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Cpp - Default Arguments

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:

// default_arguments.cpp
// compile with: /EHsc /c

// 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.

#include <iostream>
#include <math.h>
using namespace std;

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 );

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