Language in C 面接の質問と回答

Question: How can I write a general-purpose swap without using templates?

Answer: Given below is the program which uses the stringizing preprocessor directive ## for building a general purpose swap macro which can swap two integers, two floats, two chars, etc.
#define swap( a, b, t ) ( g ## t = ( a ), ( a ) = ( b ), ( b ) = g ## t )
int gint;
char gchar;
float gfloat ;
main( )
{
int a = 10, b = 20 ;
char ch1 = 'a' , ch2 = 'b' ;
float f1 = 1.12, f2 = 3.14 ;
swap ( a, b, int ) ;
printf ( "na = %d b = %d", a, b ) ;
swap ( ch1, ch2, char ) ;
printf ( "nch1 = %c ch2 = %c", ch1, ch2 ) ;
swap ( f1, f2, float ) ;
printf ( "nf1 = %4.2f f2 = %4.2f", f1, f2 ) ;
}
swap ( a, b, int ) would expand to,
( gint = ( a ), ( a ) = ( b ), ( b ) = gint )