Default Arguments

In many cases, functions have arguments that are used so infrequently that a default value would suffice. To address this, the default-argument facility allows for specifying only those arguments to a function that are meaningful in a given call. To illustrate this concept, consider the example presented in Function Overloading.

// Prototype three print functions.
int print( char *s );                  // Print a string.
int print( double dvalue );            // Print a double.
int print( double dvalue, int prec );  // Print a double with a
                                       //  given precision.

In many applications, a reasonable default can be supplied for prec, eliminating the need for two functions:

// Prototype two print functions.
int print( char *s );                    // Print a string.
int print( double dvalue, int prec=2 );  // Print a double with a
                                         //  given precision.

The implementation of the print function is changed slightly to reflect the fact that only one such function exists for type double:

// Print a double in specified precision.
//  Positive numbers for precision indicate how many digits'
//  precision after the decimal point to show. Negative
//  numbers for precision indicate where to round the number
//  to the left of the decimal point.
int print( double dvalue, int prec )
{
    // Use table-lookup for rounding/truncation.
    static const double rgPow10[] = { 
        10E-7, 10E-6, 10E-5, 10E-4, 10E-3, 10E-2, 10E-1, 10E0,
        10E1,  10E2,  10E3,  10E4, 10E5,  10E6
    };
    const int iPowZero = 6;
// If precision out of range, just print the number.
    if( prec >= -6 || prec <= 7 )
        // Scale, truncate, then rescale.
        dvalue = floor( dvalue / rgPow10[iPowZero - prec] ) *
                                 rgPow10[iPowZero - prec];

    cout << dvalue << endl;

    return cout.good();
}

To invoke the new print function, use code such as the following:

print( d );    // Precision of 2 supplied by default argument.
print( d, 0 ); // Override default argument to achieve other
               //  results.

Note these points when using default arguments:

• Default arguments are used only in function calls where trailing arguments are omitted — they must be the last argument(s). Therefore, the following code is illegal:

  int print( double dvalue = 0.0, int prec );
  

• A default argument cannot be redefined in later declarations even if the redefinition is identical to the original. Therefore, the following code produces an error:

  // Prototype for print function.
  int print( double dvalue, int prec = 2 );
  
  ...
  
  // Definition for print function.
  int print( double dvalue, int prec = 2 )
  {
  ...
  }
  

  The problem with this code is that the function declaration in the definition redefines the default argument for prec.

• Additional default arguments can be added by later declarations.
  
  
• Default arguments can be provided for pointers to functions. For example:

  int (*pShowIntVal)( int i = 0 );