Re: [eigen] proposal to use static const integer class members instead of enum values

[Marc Glisse <marc.glisse@xxxxxxxx>]

On Sat, 13 Dec 2014, Benoit Jacob wrote:

> Anyway, my proposal seems pretty much dead at this point. Eigen's constants
> have been public and passable by const reference for years, so surely
> people are depending on it now, and we shouldn't break them. As to the idea
> of inheriting from an integral_constant struct, as far as I can see, Eigen
> doesn't have anywhere to put the definition of such structs, since it is
> headers-only, contrary to libc++ / libstdc++.

template<class>struct A{
  static const int i = 42;
};
template<class T> const int A<T>::i; // (1)
void f(const int&){}
int main(){
  f(A<char>::i);
}

This works even if line (1) is in a header, as multiple such definitions 
will be merged (like inline functions). But it only works for templates. 
And it is a pain to add a definition for each (public) constant.

The main goal of deriving from integer_constant is to have only one class 
that needs the definition, as long as the constant is always called value. 
But since in Eigen the constants can have many, many different names 
(IsColVector, ExpandMem, RowsAtCompileTime, OuterSize, etc) it wouldn't 
gain much if anything.

--
Marc Glisse