Why are static members and static constant members different in initialization?

Question

class Test
{
private:
    static int data;
public:
    static constexpr int MAX{ 10 }; // OK
public:
    int getData() const { return data; }
    void setData(int rhs) { data = rhs; }
};

int Test::data{};

I have code like above. Why are static members and static constant members different in initialization? Is it just a difference in grammar?

You could do `static inline int data{};` if you want to initialize `data` *in situ*. — Eljay, Oct 22 '20 at 16:49

score 0 · Answer 1 · answered Oct 22 '20 at 18:24

That's because in C++17, constexpr implies inline, allowing in-line definitions with initialisers. A more fair comparison would be with static inline int data{};, which looks the same.

A constexpr variable has to have an initialiser (presumably so it can be used in constant expressions). Before C++17, this means that you have to write:

class Test
{
private:
    static int data;
public:
    static constexpr int MAX{ 10 };
public:
    int getData() const { return data; }
    void setData(int rhs) { data = rhs; }
};

constexpr int Test::MAX;
int Test::data{};

to be able to ODR-use MAX (essentially "take the address of"/"bind a reference to") since inline static data members didn't exist. This is also similar for constexpr and non-constexpr static data members.

Why are static members and static constant members different in initialization?

1 Answers1