Using default parameters

There are situations when you have one or more parameters that have values that are so frequently used that you want them to be treated as a default value for the parameter, while still having the option of allowing the caller to provide a different value if necessary. To do this, you provide the default value in the parameter list of the definition:

    void log_message(const string& msg, bool clear_screen = false) 
    { 
        if (clear_screen) clear_the_screen(); 
        cout << msg << endl; 
    }

In most cases, this function is expected to be used to print a single message, but occasionally the user may want to have the screen cleared first (say, for the first message, or after a pre-determined count of lines). To accommodate this use of the function, the clear_screen parameter is given a default value of false, but the caller still has the option of passing a value:

    log_message("first message", true); 
    log_message("second message"); 
    bool user_decision = ask_user(); 
    log_message("third message", user_decision);

Note that the default values occur in the function definition, not in a function prototype, so if the log_message function is declared in a header file the prototype should be:

    extern void log_message(const string& msg, bool clear_screen);

The parameters that can have default values are the right-most parameters.

You can treat each parameter with a default value as representing a separate overload of the function, so conceptually the log_message function should be treated as two functions:

    extern void log_message(const string& msg, bool clear_screen); 
    extern void log_message(const string& msg); // conceptually

If you define a log_message function that has just a const string& parameter, then the compiler will not know whether to call that function or the version where clear_screen is given a default value of false.