Convert a struct to vector of bytes

Question

In my C++ application I have this struct

typedef struct 
{
int a;
int b;
char *c
}MyStruct

and I have this instance:

MyStruct s;

Also I have this definition:

vector<byte> buffer;

I would like to insert\convert the s struct to the buffer vector.

What is the best way to do it in C++?

Thanks.

`vector buffer(sizeof(s)); memcpy(&buffer[0] &s, sizeof(s));` — Alex F, Nov 26 '13 at 15:34
Are you trying to do serialization? Do you care about endianness? Padding? Alignment? — Mark B, Nov 26 '13 at 15:34
and what do you want to do with your `char* c`? Should you copy its content? by the way, your code doesn't look like C++ — Andriy Tylychko, Nov 26 '13 at 15:37
Careful, if you are doing serialization keep in mind that the char* pointer is not going to be valid anymore in another process. — Matteo Italia, Nov 26 '13 at 15:39

Konrad Rudolph · Answer 1 · 2019-03-01T17:33:24.913

17

The best way is by using the range copy constructor and a low-level cast to retrieve a pointer to the memory of the structure:

auto ptr = reinterpret_cast<byte*>(&s);
auto buffer = vector<byte>(ptr, ptr + sizeof s);

To go the other way, you can just cast the byte buffer to the target type (but it needs to be the same type, otherwise you’re violating strict aliasing):

auto p_obj = reinterpret_cast<obj_t*>(&buffer[0]);

However, for indices ≠ 0 (and I guess technically also for index = 0, but this seems unlikely) beware of mismatching memory alignment. A safer way is therefore to first copy the buffer into a properly aligned storage, and to access pointers from there.

edited Mar 01 '19 at 17:33

answered Nov 26 '13 at 15:34

Konrad Rudolph

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Nice answer. I have a different question: I need to do the "other way around": I have a vector which is filled with values (it's size is , say 100) , and I wish to compare the first 8 bytes of the vector with a given struct , that defined as follows: struct BlockInfoHeader { uint32_t magicHeader; uint32_t thread; uint64_t timestamp; // in microseconds since start of run uint64_t offset; uint32_t len; uint32_t blockIndex; uint32_t writeIndex; uint32_t seed; }; In order to compare the first 8 bytes of the vector with some values given in advance. – Guy Avraham May 11 '17 at 11:32
1

@GuyAvraham, please ask a new question, if you still have that doubt. – alx - recommends codidact Aug 26 '21 at 09:29

Convert a struct to vector of bytes

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