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; ^

May be duplicate of this stackoverflow.com/questions/33177955/… ? — fghj
– fghj, Commented Nov 5, 2015 at 7:13
@user1034749, Not quite, I have a static constexpr member that is not a function member — asimes
– asimes, Commented Nov 5, 2015 at 7:20
Follow this question you can find out that you can not declare constexpr of type Mask inside class Mask that is your first problem. — fghj
– fghj, Commented Nov 5, 2015 at 7:22
You have 1) an incomplete type inside your type, 2) an infinite recursion of type content: each Mask contains a Mask. Try to write down the contents of a Mask object on paper. Stop when you reach the end of the page, to save trees. It never ends! — rubenvb
– rubenvb, Commented Nov 5, 2015 at 9:50

Richard Hodges · Accepted Answer · 2015-11-05 09:12:45Z

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; }

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