How to use alignas to replace pragma pack?

Question

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!

Answer 1

alignas cannot replace #pragma pack.

GCC accepts the alignas declaration, but still keeps the member properly aligned: satisfying the strictest alignment requirement (in this case, the alignment of long) also satisfies the requirement you specified.

However, GCC is too lenient as the standard actually explicitly forbids this in §7.6.2, paragraph 5:

The combined effect of all alignment-specifiers in a declaration shall not specify an alignment that is less strict than the alignment that would be required for the entity being declared if all alignment-specifiers were omitted (including those in other declarations).