Static constexpr int vs old-fashioned enum: when and why?

Question

This is maybe a basic question, but I cannot see the response by myself right now.

Consider the following code:

template
struct T {
            


        

Answer 1

I suspect it's legacy code.

enum { value = (b ? 42 : 0) };

is valid code in C++03 as well as C++11.

static constexpr int value = (b ? 42 : 0);

is valid only in C++11.

Even more, is there any case for which one of them is not a viable solution?

Both are viable solutions in C++11. The choice of which one to use depends on a team. It's going to be a matter of a policy decision.

As the answer by SergeyA indicates, enum are true constants. You cannot ODR-use them. You can ODR-use a constexpr. Depending on which of these is desirable for your application, you can decide whether to use enums or constexprs.