Does alignas affect the value of sizeof?

Question

#include <iostream>
using namespace std;

int main()
{
    alignas(double) unsigned char c[1024];   // array of characters, suitably aligned for doubles
    alignas(16) char d[100];            // align on 16 byte boundary
    cout<<sizeof(c)<<endl;
    cout<<sizeof(d)<<endl;
    constexpr int n = alignof(int);     // ints are aligned on n byte boundarie
    cout<<n<<endl;
}

Here is the code, for alignas(double) unsigned char c[1024];, it means the c should be aligned by double, the double is 8 bytes. So I think sizeof(c) should be 1024*8 bytes, but the console output is 1024.

So I am confused. Who can tell me the reason?

Answer 1

The alignas keyword can be used to dictate alignment requirements. alignas(double) for example forces the variable to have the same alignment requirements as a double. On my platform, this will mean that the variable is aligned on 8 byte boundaries.

In your example, the whole array will get the alignment requirements so it's being aligned on 8 byte boundaries but this won't affect its size.

It is however possible that alignas changes the size of a composite data type when upholding the alignment requirements requires additional padding. Here's an example:

#include <iostream>
#include <cstddef>

struct Test
{
    char a;
    alignas(double) char b;
};

int main(int argc, char* argv[])
{
    Test test;
    std::cout << "Size of Struct: " << sizeof(Test) << std::endl;
    std::cout << "Size of 'a': " << sizeof(test.a) << std::endl;
    std::cout << "Size of 'b': " << sizeof(test.b) << std::endl;
    std::cout << "Offset of 'a': " << (int)offsetof(struct Test, a) << std::endl;
    std::cout << "Offset of 'b': " << (int)offsetof(struct Test, b) << std::endl;
    return 0;
}

Output:

Size of Struct: 16
Size of 'a': 1
Size of 'b': 1
Offset of 'a': 0
Offset of 'b': 8

The size of this structure is 16 bytes on my platform even though both members are just 1 byte in size each. So b didn't become bigger because of the alignment requirement but there is additional padding after a. You can see this by looking at the size and offset of the individual members. a is just 1 byte in size but b, due to our alignment requirements, starts after a 8 byte offset.

And the size of a struct must be a multiple of its alignment, otherwise arrays don't work. So if you set an alignment requirement that's bigger than the whole struct was to begin with (for example a struct containing only a single short and you apply alignas(double) to that data member), padding must be added after it.