Standard way of overlay flexible array member - c++

Question

So the server sends the data just as packed structures, so what only need to decode is to overlay the structure pointer on the buffer. However one of the structure is a dynamic array kind of data, but I learned that flexible array member is not a C++ standard feature. How can I do it in standard C++ way, but without copying like a vector?
// on wire format: | field a | length | length of struct b |
// the sturcts are defined packed
__pragma(pack(1))
struct B {
//...
};
struct Msg {
int32_t a;
uint32_t length;
B *data; // how to declare this?
};
__pragma(pack())
char *buf = readIO();
// overlay, without copy and assignments of each field
const Msg *m = reinterpret_cast<const Msg *>(buf);
// access m->data[i] from 0 to length

Answer 1

The common way to do this in C was to declare data as an array of length one as the last struct member. You then allocate the space needed as if the array was larger.
Seems to work fine in C++ as well. You should perhaps wrap access to the data in a span or equivalent, so the implementation details don't leak outside your class.
#include <string>
#include <span>
struct B {
float x;
float y;
};
struct Msg {
int a;
std::size_t length;
B data[1];
};
char* readIO()
{
constexpr int numData = 3;
char* out = new char[sizeof(Msg) + sizeof(B) * (numData - 1)];
return out;
}
int main(){
char *buf = readIO();
// overlay, without copy and assignments of each field
const Msg *m = reinterpret_cast<const Msg *>(buf);
// access m->data[i] from 0 to length
std::span<const B> data(m->data, m->length);
for(auto& b: data)
{
// do something
}
return 0;
}
https://godbolt.org/z/EoMbeE8or

Answer 2

A standard solution is to not represent the array as a member of the message, but rather as a separate object.
struct Msg {
int a;
size_t length;
};
const Msg& m = *reinterpret_cast<const Msg*>(buf);
span<const B> data = {
reinterpret_cast<const B*>(buf + sizeof(Msg)),
m.length,
};
Note that reinterpretation / copying of bytes is not portable between systems with different representations (byte endianness, integer sizes, alignments, subobject packing etc.), and same representation is typically not something that can be assumed in network communication.

Answer 3

// on wire format: | field a | length | length of struct b |
You can't overlay the struct, because you can't guarantee that the binary representation of Msg will match the on wire format. Also int is at least 16 bits, can be any number of bits greater than 16, and size_t has various size depending on architecture.
Write actual accessors to the data. Use fixed width integer types. It will only work if the data actually point to a properly aligned region. This method allows you to write assertions and throw exceptions when stuff goes bad (for example, you can throw on out-of-bounds access to the array).
struct Msg {
constexpr static size_t your_required_alignment = alingof(uint32_t);
char *buf;
Msg (char *buf) : buf(buf) {
assert((uintptr_t)buf % your_required_alignment == 0);
}
int32_t& get_a() { return *reinterpret_cast<int32_t*>(buf); }
uint32_t& length() { return *reinterpret_cast<uint32_t *>(buf + sizeof(int32_t)); }
struct Barray {
char *buf;
Barray(char *buf) : buf(buf) {}
int16_t &operator[](size_t idx) {
return *reinterpret_cast<int16_t*>(buf + idx * sizeof(int16_t));
}
}
Barray data() {
return buf + sizeof(int32_t) + sizoef(uint32_t);
}
};
int main() {
Msg msg(readIO());
std::cout << msg.a() << msg.length();
msg.data()[1] = 5;
// or maybe even implement straight operator[]:
// msg[1] = 5;
}
If the data do not point to a properly aligned region, you have to copy the data, there is no possibility to access them using other types then char.