Default Arguments

Artykuł
11/16/2012

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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Default Arguments

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