Question

Is there a way to detect at compile-time if a class has a vtable or not? I am trying to ensure
a class is aligned to 64-byte boundaries and is 64 bytes in length. Adding a vtable increases
the class size to 128 bytes.

class __attribute__((aligned(64))) C
{
private:
    int64_t iValue;
    char iPadding[64 - sizeof(int64_t)];
};

This is fine. However

class __attribute__((aligned(64))) C
{
public:
    virtual ~C() {}

private:
    int64_t iValue;
    char iPadding[64 - sizeof(int64_t)];
};

screws things up.

Answer: aligned also pads, not just controls location. __declspec(align()) seems to do the same!

Edit: Still bamboozled. After putting a check in the constructor of C that checks that this is divisible by 64 and throw an exception if it’s not, I’m getting exceptions. Initially I though it might have to do with having instances of C on the stack, but after changing them to being heap-based the alignment check is still failing. I’ll just fall back to a factory function that calls posix_memalign and do in-place new (which is probably what std::aligned_storage eventually does)

Answer 1

Instead of adding padding bytes manually, why not just use __attribute__((aligned(64))) and let the compiler align it for you whether or not there’s a vtable present? Then you’ll always get 64 byte alignment with no other work needed, and it removes the dependency on knowing the size of the vtable.