Question

I desperately need to find a solution for the following problem:

namespace test
{
    template <int param = 0> struct Flags
    {
        int _flags;

        Flags()
        {
            _flags = 0;
        }

        Flags(int flags)
        {
            _flags = flags;
        }

        void init()
        {

        }
    };

    union example
    {
        struct
        {
            union
            {
                struct
                {
                    Flags<4096> f;
                }p1; //error: member 'test::example::<anonymous struct>::<anonymous union>::<anonymous struct> test::example::<anonymous struct>::<anonymous union>::p1' with constructor not allowed in union

                struct 
                {
                    Flags<16384> ff;
                }p2; //error: member 'test::example::<anonymous struct>::<anonymous union>::<anonymous struct> test::example::<anonymous struct>::<anonymous union>::p2' with constructor not allowed in union
            }parts;

            byte bytes[8];
        }data;

        int data1;
        int data2;
    }
}

It’s frustrating that if I add tags to p1 and p2 structs, the code will compile, but the f & ff members would not be accessible:

...
struct p1
{
    Flags<4096> f;
};

struct p2
{
    Flags<4096> ff;
};
...

void test()
{
    example ex;
    ex.data.bytes[0] = 0; //Ok
    ex.data.parts.p1.f.init(); //error: invalid use of 'struct test::example::<anonymous struct>::<anonymous union>::p1'
}

Is there any way to make this work somehow?

Answer 1

As @Als said, union cannot define non-POD as member data, there is one alternative. You can still define a pointer to the non-POD as member data of the union.

So this is allowed:

union
{
   struct
   {
      Flags<4096> *pf; //pointer to non-POD
   }p1;
   struct 
   {
      Flags<16384> *pff; //pointer to non-POD
   }p2;
}parts;

But then Boost.Variant is a better alternative.