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c++classvisual-c++alignment
ulak blade
ulak blade

Reputation: 2665

How do you use __declspec(align(16)) with a template?

I'm trying to make my class 16-byte aligned with __declspec(align(16)); however it is a template class.

If I place __declspec(align(16)) before the template keyword, it tells me that variable attributes are not allowed there.

If I place it before the class keyword, the whole class becomes invalid and all methods show errors.

So how is it done then?

Upvotes: 6

Views: 1945

Answers (2)

Martin Connell
Martin Connell

Reputation: 195

Put the alignment declaration after the class keyword and before the class's name:

template<typename xyz>
class __declspec(align(16)) myclass
{
...
};

Upvotes: 0

Daniel Trugman
Daniel Trugman

Reputation: 8501

This implementation probably answers this request:

template <class T, std::size_t Align>
struct alignas(Align) aligned_storage
{
    T a;
    T b;
};

template <class T, std::size_t Align>
struct aligned_storage_members
{
    alignas(Align) T a;
    alignas(Align) T b;
};

int main(int argc, char *argv[]) {
    aligned_storage<uint32_t, 8> as;
    std::cout << &as.a << " " << &as.b << std::endl;

    aligned_storage_members<uint32_t, 8> am;
    std::cout << &am.a << " " << &am.b << std::endl;
}

// Output: 
0x73d4b7aa0b20 0x73d4b7aa0b24
0x73d4b7aa0b30 0x73d4b7aa0b38

The first struct (which can be defined as a class of course), is 8-byte aligned, whereas the second struct is not aligned by itself, but rather each of the members is 8-byte aligned.

Upvotes: 1

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