everest% CC fptr3.C
"fptr3.C", line 43: error(1164): argument of type "void (*)(int *)" is
          incompatible with parameter of type "void (*)(void *)"
     apply(&add3, &m) ;
           ^

"fptr3.C", line 45: error(1164): argument of type "void (*)(double *)" is
          incompatible with parameter of type "void (*)(void *)"
     apply(&add5, &x) ;
           ^

"fptr3.C", line 49: error(1515): a value of type "void (*)(int *)" cannot be
          assigned to an entity of type "void (*)(void *)"
     fptr = &add3 ;
          ^

"fptr3.C", line 52: error(1515): a value of type "void (*)(double *)" cannot
          be assigned to an entity of type "void (*)(void *)"
     fptr = &add5 ;
          ^

4 errors detected in the compilation of "fptr3.C".
everest% 

everest% g++ fptr3.C
ld: WARNING 84: /usr/lib32/libm.so is not used for resolving any symbol.
everest% 

everest% a.out
After add3(&m), m = 5
After add5(&x), x = 10

Applying add3 to m, m = 8
Applying add5 to x, x = 15

Applying fptr to m, m = 11
Applying fptr to x, x = 20
everest%