Aligning Member Variables By Template Type

Question

Hello

I want to align my member variables based on a class template type but I'm not sure if it is actually possible.

The following is a (very) simple example of what I'd like to do

template<int Align>
class MyClass
{
private:
  struct MyStruct
  {
    // Some stuff
  } __declspec(align(Align));

  __declspec(align(Align)) int myAlignedVariable;
};

So what I'd like is for Align to be a per-instance variable, and only through that is the align value of the class contents decided.

Unfortunately I always get the following error

error C2975: 'test::MyClass' : invalid template argument for 'Align', expected compile-time constant expression

So, is this actually possible or can the alignment only be possible using a fixed compile time constant? If not, can anyone think of a way around this?

Thanks :)

Answer 1

Custom alignment isn't in the standard, so how the compilers deal with it is up to them - looks like VC++ doesn't like combining templates with __declspec.

I suggest a work-around using specialisation, something like this:

template<int A> struct aligned;
template<> struct aligned<1> { } __declspec(align(1));
template<> struct aligned<2> { } __declspec(align(2));
template<> struct aligned<4> { } __declspec(align(4));
template<> struct aligned<8> { } __declspec(align(8));
template<> struct aligned<16> { } __declspec(align(16));
template<> struct aligned<32> { } __declspec(align(32));

and then derive from that in your code:

template<int Align>
class MyClass
{
private:
  struct MyStruct : aligned<Align> {
    // stuff
  };
};

This unfortunately breaks the POD-ness of MyStruct. It also doesn't work with built-in/existing types, so you'll have to use a wrapper for those.

aligned_t<int, 4> myAlignedVariable;

Answer 2

Boost has solved this problem already. They use the technique in boost::optional (link to header), which has to hold enough space for an aligned, arbitrary type but can't (won't) actually instantiate that object on construction.

Their solution was to allocate a simple pool of bytes (char array) and use in-place new to construct objects at the desired location. The address given to in-place new can be of any arbitrary alignment.

That being said, you're saying that you're giving a very simple example in your question. What is the actual problem that you're trying to solve by implementing a class where every member has a user-specified alignment that doesn't vary per member but can vary per class instance?