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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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