the undef directivethis directive undefines a

Question

The #undef Directive

This directive undefines a previously explained macro. For, example the following will give an error since PI is undefined.

                #define PI 3.14285

                void main()

                 {

                                cout<< PI;                                           // no error here

                  # undef PI;

 

cout << PI;                          // error after undef.

                 }