I am trying to understand how alignas should be used, I wonder if it can be a replacement for pragma pack, I have tried hard to verify it but with no luck. Using gcc 4.8.1 (http://ideone.com/04mxpI) I always get 8 bytes for below STestAlignas, while with pragma pack it is 5 bytes. What I would like ot achive is to make sizeof(STestAlignas) return 5. I tried running this code on clang 3.3 (http://gcc.godbolt.org/) but I got error:
!!error: requested alignment is less than minimum alignment of 8 for type 'long' - just below alignas usage.
So maybe there is a minimum alignment value for alignas?
below is my test code:
#include <iostream>
#include <cstddef>
using namespace std;
#pragma pack(1)
struct STestPragmaPack {
char c;
long d;
} datasPP;
#pragma pack()
struct STestAttributPacked {
char c;
long d;
} __attribute__((packed)) datasAP;
struct STestAlignas {
char c;
alignas(char) long d;
} datasA;
int main() {
cout << "pragma pack = " << sizeof(datasPP) << endl;
cout << "attribute packed = " << sizeof(datasAP) << endl;
cout << "alignas = " << sizeof(datasA) << endl;
}
results for gcc 4.8.1:
pragma pack = 5
attribute packed = 5
alignas = 8
[26.08.2019]
It appears there is some standardisation movement in this topic. p1112 proposal - Language support for class layout control - suggest adding (among others) [[layout(smallest)]] attribute which shall reorder class members so as to make the alignment cost as small as possible (which is a common technique among programmers - but it often kills class definition readability). But this is not equal to what pragma(pack) does!
#pragma packwill forever be a non-portable extension. Some architectures don't support unaligned memory accesses. – Crompton