struct Padding

{

    size_t padding;

}

​

template staticPaddedStruct(S)

{

    struct inner

    {

        import std.conv : to;

        static foreach (member; __traits(derivedMembers, S))

        {

            import std.traits : hasUDA, getUDAs;

            mixin("typeof(S." ~ member ~ ")" ~ member ~ ";");

            static if (hasUDA!(__traits(getMember, S, member), Padding))

            {

                enum __PADDING_SIZE__ = getUDAs!(__traits(getMember, S, member), Padding)[0].padding;

                mixin("ubyte[" ~ to!string(__PADDING_SIZE__) ~ "] __PADDING__" ~ member  ~ ";");

            }

        }

    }

    alias staticPaddedStruct = inner;

}

​

template staticSizedStruct(S, size_t size)

{

    alias padded = staticPaddedStruct!S;

    pragma(msg, padded.sizeof); // Prints 12 which is correct.

    struct inner

    {

        import std.conv : to;

        static foreach (member; __traits(derivedMembers, padded))

        {

            mixin("typeof(padded." ~ member ~ ")" ~ member ~ ";");

            pragma(msg, member); // Only prints bar and foo.

        }

        static if (padded.sizeof < size)

        {

            import std.conv : to;

            mixin("ubyte[" ~ to!string(size - padded.sizeof) ~ "] __PADDING__;");

        }

    }

    pragma(msg, inner.sizeof); // Prints 8 which is incorrect.

    alias staticSizedStruct = inner;

}

​

struct FooImpl

{

    int bar;

    @Padding(4)

    int foo;

}

​

alias Foo = staticSizedStruct!(FooImpl, 12);

​

void main()

{

   assert(Foo.sizeof == 12);

   assert(FooImpl.sizeof == 8);

}