Assigning static constexpr member equal to static constexpr function?

Question

I have a C++ class with a static constexpr array. I'd like to assign it equal to another class member function. Right now I have the current solution, but I feel it's pretty suboptimal, and the function is no longer scoped to the class.

static constexpr std::array<Degrees, 270> angles()
{
    std::array<Degrees, 270> angles;
    for (size_t i = 0; i < angles.size(); i++)
    {
        angles[i] = static_cast<double>(i - 90.0) * (std::numbers::pi / 180.0);
    };
    return angles;
}

class MyClass
{
    static constexpr std::array<Degrees, 270> angles = angles();
}

Is there a better approach for initialising a static constexpr class member to a function like this? Ideally I'd do this using a member function of MyClass, with the implementation defined in the .cpp instead of the header.

"equal to a function" or "equal to the result of a function call"? — Ben Voigt
– Ben Voigt, Commented Oct 5, 2021 at 15:13
And no, you can't hide implementation details of constexpr code from the public header file. — Ben Voigt
– Ben Voigt, Commented Oct 5, 2021 at 15:14

Timo · Accepted Answer · 2021-10-05 21:58:01Z

2

If you want your member to be constexpr, you have to put the initialization in the header file.

Regarding your initialization, you can inline the function call by exploiting a lambda like so:

class MyClass
{
    static constexpr auto angles = []
    {
        std::array<Degrees, 270> angles;
        for (size_t i = 0; i < angles.size(); i++)
        {
            angles[i] = static_cast<double>(i - 90.0) * (std::numbers::pi / 180.0);
        };
        return angles;
    }();
};

edited Oct 5, 2021 at 21:58

answered Oct 5, 2021 at 15:25

Timo

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2 Comments

StoryTeller - Unslander Monica Over a year ago

I'd auto the type of the variable. Keeps it a bit DRYer since it must be specified in the lambda anyway.

Timo Over a year ago

@StoryTeller-UnslanderMonica fixed

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