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 http://stackoverflow.com/questions/33177955/static-constexpr-of-class-inside-class-link-problems ? — fghj, Nov 05 '15 at 07:13
@user1034749, Not quite, I have a `static constexpr` member that is not a function member — asimes, Nov 05 '15 at 07:20
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, Nov 05 '15 at 07:22
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, Nov 05 '15 at 09:50

score 1 · Accepted Answer · answered Nov 05 '15 at 09:12

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;
}

Move global constexpr class into class as static constexpr

1 Answers1

* Mask.hpp