calling a function with default parameters and calling various forms of an overloaded function.

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