View Compilation Error

0_0_3135945_15833.cpp: In function `TPoint calcGC(const std::vector<TPoint, std::allocator<TPoint> >&)':
0_0_3135945_15833.cpp:29: error: `cross' undeclared (first use this function)
0_0_3135945_15833.cpp:29: error: (Each undeclared identifier is reported only once for each function it appears in.)