Re: [eigen] "plain": a new keyword for auto with implicit evaluation

[David Tellenbach <david.tellenbach@xxxxxxxxxxxxx>]

Hello,

I was thinking about:

Matrix M = ...;

SparseMatrix S = ....;

Array A = ....;

Ah, I see! I think something like this would be really great. The STL follows a similar 

approach by providing deduction guides for standard containers such that something

like

std::array {1, 2, 3, 4);

works now.

2. Using plain (using the implementation you provided) can have performance

penalties in the case of object slicing: Object slicing prevents copy elision!

If the following paper gets into C++20, then we could define "plain" as an alias template:

http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2018/p1021r2..html

Regarding this C++20 approach or a C++17 approach, I think we should start discussing

how and to what extend Eigen wants to allow functionality provided by newer standards. 

Officially we support C++03 and to some limited extend C++11. If we add additional functionality

code and documentation get more and more complicated (this feature is available using C++11,

that one using C++14 and this one using C++20).

How about some sort of collection of features we would like to see in newer versions of Eigen?

Based on something like this we could then decide if, e.g., Eigen 4 supports C++11 and some

C++14/17/20 features.

Cheers,

DT