Initialize array with variable OR pointer to struct

I'm trying to write a library that will use a struct, but depending on the specific use, the size of an array in the structure needs to vary.

I’m pretty sure that C/C++ structs have fixed sizes. You can have a pointer to the array in your struct (and a length, presumably), or you can do rather gross things if you put the array last in the struct, making use of the lack of range-checking on array indexing, or a union of the structs, with different array sizes.

Maybe you can work with this guy: Initialize array with variable OR pointer to struct - Programming Questions - Arduino Forum

westfw:
I'm pretty sure that C/C++ structs have fixed sizes. You can have a pointer to the array in your struct (and a length, presumably), or you can do rather gross things if you put the array last in the struct, making use of the lack of range-checking on array indexing, or a union of the structs, with different array sizes.

What about VA_ARGS ?