undef directive
If you want to nullify the effect of the define directive and specify a new value, then you can use the undef directive.
Example
#include <stdio.h>
#define VAL 40; //A
#undef VAL //B
#define VAL 40 //C
int main()
{
printf ("%d\n", VAL); //D
getchar();
return 0;
}
Explanation
Statement A defines VAL as 40, that is, an erroneous definition.
Statement B indicates that the afore mentioned definition no longer exists.
Statement C allows a new definition.
Statement D uses new definition of 40.
Point to Remember
The undef directive nullifies the effect of an earlier definition.
