View Compilation Error

0_0_13996685_28824.cpp: In function 'int compare(const void*, const void*)':
0_0_13996685_28824.cpp:51:30: error: invalid conversion from 'const void*' to 'const Person*' [-fpermissive]
     const struct Person *a = p;
                              ^
0_0_13996685_28824.cpp:52:30: error: invalid conversion from 'const void*' to 'const Person*' [-fpermissive]
     const struct Person *b = q;
                              ^