C++ Safe way to convert vector of char to size_t

Question

Here is my vector of char:

std::vector<char> size;

Here is the size_t value:

size_t body_size{0};

Here is how I convert std::vector<char> size to size_t body_size

_packet.body_size = *(reinterpret_cast<int*>(_packet.size.data()));

Is there another better/safer way to make the conversion?

Do you know the byte-order of the bytes in the vector? Is it the same as on your system? And please use `unsigned char` (or better yet `uint8_t` or even [`std::byte`](https://en.cppreference.com/w/cpp/types/byte)) for raw bytes. — Some programmer dude, Oct 20 '20 at 06:40
You should also make sure that `size.length()` isn't smaller than `sizeof body_size`. — Some programmer dude, Oct 20 '20 at 06:42
@Someprogrammerdude you mean `uint8_t` instead of `size_t` ? — Venelin, Oct 20 '20 at 06:47
@Someprogrammerdude I've made `std::vector size;` and it works just fine. How can I convert from `std::vector size;` to `size_t body_size{0};` ? — Venelin, Oct 20 '20 at 06:53
Looks very like [this question](https://stackoverflow.com/q/64426995/10871073) and the answers given there. — Adrian Mole, Oct 20 '20 at 09:10

score 3 · Accepted Answer · answered Oct 20 '20 at 06:40

3

Use std::memcpy(&body_size, &size[0], sizeof (body_size));. That's safe.

answered Oct 20 '20 at 06:40

wilx

If I do that instead of `_packet.body_size = *(reinterpret_cast(_packet.size.data()));` and I run it several times it gives me `Exception: std::bad_alloc` Do I need to do anything else before memcpy ? – Venelin Oct 20 '20 at 06:44
Plus it gives some strange value for `_packet.body_size` – Venelin Oct 20 '20 at 06:45
1

Do you have enough bytes in your vector? You are probably overwriting some memory. – wilx Oct 20 '20 at 06:49
How can I prevent that @wilx ? – Venelin Oct 20 '20 at 06:51
@Venelin reserve the memory for the vector first `std::vector size(sizeof body_size);` – phuclv Oct 20 '20 at 08:22

1 Answers1