Move global constexpr class into class as static constexpr

Question

I have some global constexpr Masks that I would like to make a part of my Mask class as a static constexpr to reduce globals in main.cpp

Currently this works:

• main.cpp has:

  constexpr Mask cg_completeMask(0xffffffffffffffffull, 0x1ffff);
  

• Mask.hpp has (reduced for SO):

  class Mask {
      unsigned long long m_64;
      unsigned int       m_32;
  
  public:        
      constexpr Mask(const unsigned long long ac_64, const unsigned int ac_32) :
          m_64(ac_64),
          m_32(ac_32)
      {}
  };
  

What I tried to move the global Masks from main.cpp:

• Mask.hpp has:

  class Mask {
      unsigned long long m_64;
      unsigned int       m_32;
  
  public:        
      static constexpr Mask completeMask;
  
      constexpr Mask(const unsigned long long ac_64, const unsigned int ac_32) :
          m_64(ac_64),
          m_32(ac_32)
      {}
  };
  

• Mask.cpp has:

  constexpr Mask Mask::completeMask(0xffffffffffffffffull, 0x1ffff);
  

What I tried produces these errors:

In file included from main.cpp:3:0:
Mask.hpp:12:27: error: constexpr static data member 'completeMask' must have an initializer
     static constexpr Mask completeMask;
                           ^
In file included from Mask.cpp:1:0:
Mask.hpp:12:27: error: constexpr static data member 'completeMask' must have an initializer
     static constexpr Mask completeMask;
                           ^

Answer 1

Best I could get was this, which ought to do what you want - but does unfortunately expose your constants in the interface. This is unavoidable I'm afraid.

* Mask.hpp

class Mask {
    unsigned long long m_64;
    unsigned int       m_32;


    // private constructor
    constexpr Mask(const unsigned long long ac_64, const unsigned int ac_32) :
    m_64(ac_64),
    m_32(ac_32)
    {}

public:

    // public declarations
    inline static constexpr Mask complete();
    inline static constexpr Mask partial();

};

// inline definitions
constexpr Mask Mask::complete()
{
    return Mask(0xffffffffffffffffull, 0x1ffff);
}

constexpr Mask Mask::partial()
{
    return Mask(0xf0f0f0f0f0f0f0f0ull, 0x1ffff);
}

example of use:

auto main() -> int
{
    auto m1 = Mask::complete();
    auto m2 = Mask::partial();

    return 0;
}