View Compilation Error

0_0_23379507_1426.cpp: In function 'int cmp_point(const void*, const void*)':
0_0_23379507_1426.cpp:23:4: error: invalid conversion from 'const void*' to 'const point*' [-fpermissive]
  a = xa;
    ^
0_0_23379507_1426.cpp:24:4: error: invalid conversion from 'const void*' to 'const point*' [-fpermissive]
  b = xb;
    ^