实例讲解在C++的函数中变量参数及默认参数的使用

// variable_argument_lists.cpp
#include <stdio.h>
#include <stdarg.h>

// Declaration, but not definition, of ShowVar.
void ShowVar( char *szTypes, ... );
int main() {
  ShowVar( "fcsi", 32.4f, 'a', "Test string", 4 );
}

// ShowVar takes a format string of the form
//  "ifcs", where each character specifies the
//  type of the argument in that position.
//
// i = int
// f = float
// c = char
// s = string (char *)
//
// Following the format specification is a variable 
// list of arguments. Each argument corresponds to 
// a format character in the format string to which 
// the szTypes parameter points 
void ShowVar( char *szTypes, ... ) {
  va_list vl;
  int i;

  // szTypes is the last argument specified; you must access 
  // all others using the variable-argument macros.
  va_start( vl, szTypes );

  // Step through the list.
  for( i = 0; szTypes[i] != '\0'; ++i ) {
   union Printable_t {
     int   i;
     float  f;
     char  c;
     char  *s;
   } Printable;

   switch( szTypes[i] ) {  // Type to expect.
     case 'i':
      Printable.i = va_arg( vl, int );
      printf_s( "%i\n", Printable.i );
     break;

     case 'f':
       Printable.f = va_arg( vl, double );
       printf_s( "%f\n", Printable.f );
     break;

     case 'c':
       Printable.c = va_arg( vl, char );
       printf_s( "%c\n", Printable.c );
     break;

     case 's':
       Printable.s = va_arg( vl, char * );
       printf_s( "%s\n", Printable.s );
     break;

     default:
     break;
   }
  }
  va_end( vl );
}
//Output: 
// 32.400002
// a
// Test string

// 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.

// 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.

// 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();
}

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

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

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

...

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

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