Feature request: SizeOfSelf for structs #1120

The number one sore spot for me when using construct is the absence of specifying that some field carries the size of the enclosing struct.

Consider this C struct:

struct header {
    int magic;
    int hdr_size;
    int version_major;
    int version_minor;
};

struct header_ext {
    struct header hdr;
    char name[30];
    char surname[30];
};

the intention of struct header is that it can be extended.

if i want to parse it generically out of a stream, i'm currently writing something like this:

Header = c.Struct(
    "_start" / c.Tell,
    "magic" / c.Int32ul,
    "hdr_size" / c.Int32ul,
    "version_major" / c.Int32ul,
    "version_minor" / c.Int32ul,
    "_end" / c.Tell,
    "trailing_data" / # the abomination!
        c.Bytes(c.this.hdr_size - c.this._end + c.this._start)
)

but then I can't build that from scratch!
i can either

• hard-code that the size of the common prefix is, in this case, 16 bytes, which will break if i ever modify the common header
• say that trailing_data is actually c.Default(c.Bytes(<the horrible expression>), b"\x00 * <the horrible expression>, then not provide the trailing data, set hdr_size to something big enough, read out the trailing data ......... blech

consider the alternative:

Header = c.Struct(
    "magic" / c.Int32ul,
    "hdr_size" / c.SizeOfSelf(c.Int32ul),
    "version_major" / c.Int32ul,
    "version_minor" / c.Int32ul,
    "trailing_data" / c.GreedyBytes,
)

when parsing hdr_size, the parser will limit the current struct to hdr_size - <size of already parsed>, and raise if that ends up negative
and when building, the builder will just do what it does, then go back Pointer-like to the right place and write the final value