In a type trait, why do people use enum rather than static const for the value?

Question

For example, this is how I would write it, and it compiles and works just fine:

template struct is_pointer {
  static const bool valu         


        

Answer 1

Some people write the less obvious enum rather than static bool const because they don't realize that there are other changes they should make.

C++ requires the object to be defined if it's address is needed, for example if it's passed to this function foo:

void foo(bool const &);

However, solving the issue by defining the object is actually not the correct fix for this problem. Here are some alternatives:

1. Small objects should not be passed by reference. The change should be to remove const & from the function signature, not add a definition for the object.

2. Where the function signature cannot be changed, a temporary can be created explicitly in the call: foo( bool { Cls::mbr } )

3. However, this is compile time information! Therefore foo should be a template with a T and T* overload, or be specialized with bool.

This 3rd solution has the benefit of removing an unnecessary run time check (hopefully optimized by the compiler) and also allowing for the pointer and non-pointer case to be handled independently, possibly making the code clearer.