View Compilation Error

0_0_13083372_15872.cpp: In function 'int cmp(const void*, const void*)':
0_0_13083372_15872.cpp:17:14: error: invalid use of incomplete type 'struct cmp(const void*, const void*)::road'
     return p3->w-p4->w;
              ^
0_0_13083372_15872.cpp:15:12: error: forward declaration of 'struct cmp(const void*, const void*)::road'
     struct road * p3 = ( struct road * )p1;
            ^
0_0_13083372_15872.cpp:17:20: error: invalid use of incomplete type 'struct cmp(const void*, const void*)::road'
     return p3->w-p4->w;
                    ^
0_0_13083372_15872.cpp:15:12: error: forward declaration of 'struct cmp(const void*, const void*)::road'
     struct road * p3 = ( struct road * )p1;
            ^