Initializing a static constexpr from an incomplete type because of a template base class

Question

I have a template base class, whereby subclasses are expected to pass themselves as the template parameter.

It looks a little like this:

template<         


        

Answer 1

I think you shall use lazy initialization. Actually Derived is still incomplete type; because the compiler don't know its size yet.

So the code shall be:

struct Derived : public Base<Derived>
{
    constexpr Derived(int x) : Base(x) {}

    static constexpr Derived getLIFE()
    {
        return Derived(34);
    }
};

EDIT: Same incomplete type behavior can be reproduced using this snippet:

struct MyStruct
{
    static constexpr int x = sizeof(MyStruct);
};

Answer 2

You can simply add constexpr to the definition of LIFE:

constexpr Derived Derived::LIFE = Derived(42);

Until recently GCC had a bug where it rejected this; you'll need to use either Clang or GCC 4.9.