Allocate the memory of struct more efficiently in C++

Question

I'm trying the construct a struct in C++ as below:

struct kmer_value {
    uint32_t count : 32;
    uint32_t  path_length : 32;
    uint8_t acgt_prev : 4;
    uint8_t  acgt_next : 4;
}

The struct currently takes the memory of 12 bytes, but I want to reduce the size to 9 bytes. Is there any way to realize it?

Thank you.

@AlexanderHuszagh `acgt_prev` and `acgt_next` share `uint8_t` — NathanOliver, Jul 01 '17 at 03:21
Sure, just compress the dynamic range of `count` or `path_length`. If they don't each need 32 bits, allocate fewer for each. — Brian Cain, Jul 01 '17 at 03:22
see this Q: https://stackoverflow.com/questions/1455458/c-struct-alignment-question — NathanOliver, Jul 01 '17 at 03:24
What does `uint32_t count : 32;` do that `uint32_t count;` doesn't? — Borgleader, Jul 01 '17 at 03:25
What's the use case where there will be benefit having the struct not end on a word boundary? — dlasalle, Jul 01 '17 at 04:06
@Borgleader You wouldn't believe me if I told you. Those first 23 bits... wow. — user4581301, Jul 01 '17 at 04:11
@user4581301 What about those first *23* bits? I honestly don't know what you're on about. — Borgleader, Jul 01 '17 at 04:26

score 9 · Answer 1 · answered Jul 01 '17 at 03:25

There's no portable solution. For GCC that would be

struct __attribute__((packed)) kmer_value {
  uint32_t count : 32;
  uint32_t path_length : 32;
  uint8_t acgt_prev : 4;
  uint8_t acgt_next : 4;
};

In MSVC #pragma pack can achieve the same effect.

Consult your compiler's documentation.

1 Answers1