static constexpr variable vs function

Question

Is there a difference between declaring floating point constant as a static constexpr variable and a function as in example below, or is it just a matter of style?

class MY_PI
{
public:
    static constexpr float MY_PI_VAR = 3.14f;
    static constexpr float MY_PI_FUN() { return 3.14f; }
}

Answer 1

constexpr functions

Functions have an advantage that free variables do not have (until C++14 that is): they can easily be templated without some class boilerplate. That means you can have your pi with a precision depending on a template argument:

template<typename T>
constexpr T pi();

template<>
constexpr float pi() { return 3.14f; }

template<>
constexpr double pi() { return 3.1415; }

int main()
{
    constexpr float a = pi<float>();
    constexpr double b = pi<double>();
}

However, if you decide to use a static member function instead of a free function, it won't be shorter nor easier to write than a static member variable.

constexpr variables

The main advantage of using a variable is that... well. You want a constant, right? It clarifies the intent and that may be one of the most important points here.

You could still have an equivalent behaviour with a class, but then, you would have to use it like this if your class is a class containing miscellaneous mathematics constants:

constexpr float a = constants<float>::pi;

Or like this if your class is only meant to represent pi:

constexpr double = pi<double>::value;

In the first case, you may prefer to use variables since it will be shorter to write and that will really show that you are using a constant and not trying to compute something. If you just have a class representing pi, you could however go with a free constexpr function instead of a whole class. It would IMHO be simpler.

C++14: constexpr variable templates

However, note that if you choose to use C++14 instead of C++11, you will be able to write the following kind of constexpr variable templates:

template<typename T>
constexpr T pi = T(3.1415);

That will allow you to write your code like this:

constexpr float a = pi<float>;

Starting from C++14, this may be the preferred way to do things. If you are using an older version of the standard, the first two paragraphs still hold.