Forward declaration at line 9 should looks like “struct FVertex;”, but not as “typedef struct FVertex FVertex;”. This is a C++ structure, but forward declaration refers to “C” syntax. Regarding “why structure but not class”, struct type is ok here. There’s no difference between struct and class declarations in C++, except struct will have all fields “public” by default. Unreal Header Tool works well with either declaration type, so there should be no errors with it.