C++ (Somehow) limit struct to parent union size

Question

I'm attempting to create a color class of variable size- given a template-determined array of values, I'd like to create named aliases of each value in the array, ie:

template<int C = 3, typename T = unsigned char>
class Color {
public:
  union {
    T v[C];
    struct {
      T r, g, b, a;
    };
  };
};

However, if I try to use the same class for C=3, the union mandates a size of 4 bytes (the 'a' member). Alternatively, using a mathematically expressed bitfield size for a (struct named a, anonymous T member, size evaluates to 1 at C>3), the compiler issues a permissive warning (non-suppressible, as per In gcc, how to mute the -fpermissive warning? ), something unsuitable for a larger-scale API.

How would I go about allowing a single class to handle different numbers of variables, while retaining per-variable names and without implementing recursive-include macro magic (tried this, shouldn't have). Thanks in advance!

Edit: To clarify the question, an answer to any of the following will solve this problem:

• Suppress GCC's -fpermissive errors (#pragma diagnostic ignored doesn't work for permissive)
• Set maximum size of union or child struct not to exceed C bytes
• Allow bitfield length of 0 for members not covered by C bytes (GCC allows mathematical expressions for bitfield length, such as (C-3 > 0)?8:0; )
• Disable members not covered by C bytes by some other means (ie, mythical static_if() )

Answer 1

You could make a specialization of the struct for different cases of C:

template <int C = 3, typename T = unsigned char> union Color;

template <typename T>
union Color<3,T> {
  T v[3];
  struct {
    T r,g,b;
  };
};

template <typename T>
union Color<4,T> {
  T v[4];
  struct {
    T r,g,b,a;
  };
};

Note that anonymous structs are non-standard.

If using member functions is a possibility, I think that would be a better way to go:

template <int C,typename T>
class Color {
  public:
    using Values = T[C];

    Values &v() { return v_; }

    const Values &v() const { return v_; }

    T& r() { return v_[0]; }
    T& g() { return v_[1]; }
    T& b() { return v_[2]; }

    template <int C2 = C,
      typename = typename std::enable_if<(C2>3)>::type>
    T& a()
    {
      return v_[3];
    }

    const T& r() const { return v_[0]; }
    const T& g() const { return v_[1]; }
    const T& b() const { return v_[2]; }

    template <int C2 = C,
      typename = typename std::enable_if<(C2>3)>::type>
    const T& a() const
    {
      return v_[3];
    }

  private:
    Values v_;
};

You can then use it like this:

int main()
{
  Color<3,int> c3;
  Color<4,int> c4;

  c3.v()[0] = 1;
  c3.v()[1] = 2;
  c3.v()[2] = 3;

  std::cout <<
    c3.r() << "," <<
    c3.g() <<"," <<
    c3.b() << "\n";

  c4.v()[0] = 1;
  c4.v()[1] = 2;
  c4.v()[2] = 3;
  c4.v()[3] = 4;

  std::cout <<
    c4.r() << "," <<
    c4.g() << "," <<
    c4.b() << "," <<
    c4.a() << "\n";
}

Answer 2

Okay, so now @VaughnCato had this one out before me, but I'll still post my answer using std::enable_if. It declares Color as struct, because there is really no point in having a class when everything is public (and there is no good reason to declare the data member [v in the question] private), adds template aliases for a bit more syntactic sugar and uses static_assert to make sure the user doesn't use weird values for the template parameters. It also uses std::enable_if in a slightly different manner, which I would argue is a bit more readable.

#include <type_traits>
#include <cstdint>

template<unsigned nChans, typename T = std::uint8_t>
struct Color
{
    static_assert(nChans >= 3 || nChans <= 4,   "number of color channels can only be 3 or 4");
    // allow integral types only
    //static_assert(std::is_integral<T>::value,   "T has to be an integral type");
    // also allow floating-point types
    static_assert(std::is_arithmetic<T>::value, "T has to be an arithmetic (integral or floating-point) type");

    T data[nChans];

    T& r() { return data[0]; }
    T& g() { return data[1]; }
    T& b() { return data[2]; }

    //template<typename U = T, typename EnableIfT = std::enable_if<(nChans == 4), U>::type> // C++11
    template<typename U = T, typename EnableIfT = std::enable_if_t<(nChans == 4), U>> // C++14
    T& a() { return data[3]; }

    const T& r() const { return data[0]; }
    const T& g() const { return data[1]; }
    const T& b() const { return data[2]; }

    //template<typename U = T, typename EnableIfT = std::enable_if<(nChans == 4), U>::type>
    template<typename U = T, typename EnableIfT = std::enable_if_t<(nChans == 4), U>>
    T const& a() const { return data[3]; }
};

template<typename T = std::uint8_t> using RgbColor  = Color<3, T>;
template<typename T = std::uint8_t> using RgbaColor = Color<4, T>;