Why flatbuffer struct fields can not be vector/table/string?

Question

Structs may only contain scalars or other structs. But as I konw , only offset is writed to buffer when writing vector/table/string field into table data (the data of vector/table/string is wirited before table data). So struct contains vector/table/string field still can has fix size. Why flatbuffer do the limit that struct can only contain scalars or other structs?

Answer 1

The idea behind a struct is that it is a self-contained piece of memory that always has the same layout and size and as such can easily be copied around by itself, especially in languages that support such types natively, like C/C++/Rust etc.

If it could contain strings, then it would be at least two pieces of memory, whose distance and size would be variable, and thus not efficient to copy and easy to manage. We have table for such cases.

If you must have a vector or string inside a struct, some languages already support an "array" type, which is of fixed length. You could put that, plus a length field in a struct to emulate vectors and strings, of course with the downside that the space allocated for them is always the same.